Formation of complex organic molecules in methanol and methanol-carbon monoxide ices exposed to ionizing radiation - A combined FTIR and reflectron time-of-flight mass spectrometry study

Article abstract of DOI:10.1039/c4cp04149f

The radiation induced chemical processing of methanol and methanol-carbon monoxide ices at 5.5 K exposed to ionizing radiation in the form of energetic electrons and subsequent temperature programmed desorption is reported in this study. The endogenous formation of complex organic molecules was monitored online and in situ via infrared spectroscopy in the solid state and post irradiation with temperature programmed desorption (TPD) using highly sensitive reflectron time-of-flight (ReTOF) mass spectrometry coupled with single photoionization at 10.49 eV. Infrared spectroscopic analysis of the processed ice systems resulted in the identification of simple molecules including the hydroxymethyl radical (CH₂OH), formyl radical (HCO), methane (CH₄), formaldehyde (H₂CO), carbon dioxide (CO₂), ethylene glycol (HOCH₂CH₂OH), glycolaldehyde (HOCH₂CHO), methyl formate (HCOOCH₃), and ketene (H₂CCO). In addition, ReTOF mass spectrometry of subliming molecules following temperature programmed desorption definitely identified several closed shell C/H/O bearing organics including ketene (H₂CCO), acetaldehyde (CH₃COH), ethanol (C₂H₅OH), dimethyl ether (CH₃OCH₃), glyoxal (HCOCOH), glycolaldehyde (HOCH₂CHO), ethene-1,2-diol (HOCHCHOH), ethylene glycol (HOCH₂CH₂OH), methoxy methanol (CH₃OCH₂OH) and glycerol (CH₂OHCHOHCH₂OH) in the processed ice systems. Additionally, an abundant amount of molecules yet to be specifically identified were observed sublimating from the irradiated ices including isomers with the formula C₃H_(x=4,6,8)O, C₄H_(x=8,10)O, C₃H_(x=4,6,8)O₂, C₄H_(x=6,8)O₂, C₃H_(x=4,6)O₃, C₄H₈O₃, C₄H_(x=4,6,8)O₄, C₅H_(x=6,8)O₄ and C₅H_(x=6,8)O₅. The last group of molecules containing four to five oxygen atoms observed sublimating from the processed ice samples include an astrobiologically important class of sugars relevant to RNA, phospholipids and energy storage. Experiments are currently being designed to elucidate their chemical structure. In addition, several reaction pathways were identified in the irradiated ices of mixed isotopes based upon the results of both in situ FTIR analysis and TPD ReTOF gas phase analysis. In general, the results of this study provide crucial information on the formation of a variety of classes of organics including alcohols, ketones, aldehydes, esters, ethers, and sugars within the bulk ices upon exposure to ionizing radiation that are relevant to the molecular clouds within the interstellar medium.

Full text of DOI:10.1039/c4cp04149f

View Article Online
View Journal
PCCP
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Maity, R. I.
Kaiser and B. M. Jones, Phys. Chem. Chem. Phys., 2014, DOI: 10.1039/C4CP04149F.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/pccp

Page 1 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
Formation of Complex Organic Molecules in Methanol and Methanol - Carbon Monoxide
Ices Exposed to Ionizing Radiation - A Combined FTIR and Reflectron Time-of-Flight
Mass Spectrometry Study
Surajit Maity, Ralf I. Kaiser* and Brant M. Jones*
Department of Chemistry
W.M. Keck Research Laboratory in Astrochemistry
University of Hawai’i at Manoa
Honolulu, HI 96822, USA
1

Physical Chemistry Chemical Physics
Page 2 of 77
View Article Online
DOI: 10.1039/C4CP04149F
Abstract
The radiation induced chemical processing of methanol and methanolꢀcarbon monoxide ices at
.5 K exposed to ionizing radiation in the form energetic electrons and subsequent temperature
5
programmed desorption is reported in this study. The endogenous formation of complex organic
molecules was monitored online and in situ via infrared spectroscopy in the solid state and post
irradiation with temperature programmed desorption (TPD) using highly sensitive reflectron
timeꢀofꢀflight (ReTOF) mass spectrometry coupled with single photoionization at 10.49 eV.
Following infrared spectroscopic analysis of the processed ice systems resulted in the
identification of simple molecules including the hydroxymethyl radical (CH OH), formyl radical
2
(
(
(
HCO), methane (CH ), formaldehyde (H CO), carbon dioxide (CO ), ethylene glycol
4 2 2
HOCH CH OH), glycolaldehyde (HOCH CHO), methyl formate (HCOOCH ), and ketene
2
2
2
3
H CCO). In addition, ReTOF mass spectrometry of subliming molecules following temperature
2
program desorption definitely identified several closed shell C/H/O bearing organics including
ketene (H CCO), acetaldehyde (CH COH), ethanol (C H OH), dimethyl ether (CH OCH ),
2
3
2
5
3
3
glyoxal (HCOCOH), glycolaldehyde (HOCH CHO), etheneꢀ1,2ꢀdiol (HOCHCHOH), ethylene
2
glycol
(HOCH CH OH),
methoxy
methanol
(CH OCH OH)
and
glycerol
2
2
3
2
(
CH OHCHOHCH OH) in the processed ice systems. Additionally, an abundant amount of
2
2
molecules yet to be specifically identified were observed sublimating from the irradiated ices
including isomers with formulae: C H O, C H O, C H O , C H O ,
3
(x=4,6,8)
4
(x=8,10)
3
(x=4,6,8)
2
4
(x=6,8)
2
C H
O , C H O , C H
O , C H
O and C H
O . The last groups of molecules
(x=6,8) 5
3
(x=4,6)
3
4
8
3
4
(x=4,6,8)
4
5
(x=6,8)
4
5
containing four to five oxygen atoms observed sublimating from the processed ice samples
include astrobiologically important class of sugars; relevant in RNA, phospholipids and energy
storage. Experiments are currently being designed to elucidate their chemical structure. In
addition, several reaction pathways were identified in the irradiated ices of mixed isotopes based
upon the results of both in situ FTIR analysis and TPD ReTOF gas phase analysis. In general,
the results of this study provide crucial information on the formation of a variety of classes of
organics including alcohols, ketones, aldehydes, esters, ethers, and sugars within the bulk ices
upon exposure to ionizing radiation that are relevant to the molecular clouds within the
interstellar medium.
2

Page 3 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
1. Introduction
During the last decades, cold molecular clouds and star forming regions have been extensively
explored for complex organic molecules as these species serve as a ‘molecular clock’ and thereꢀ
by aid in an understanding of the chemical evolution of the interstellar medium. In these extreme
1
environments, approximately 60 amongst the currently detected 180 molecules contain six or
more atoms with at least one carbon atom and are labelled as complex organic molecules
2
, 3
(
COMs). Furthermore, those molecules containing oxygen such as acetaldehyde (CH CHO),
3
acetic acid (CH COOH), glycolaldehyde (HOCH CHO), formamide (HCONH ), and acetamide
3
2
2
(
CH CONH ) have received considerable interest from the astrochemistry and astrobiology comꢀ
3 2
4
ꢀ6
munities as these are considered as key precursors and building blocks of biologically imporꢀ
7
8
6, 9
tant molecules like carbohydrates, amino acids, and polypeptides. The discoveries of several
sugar related molecules such as dihydroxyacetone (simple sugar), gylcerol (sugar alcohol), and
9
10, 11
gylceric acid (sugar acid) along with amino acids
in carbonaceous meteorites such as
12,
Murchison encouraged the search for their interstellar origin as is hinted by their isotope ratios.
1
3
14ꢀ18
In addition, glycolaldehyde (the simplest form of a sugar, HOCH CHO)
and ethylene
have been detected in distinct astrophysical
environments ranging from hot cores to the galactic center molecular clouds. Although searches
2
1
7, 19, 20
glycol (a sugar alcohol, HOCH CH OH)
2
2
2
1
for higher mass sugars such as glyceraldehyde (HOCH CH(OH)CHO)
and 1,3ꢀ
2
22ꢀ24
dihydroxyacetone ((HOCH ) CO)
have been carried out, these molecules have still remained
2
2
elusive to date.
A current catalogue of observed oxygen bearing complex organic molecules is listed in the
supporting information Table S1 along with the locations of their detection and molecular
abundances with most of these molecules detected in the Sgr B2(N) hot core. Recently however,
RequenaꢀToress et al. detected several of these molecules in the cold molecular clouds (MC Gꢀ
1
7, 25
0.11ꢀ0.08, MC Gꢀ0.02ꢀ0.07, and MC G+0.69ꢀ0.03) within the Galactic Center.
Despite wellꢀ
established detections of these interstellar molecules, a detailed understanding of the formation
pathways has remained elusive. Often, the abundances of complex organics observed toward
protostellar cores at temperatures of up to 100 K cannot be explained by networks of gas phase
2
, 3, 26
processes via neutralꢀneutral and/or ionꢀmolecule reactions.
For example, the formation of
3

Physical Chemistry Chemical Physics
Page 4 of 77
View Article Online
DOI: 10.1039/C4CP04149F
complex organic molecules in the gas phase often requires the involvement of an internally
(rovibrationally) excited intermediate, which is highly short lived on the picosecond timescale
2
without a third body for collision induced relaxation. Subsequent models have attempted to
boost the production rates of COMs by incorporating grainꢀsurface chemistry, often involving
2
7, 28
the use of radicalꢀradical reactions on interstellar grains.
In addition, chemical network
models attempting to explain the observed abundances of complex organics in the hot molecular
core have acknowledged the requirement to include induced suprathermal chemical reactions
2
9ꢀ33
within icy mantles via UV photons and galactic cosmic rays.
The significance of nonꢀ
traditional chemistry within the bulk ice is again emphasized based on the relative abundances of
COMs in the cold Central Molecular Zone and the Galactic Disk, where observations strongly
suggest that the COMs are formed in the icy mantles followed by ejection to the gas phase by
1
7, 25
shock waves.
Further, the fractional abundances of molecules such as methyl formate
(
HCOOCH ) and ethanol (C H OH) in the outflow of L1157 suggest that the time scale for the
3
2
5
gas phase synthesis is too short, i.e. less than 2,000 years are required for the formation of these
COMs following the proposed reaction network and that these complex molecules were therefore
3
4
most probably formed in the icy mantles followed with desorption due to outflow shocks.
3
5, 36
Of the many successes from the Infrared Space Observatory
and the Spitzer c2d ice survey,
was the confirmation that interstellar grains are coated with an icy mantle consisting mainly
of water (H O) followed by methanol (CH OH), carbon monoxide (CO), carbon dioxide (CO ),
3
7ꢀ41
2
3
2
methane (CH ) ammonia (NH ), and formaldehyde (H CO) holding a thicknesses of up to a few
4
3
2
hundreds of nanometers. The interaction of these ices with energetic ions simulating galactic
cosmic rays (GCRs) and with ultraviolet photons (UV) simulating the internal ultraviolet field
inside cold molecular clouds has repeatedly been demonstrated to chemically modify ices via
nonꢀthermal, nonꢀequilibrium chemistry involving radical reactants such as hydrogen, oxygen,
42ꢀ47
nitrogen, and carbon atoms, which are not in thermal equilibrium with the 10 K ices.
Therefore, in cold molecular clouds, the interaction of ionizing radiation with ice coated
nanoparticles is expected to lead to the synthesis of complex organic molecules, which cannot be
2
, 3
explained by classical thermal chemistry. Once the molecular cloud collapses and transforms
to a starꢀforming region, the elevated temperatures result in a sublimation of these newly formed
organic molecules from the grains into the gas phase thereby explaining the abundances of these
molecules in the hot cores and corinos. In addition, nonꢀthermal desorption via cosmic ray
4

Page 5 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
particles, grainꢀgrain collisions, and/or shocks may result in the ejection of COMs into the gas
2
, 3
phase of the cold cloud cores.
Previous laboratory experiments mimicking interstellar ices exposed to ionizing radiation have
provided compelling evidence on the formation of complex organic molecules in the ices at
temperatures at low as 5 K. Acetaldehyde for instance has been shown to be formed in methane
4
8
(
CH ) ꢀ carbon monoxide (CO) and ethylene (C H ) ꢀ carbon dioxide (CO ) ices upon electron
4 2 4 2
irradiation at 10 K thus simulating the effects of secondary electrons within the track of GCRs
4
9
once penetrating the ice coated interstellar grains. The thermodynamically less stable C H O
2
4
isomers, vinyl alcohol (CH CHOH) and ethylene oxide (cꢀC H O), were found as well in the
2
2
4
4
9
ethylene (C H ) ꢀ carbon dioxide (CO ) irradiated ice. Structural isomers propynal (HCCCHO)
2
4
2
and cyclopropenone (cꢀC H O) were formed in an icy mixture of acetylene (C H ) and carbon
3
2
2
2
5
0
monoxide (CO) at 10 K upon radiolysis. Additionally, formic acid (HCOOH) and acetic acid
(
CH COOH) were found to be endogenous products in a water (H O) – carbon monoxide (CO)
3
2
5
1
and methane (CH ) – carbon dioxide (CO ) ices upon exposure to ionizing radiation,
4
2
5
2
respectively. Following the first detection of glycolaldehyde (HOCH CHO) in the galactic
2
1
4
center, irradiation experiments with ices consisting of methanol (CH OH) and methanol
3
(
CH OH) ꢀ carbon monoxide (CO) were conducted in hopes of explaining the origin of this sugar
3
together with observed isomers i.e., methyl formate (HCOOCH ) and acetic acid (CH COOH) as
3
3
both methanol (CH OH) and carbon monoxide (CO) are the two most abundant species (relative
3
3
5ꢀ41
53
to water) in the interstellar ices.
Indeed, experiments on methanol ice and a binary ices of
54
methanol and carbon monoxide exposed to ionizing radiation resulted in the identification of
glycolaldehyde (HOCH CHO), methyl formate (HCOOCH3), and ethylene glycol
2
(HOCH CH OH) in addition to the formyl radical (HCO), hydroxyl radical (OH), methane
2 2
(
CH ), carbon dioxide (CO ), and formaldehyde (H CO) at 10 K. Similar work was conducted
4
2
2
utilizing broad band UV photons produced via a hydrogen microwave discharge lamp and found
evidence of methyl formate (HCOOCH ) and glycolaldehyde (HOCH CHO) as a potential
3
2
55
products post photolysis of methanol (CH OH) and mixtures with carbon monoxide (CO);
a
3
subsequent study on UV processed methanol and methanolꢀcarbon monoxide ices identified
methyl formate (HCOOCH ), ethylene glycol (HOCH CH OH), ethanol (C H OH), and
3
2
2
2
5
5
6
dimethyl ether (CH OCH ) Methoxymethanol (CH OCH OH) was first identified in the
3
3
3
2
exposure of methanol (CH OH) ices to low energy electron and later confirmed along with other
3
5

Physical Chemistry Chemical Physics
Page 6 of 77
View Article Online
DOI: 10.1039/C4CP04149F
COMs such as ethylene glycol (HOCH CH OH), glycolaldehyde (HOCH CHO), and possibly
2
2
2
5
7
glycolic acid (HOCOCH OH). Additional experiments via 200 keV proton bombardment upon
2
frozen methanol (CH OH) and mixtures with carbon monoxide (CO) resulted once again in the
3
5
8, 59
58, 59
identification of glycolaldehyde (HOCH CHO),
methyl formate (HCOOCH3)
and
2
5
9
ethylene glycol (HOCH CH OH). Chen et al. reported formation of diemthylether (CH OCH )
2
2
3
3
glycolaldehyde (HOCH CHO), methyl formate (HCOOCH ), acetic acid (CH COOH), and
2
3
3
ethylene glycol (HOCH CH OH) in irradiated methanol ices using soft Xꢀrays containing peak
2
2
6
0
energies of 300 eV and 550 eV over a broad band spectrum (250ꢀ1200 eV). To summarize,
multiple experiments have been conducted over the last decades regarding the chemical
modification of methanol ices upon exposure to ionizing radiation simulating astrophysical
conditions. Within these studies, a consensus on the formation of small molecules e.g., carbon
monoxide (CO), carbon dioxide (CO ), formaldehyde (H CO), methane (CH ) and for the most
2
2
4
part ethanol (C H OH), glycolaldehyde (HOCH CHO), ethylene glycol (HOCH CH OH),
2
5
2
2
2
methyl formate (HCOOCH ), and acetic acid (CH COOH) have been ascertained via in situ
3
3
infrared spectroscopy. Unfortunately, relying only on this technique can lead often to ambiguous
assignments of large organics with similar functional groups due to the overlap of their group
frequencies, in particular of the important carbonyl group.
Recently, utilizing single photoionization reflectron timeꢀofꢀflight (ReTOFꢀPI) mass spectroꢀ
metry in addition to solid state FTIR spectroscopy, significant progress has been made toward a
detailed understanding of several key classes of complex organic molecules carrying the
6
1, 62
carbonyl functional group.
Moreover, we have reported on the detection of glycolaldehyde
(
HOCH CHO) in irradiated methanol and methanolꢀcarbon monoxide ices exposed to energetic
2
6
3
electrons via ReTOFꢀPI mass spectrometry. The continued success of ReTOFꢀPI mass
spectrometry approach in identifying astrochemically relevant complex organics synthesized in
simple ices of methanol and methanolꢀcarbon monoxide is explored here. In this study, we
present compelling evidence on the formation of key classes of complex organic molecules
synthesized in irradiated ices of methanol (CH OH) and methanol (CH OH) – carbon monoxide
3
3
(CO) from infrared spectral data correlated with temperature program desorption (TPD) studies
exploiting ReTOFꢀPI gas phase detection at doses relevant to the lifetime of an interstellar icy
grain within a cold molecular cloud prior to the warm up (star formation) phase.
6

Page 7 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
2. Experimental
The experiments were carried out in a novel, contaminationꢀfree ultraꢀhigh vacuum (UHV)
chamber at the W.M. Keck Research Laboratory in Astrochemistry. A detailed description of the
6
1ꢀ64
instrumentation has been described previously.
Briefly, the main chamber is evacuated down
ꢀ11
to a base pressure of a few 10 torr by oilꢀfree magnetically suspended turbomolecular pumps
backed with dry scroll pumps. A closedꢀcycle helium refrigerator (Sumitomo Heavy Industries,
RDKꢀ415E) cools a polished silver substrate mounted to the cold finger to final temperature of
5.5 ± 0.1 K. The entire cold finger assembly is freely rotatable within the horizontal center plane
and translatable in the vertical direction via an UHV compatible bellow (McAllister, BLT106)
and a differential pumped rotational feed through (Thermoionics Vacuum Products, RNNꢀ
600/FA/MCO). The corresponding gases (methanol vapor and premixed methanolꢀcarbon
monoxide gas) were then deposited through a glass capillary array with a background pressure
ꢀ8
reading in the main chamber of about 5 × 10 torr for approximately 3 minutes. This yielded ice
samples with thicknesses of 510 ± 10 nm for pristine methanol ices and 495 ± 10 nm for mixed
ices of methanolꢀcarbon monoxide. The thickness of the samples was determined in situ using
6
5, 66
laser interferometry
with a heliumꢀneon (HeNe) laser (CVI MellesꢀGriot, 25ꢀLHPꢀ213) at
632.8 nm at an incident angle of 4°. From this technique, based upon the ratios of peak to peak
67
intensities, we derived an index of refraction n = 1.34 ± 0.02 for pure methanol, in agreement
f
6
8
with published data of 1.35, and n = 1.35 ± 0.02 for the binary CH OHꢀCO mixture.
f
3
In order to determine the ratios of the components in the methanol – carbon monoxide ices,
additional calibration experiments were performed. Here, methanol ices with distinct thicknesses
were deposited under identical experimental conditions. Subsequently, the ices were annealed at
ꢀ1
0
.5 K min allowing the ices to sublime with gas phase molecules monitored using a quadrupole
mass spectrometer (QMS; Extrel, Model 5221) operated as a residual gas analyzer exploiting
electron impact ionization with 100 eV electrons at a current of 1 mA. The resultant total QMS
+
signal at m/z = 32 amu (CH OH ) was then integrated and correlated as a function of the
3
deposited molecules determined from the ice thicknesses and known density of the pure
methanol samples. As such, we can determine the total number of methanol molecules in the
mixed ices of methanolꢀcarbon monoxide by comparing the QMS ion signal at m/z = 32 amu
1
7
with the calibration curve. Here, a total of (4.6 ± 0.5) × 10 methanol molecules were
7

Physical Chemistry Chemical Physics
Page 8 of 77
View Article Online
DOI: 10.1039/C4CP04149F
6
9
determined; this results in a thickness of 240 ± 20 nm. In the limit of volume additivity, we can
calculate a thickness of (255 ± 30) nm for carbon monoxide by subtracting the methanol
1
7
thickness (240 ± 20 nm) from total thickness resulting in an estimated value of (6.3 ± 0.6) × 10
carbon monoxide molecules. Consequently, the mixed methanolꢀcarbon monoxide ice was found
to be in the ratio of (4.0 ± 0.2) : (5.0 ± 0.2). Here, the densities of methanol and carbon monoxide
ꢀ3
ꢀ3
54
ices were used as 1.020 g cm , 1.029 g cm , respectively. In addition, isotopically labeled
1
3
18
CD OD, CH OH, and CH OH ices (SigmaꢀAldrich) for pure methanol irradiation
3
3
3
1
3
18
3
13
18
18
experiments and CD ODꢀCO, CH OHꢀCO, CH OHꢀCO, CD ODꢀ CO, CH OHꢀC O, and
3
3
3
3
18
CH OHꢀC O (labelled CO was supplied by Cambridge Isotope Labs) mixed ices for methanolꢀ
3
carbon monoxide irradiation experiments were also conducted to confirm the identified products
via isotope shifts of the infrared absorption bands and in the reflectron timeꢀofꢀflight data.
The ice systems were then irradiated with 5 keV electrons isothermally at 5.5 ± 0.1 K for one
2
hour at 30 nA over an area of 1.0 ± 0.1 cm and an angle of incidence of 70° relative to the
surface normal of the ice. The total dose deposited into the ice sample was determined from
7
0, 71
Monte Carlo simulations (CASINO)
taking into consideration the back scattering coefficient,
the energy deposited from the back scattered electrons, and the average penetration depth (SI
Table S2). The total energy deposited into the ice was 6.5 ± 0.8 eV per CH OH molecule, and
3
5
.2 ± 0.8 eV per molecule on average for the irradiation experiments of the binary CH OHꢀCO
3
(4:5) ice mixture. It should be noted here, that in determining the applied dose of the isotopic
analogs, that both the index of refraction and density were assumed to be that their respective
normal counterparts as most of these data are not empirically available. Furthermore, the density
ꢀ3
of the CH OHꢀCO ice mixture was calculated as 1.026 g cm based on the weighted fraction of
3
69
their respective pure densities.
For the on line and in situ identification of new molecular band carriers of the ices during
irradiation, a Fourier transform infrared spectrometer (Nicolet 6700) monitored the samples
throughout the duration of the experiment with an IR spectrum collected every two minutes in
ꢀ1
ꢀ1
the range of 6000 – 400 cm at a resolution of 4 cm . Each FTIR spectrum was recorded in
absorption–reflection–absorption mode (reflection angle of 45°) for two minutes resulting in a set
of 30 infrared spectra during the radiation exposure (one hour) for each system. After the
irradiation, the sample was kept at 5.5 K for one hour; then, temperature programmed desorption
8

Page 9 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
ꢀ1
(
TPD) studies were conducted by heating the irradiated ices at a rate of 0.5 K min to 300 K.
Throughout the thermal sublimation process, the ice samples were monitored via infrared
6
1, 62
spectroscopy and single photon ionization reflectron timeꢀofꢀflight mass spectrometry
separately. The products were ionized upon sublimation via single photon ionization exploiting
pulsed (30 Hz) coherent vacuum ultraviolet (VUV) light at 118.2 nm (10.49 eV). Here, the third
harmonic (354.6 nm) of a highꢀpower pulsed Nd:YAG laser (Spectra Physics, PRO – 250; 30 mJ
per pulse) was frequency tripled to produce VUV photons utilizing xenon (Xe) gas as the
7
2
nonlinear medium. A pulsed valve directed the xenon gas (99.999 %; Specialty Gases of
America) at a backing pressure of 1266 Torr into a Tꢀshaped stainless steel adapter with 1 mm
diameter hole and 25 mm in length in line with the propagating laser beam. The generated VUV
light was then separated and directed to about 1 mm above the ice surface utilizing an offꢀaxis,
7
3
differentially pumped lithium fluoride (LiF) lens, where the sublimating molecules are then
photoionized. The molecular ions were detected utilizing a multichannel plate with a dual
chevron configuration following a fast preamplifier (Ortec 9306) and shaped with a 100 MHz
discriminator (Advanced Research Instruments, Fꢀ100TD). The ReTOF spectra were then
recorded with a personalꢀcomputerꢀbased multichannel scaler (FAST ComTec, P7888ꢀ1 E) using
a bin width of 4 ns, triggered at 30 Hz with 3600 sweeps per mass spectrum reflecting a 1 K
change in temperature. Additionally, the subliming molecules were also probed via a quadrupole
mass spectrometer (Extrel, Model 5221) operating in a residualꢀgas analyzer mode in the mass
range of 1 − 500 amu with electron impact ionization of 100 eV and an emission current of 1
mA.
To assist in the identification of a specific molecules sublimating in the temperature program
desorption spectra utilizing ReTOFꢀPI mass spectrometry, calibration experiments were
performed under identical experimental conditions and annealing rates. Here, the vapor of a
specific molecule was premixed in pure methanol and/or methanolꢀcarbon monoxide and
subsequently deposited onto the substrate kept at 5.5 K. Then, TPD was conducted by heating
ꢀ1
the premixed ices at the same rate of 0.5 K min up to 300 K and subsequently detected using
ReTOFꢀPI mass spectrometry. In summary, a total of 12 calibration experiments was performed
with 17 different molecules in both methanol and methanolꢀcarbon monoxide ices. The details of
the calibration samples and the relative percent amount in the premixed ices are listed in SI Table
S3.
9

Physical Chemistry Chemical Physics
Page 10 of 77
View Article Online
DOI: 10.1039/C4CP04149F
3. Results
3.1. Infrared Spectroscopy
1
3
The infrared spectra of the isotopologue ice systems of methanol (CH OH, CD OD, CH OH
3
3
3
18
and CH OH) and mixed ice systems of methanolꢀcarbon monoxide (CH OHꢀCO, CD ODꢀCO,
3
3
3
18
18
13
13
18
18
CH OHꢀC O, CH OHꢀCO, CD ODꢀ CO, CH OHꢀCO and CH OHꢀC O) recorded before
3
3
3
3
3
and after the irradiation are shown in Figures 1A and 1B as well as in SI Figure S1, respectively.
ꢀ1
ꢀ1
The newly formed products are also shown in the 2200 ꢀ 1600 cm and 1400 ꢀ 800 cm regions
of interest along with assignments. Infrared absorption features of the pristine ices and new
absorption features that emerged following irradiation are summarized in Table 1 and Table 2,
respectively. Upon exposure to ionizing radiation several products were observed in situ within
the bulk ice. Here, in both the methanol and the mixed methanolꢀcarbon monoxide ices the
ꢀ1
hydroxymethyl radical (CH OH) was identified via the ν fundamental at 1192 cm and 1193
2
4
ꢀ1
ꢀ1
cm , respectively; the formyl radical (HCO) was gauged from the ν fundamental at 1842 cm in
3
ꢀ1
both irradiated ices; methane (CH ) was detected via the ν fundamental at 1304 cm and 1303
4
4
ꢀ1
cm . Formation of formaldehyde (H CO) was confirmed from the ν , ν , and ν fundamentals at
2
2
3
4
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
1
246 cm , 1499 cm and 1726 cm in irradiated CH OH ices and at 1249 cm , 1497 cm , and
3
ꢀ1
1
726 cm in the irradiated CH OHꢀCO ices. These absorption frequencies are in agreement with
3
5
3ꢀ55, 60, 74
previous studies.
Formation of carbon monoxide (CO) was confirmed via the ν1
ꢀ1
fundamental at 2135 cm in the irradiated CH OH ice systems. Carbon dioxide (CO ) was as
3
2
well observed in both the irradiated CH OH and mixed CH OHꢀCO ices as confirmed by the ν
3
3
3
ꢀ1
ꢀ1
band at 2339 cm and 2342 cm , respectively. Ethylene glycol (HOCH CH OH) was identified
2
2
ꢀ1
in both the irradiated ices via the ν fundamental at 1094 cm based on the assignment of this
9
5
3, 74
ꢀ1
ꢀ1
molecule in previous studies of irradiated methanol ices
that all other relatively stronger infrared absorption bands of ethylene glycol coincidentally
at 1090 cm and 1088 cm . Note
5
3, 54
overlap with the methanol absorptions
and therefore are masked. The assignments of these
absorptions were also confirmed via their isotopic shifts in irradiated ices consisting of CD OD,
3
1
1
3
3
18
18
18
CH OH, CH OH and irradiated binary mixed ices consisting of CD ODꢀCO, CH OHꢀC O,
3
3
3
3
13
18
3
18
CH OHꢀCO, CD ODꢀ CO, CH OHꢀCO and CH OHꢀC O as compiled in Table 2. We have
3
3
3
1
3
18
3
also identified ketene (H CCO) in CH OH and CH OH ices via the observation of ν
2
2
3
ꢀ1
13 13
ꢀ1
18
fundamental at 2067 cm (H C CO) and 2107 cm (H CC O), respectively, as shown in
2
2
Figure 1A. The assignment of ketene agrees with previously reported observations of the ν2
10

Page 11 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
ꢀ
1
13 13
ꢀ1
18
64, 75
fundamental at 2071 cm for H C CO and 2107 cm for H CC O, respectively.
In the
2
2
case of methanolꢀcarbon monoxide ices, ν absorption features of ketene were also identified at
2
ꢀ1
18
18
18
ꢀ1
18
18
2
107 cm (H CC O) in CH OHꢀC O ice and at 2106 cm (H CC O) in CH OHꢀC O ice in
2 3 2 3
6
4, 75
agreement with the literature values.
Note that only isotopologues of ketene are observed as
the absorption features of the natural H CCO isotopomer and the ν absorption band of carbon
2
1
ꢀ1
monoxide directly overlap at ~2135 cm .
.1.1. Infrared Absorption Spectra of the Carbonyl Functional Group
The new absorption features connected to the carbonyl functional group in the 1800ꢀ1600 cm
3
ꢀ1
region deserve special attention. These bands are very broad (Figure 1C and 1D) and subsequent
assignment to only one molecular carrier is erroneous. Further evidence suggesting more than
one molecular carrier to this functional group is gained from the additional infrared absorption
features of the carbonyl band regions in the irradiated mixed isotopic ice systems of methanolꢀ
1
3
13
18
18
carbon monoxide ( CH OHꢀCO, CD ODꢀ CO, CH OHꢀCO and CH OHꢀC O) as shown in
3
3
3
3
ꢀ1
Figure 1D. Therefore, a deconvolution to the absorption features in 1800–1600 cm region was
performed with the peak positions and their associated assignments shown in Figure 1 part C and
D, and listed in Tables 2 and 3. In both the irradiated ices of methanol (CH OH) and methanolꢀ
3
carbon monoxide (CH OHꢀCO), the deconvolution identified four distinct bands centered at
3
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
1
743 cm , 1726 cm , 1714 cm , and 1697 cm . The band at 1743 cm has been assigned to the
ν14 band of glycolaldehyde (HOCH CHO) based on previous identification of this molecule
2
ꢀ1 53
within irradiated methanol ices at 1747 cm , irradiated methanolꢀcarbon monoxide ices at 1757
ꢀ1 54
ꢀ1 76, 77
ꢀ1
cm , and matrix isolation studies of glycolaldehyde at 1747 cm .
was assigned to the ν fundamental of methyl formate (HCOOCH ); in agreement to previous
The band at 1714 cm
1
4
3
53, 54
electron irradiation experiments of methanol and methanolꢀcarbon monoxide ices,
UV
5
5
58, 59
photolysis of methanol ices, and with pure frozen methyl formate at 16 K.
The absorption
ꢀ1
at 1726 cm has been assigned to the ν fundamental of formaldehyde (H CO) again, based on
4
2
5
3ꢀ55, 58, 59
ꢀ1
previous literature values.
Finally, the band observed at 1697 cm can be attributed to
the Fermi resonance splitting of the ν14 fundamental and the 2ν overtone band of
6
5
4, 60, 77
glycolaldehyde.
Further support for the above assignments is gained through observation
of the frequency shifts in the infrared absorption features of the isotopically labeled ices of
11

Physical Chemistry Chemical Physics
Page 12 of 77
View Article Online
DOI: 10.1039/C4CP04149F
13
18
3
18
3
18
CD OD, CH OH, and CH OH and in the binary ices of CD ODꢀCO, CH OHꢀC O,
3
3
3
1
3
13
18
18
CH OHꢀCO, CD ODꢀ CO, CH OHꢀCO and CH OHꢀC O as compiled in Table 3.
3
3
3
3
Recently, we have reported on the formation of a series of saturated and unsaturated aldehyde/
6
1
ketones identified in methaneꢀcarbon monoxide ices exposed to ionizing radiation. As both
irradiated ices in the present study reveal broad complex structure associated with the carbonyl
stretching mode, we present a comparison of the deconvoluted band positions in: (i) the pertinent
regions observed in the present experiments and (ii) the carbonyl stretching vibrations of the
methaneꢀcarbon monoxide ices in Table 3. The infrared absorption due to the ν fundamental
4
(
CO stretching) of formaldehyde (H CO) in both CH OH and CH OHꢀCO ices were observed at
2 3 3
ꢀ1
ꢀ1
1
726 cm . Acetaldehyde (CH CHO) was attributed to the band (ν ) at 1727 cm in irradiated
3
4
6
1
ꢀ1
CH ꢀCO ices; as such, acetaldehyde may contribute to the observed band at 1726 cm here as
4
well. Further support for the assignment of acetaldehyde stems from the identification of the ν4
fundamental of the isotopically labeled acetaldehyde in both the irradiated isotopologue
methanol and mixed methanolꢀcarbon monoxide ices. Here, deuterated acetaldehyde (CD CDO)
3
ꢀ1 61
was previously observed at 1715 cm
and therefore contributes to the infrared absorption
ꢀ1
ꢀ1
bands witnessed here at 1711 cm in irradiated CD OD and at 1714 cm in irradiated CD ODꢀ
3
3
18
ꢀ
CO ices. In addition, acetaldehyde isotopomer (CH CH O) was previously assigned at 1694 cm
3
1
61
ꢀ1
ꢀ1
ꢀ1
and consequently was attributed to the observed bands at 1693 cm , 1693 cm , 1692 cm
ꢀ1
18
18
18
18
18
and 1695 cm in the processed CH OH, CH OHꢀC O, CH OHꢀCO and CH OHꢀC O ices
3
3
3
3
1
8
18
respectively, as well. Further, following irradiation of the O isotopically mixed ices, CH OHꢀ
3
1
8
CO and CH OHꢀC O, evidence suggesting the formation of two acetaldehyde isotopomers
3
18
ꢀ1
ꢀ1
(
CH CHO and CH CH O) is found in the observed bands at 1724 cm and 1694 cm ,
3 3
respectively. Further evidence on the formation of acetaldehyde is also confirmed using ReTOF
mass spectrometry following TPD studies as discussed below (Section 3.2.).
ꢀ1
Glycolaldehyde (HOCH CHO) is observed at 1743 cm (ν ) in both irradiated CH OH and
2
14
3
ꢀ1
mixed CH OHꢀCO ices. However, the infrared absorption band at 1743 cm may also have
3
contribution from saturated aldehydes such as propanal (CH CH CHO) and butanal (C H CHO),
3
2
3
7
ꢀ1
as the carbonyl stretching mode of these molecules has been attributed previously at 1746 cm
6
1
as well. Further evidence in support of saturated aldehydes contributing to this carbonyl band
again stems from the isotopically labeled irradiated ices and the assignments of the deconvoluted
12

Page 13 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
1
8
bands of the corresponding carbonyl stretching region (Table 3). Here, the O isotope labeled
1
8
ꢀ1
saturated aldehydes (RCH O) were observed at 1717 cm in the irradiated methaneꢀcarbon
6
1
monoxide ices as previously reported, and therefore may contribute here to the ν14 of
1
8
ꢀ1
18
3
ꢀ1
18
3
glycolaldehyde (HOCH CH O) observed at 1713 cm in CH OH, 1715 cm in CH OHꢀ
2
18
ꢀ1
18
ꢀ1
18
C O, 1708 cm in CH OHꢀCO and 1707 cm in CH OHꢀC O ices. Similarly, the ν
14
3
3
ꢀ1
fundamental of methyl formate (HCOOCH ) observed at 1714 cm in both the irradiated
3
ꢀ1
CH OH and CH OHꢀCO ices can also have contribution from saturated ketones (1717 cm ) e.g.
3
3
1
8
acetone (CH COCH ) and butanone (C H COCH ). Here, the carbonyl absorption band of
O
3
3
2
5
3
ꢀ1
labeled saturated ketones (1683 cm ) was also identified in the isotopically labeled irradiated
1
8
ꢀ1
18
18
ꢀ1
18
ꢀ1
ices of CH OH (1682 cm ), CH OHꢀC O (1680 cm ), CH OHꢀCO (1680 cm ) and
3
3
3
18
ꢀ1
CH OHꢀC O (1684 cm ), (Table 3). In summary, we wish to stress that FTIR spectroscopy can
3
only elucidate particular vibrational modes of complex organics synthesized in situ of bulk ices
from exposure to ionizing radiation and very rarely, actual molecular isomers. Consequently, we
then turn our attention to a more sensitive technique allowing for the identification of individual
molecules via their molecular formula, temperature program desorption coupled with single
photoionization reflectron timeꢀofꢀflight mass spectrometry.
3.2. Reflectron Time-of-Flight Mass Spectra
Following the in situ identification of small molecules and vibrational modes of more complex
organics as described above, we employed the use of temperature programmed desorption (TPD)
to monitor the products sublimating via ReTOFꢀPI mass spectrometry. The full product spectrum
of processed methanol and methanolꢀcarbon monoxide isotopologue ices are displayed in Figure
2A and Figure 2B, respectively, as a function of temperature during the postꢀirradiation warmꢀup
stage. In the case of methanol ice, molecules with a massꢀtoꢀcharge ratio (m/z) up to 90 amu are
observed (Figure 2A). However, in the case of methanolꢀcarbon monoxide ices, large molecules
up to 150 amu are observed (Figure 2B). This observation alone implies the presence of rich and
complex chemistry in the mixed methanolꢀcarbon monoxide ices compared pure methanol ices
despite the similar doses deposited into both systems.
Products identified utilizing ReTOF mass spectrometry following photoionization at E = 10.49
hv
eV are described below for the irradiated methanol and mixed methanolꢀcarbon monoxide ices,
respectively. Here, the detected products in the methanol ice are also observed in the irradiated
13

Physical Chemistry Chemical Physics
Page 14 of 77
View Article Online
DOI: 10.1039/C4CP04149F
methanolꢀcarbon monoxide system. The corresponding massꢀtoꢀcharge ratios (m/z) of the idenꢀ
tified products and their isotopomers are listed in Tables 4A and 4B. Note that in the methanol
1
3
18
(
CH OH, CD OD, CH OH and CH OH) ices, only one isotope of each element is present and
3 3 3 3
thus only one isotopic mass for each product are observed as expected. As an example, C H O
2
4
1
3
isotopomers were detected at m/z = 44 amu (C H O), 48 amu (C D O), 46 amu ( C H O) and
2
4
2
4
2
4
1
8
13
18
4
6 amu (C H O) in the exposed CH OH, CD OD, CH OH and CH OH systems,
2 4 3 3 3 3
respectively. Similarly, in the mixed isotopically pure methanolꢀcarbon monoxide ices, e.g.
1
3
8
18
CH OHꢀCO, CD ODꢀCO, and CH OHꢀC O only one isotopomer of each product is detected.
3
3
1
3
13
18
3
However, in the case isotopically mixed ices such as CH OHꢀCO, CD ODꢀ CO, CH OHꢀCO
3
3
1
8
and CH OHꢀC O, several isotopomers corresponding to one molecular formula are observed
3
(
Table 4B). Again for the sake of clarity, consider the three distinct C H O isomers that were
2 4
1
3
13
observed in CH OHꢀCO processed ice; here at, m/z = 44 amu (C H O), 45 amu ( CCH O),
3
2
4
4
1
3
and 46 amu ( C H O).
2
4
Furthermore, we should mention here that several mass peaks were observed simultaneously at
about 124 K, 145 K and 203 K as can be seen in Figures 2A and 2B in both irradiated methanol
and methanolꢀcarbon monoxide isotopologue ices. Note for the simplicity that the uncertainty
associated with peak sublimation temperatures is ± 2 K, unless otherwise noted. For example, the
sublimation of massꢀtoꢀcharge ratios corresponding to C H O, C H O, C H O, C H O, C H O
2
2
2
4
2
6
3
6
3
8
are all observed at 124 K in both irradiated methanol and methanolꢀcarbon monoxide ices. The
above observations are indication of either coꢀsublimation of several different products at a
specific temperature and/or fragmentation of a high mass organic from photoionization. During
the warmꢀup phase, the amorphous ices of methanol experiences a phase change within
5
3, 63, 78, 79
temperature range 100ꢀ125 K as reported earlier
and may trigger the sublimation of the
products formed via a ‘molecular volcano’ process as reported before with frozen amorphous
8
0
water resulting in the observation of several products with identical sublimation temperatures
of 124 K. Further, peak methanol sublimation occurs at 145 K in both irradiated methanol and
methanolꢀcarbon monoxide ice systems. As such, the observed sublimation of C H O, C H O,
2
4
3
6
C H O, C H O, C H O and C H O at 145 K can be correlated to the coꢀsublimation with
4
8
2
6
3
8
2
2
2
methanol molecules. Similarly, the sublimation of a group of products peaking at 203 K is
correlated with coꢀsublimation of ethylene glycol (formed in the radiolysis process). A detailed
discussion of these products is discussed in the following sections.
14

Page 15 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
3
.2.1. Molecules with Definitive Assignments
.2.1.1. Single Oxygen Bearing Molecules
3
Evidence from the ReTOF mass spectrometry data combined with isotopic labeling led to the
identification of molecules which can be formally classified as a carbonyl, alcohol, and/or ether.
The corresponding sublimation profiles along with their respective shifted isotopologue masses
are shown in Figures 3A and SI Figures S2, S3. In the case of methanolꢀcarbon monoxide ices
we have also identified two additional high mass organics C H O and C H O as shown in
3
4
4
10
Figure 3B and SI Figures S4, S5. A detailed analysis on the identification of these products is
discussed in the following section, please note that the stated energy in electron volts (eV) is the
adiabatic ionization energy (I.E.) of the molecule for sake of clarity.
Ketene (H CCO)
2
Temperature program desorption spectra at m/z = 42 amu in the irradiated ice samples is
8
1
attributed to molecular formula C H O and has been assigned to ketene (H CCO; 9.6 eV ). The
2
2
2
sublimation profile at m/z = 42 amu in irradiated methanol ice is observed with a single peak
centered at 121 K as shown in Figures 3A along with the mass shifted isotopomers at m/z = 44
18
18
13 13
13
amu for D CCO in CD OD ice, H CC O in CH OH ice and H C CO in CH OH ice. All
2
3
2
3
2
3
are in excellent agreement with each other as shown, confirming the assignment of this
molecular formula. With regards to the irradiated methanolꢀcarbon monoxide ice systems, the
sublimation profile at m/z = 42 amu (H CCO) in CH OHꢀCO ice (Figure 3B) also depicts a
2
3
single peak centered at 123 K. Similarly, ketene isotopomers were observed in the irradiated
labeled ices as shown in Figure 3B. Recall that, we have also identified ν fundamental of ketene
2
18
18
13 13
13
isotopomers H CC O in irradiated CH OH ices and H C CO in irradiated CH OH using
2
3
2
3
infrared spectroscopy (Figure 1A, Table 2), along with the observation of ν fundamental of
2
18
18
3
18
18
ꢀ1
ketene isotopomers H CC O in both CH OHꢀC O and CH OHꢀC O ices at 2107 cm and
2
3
ꢀ1
2
106 cm , respectively as shown in Figure 1B, Table 2 as discussed previously. Note that all
masses corresponding to the in situ identified isotopomers of ketene via FTIR spectroscopy were
observed following warm up.
15

Physical Chemistry Chemical Physics
Page 16 of 77
View Article Online
DOI: 10.1039/C4CP04149F
Acetaldehyde (CH CHO)
3
Sublimation profiles for masses corresponding C H O isomers formed in both irradiated
2
4
methanol and methanolꢀcarbon monoxide isotopologue ices are shown in Figure 3A and 3B,
respectively. In both irradiated ice systems the agreement between the sublimation profiles of
C H O isotopomers confirm identification of radiolytic product with molecular formula C H O.
2
4
2
4
Here we should mention that, among the three possible isomers with molecular formula C H O
2
4
[
ethylene oxide (cꢀC H O, I.E. = 10.56 eV), acetaldehyde (CH CHO, I.E.=10.23 eV) and vinyl
2 4 3
alcohol (CH CHOH), I.E. = 9.33 eV] that only the latter two can contribute to signal at the
2
corresponding masses observed. Sublimation profiles of C H O isotopomers from the irradiated
2
4
methanol ices reveal five distinct peaks centered at 122 K, 147 K, 170 K, 203 K and 238 K. The
multiple sublimation peak temperatures imply the possibility of (i) sublimation of different
isomers with different functional groups and overall polarity, (ii) coꢀsublimation of a specific
isomer at different temperatures due to intermolecular interactions with neighboring molecules
and/or (iii) photoꢀfragmentation of a higher mass product producing a fragment ion related to
+
C H O . As mentioned above, only two isomers, acetaldehyde (CH CHO) and vinyl alcohol
2
4
3
(
CH CHOH) can be ionized at 10.49 eV. Here, the sublimation of acetaldehyde is expected at a
2
lower temperature than vinyl alcohol (if formed) due to the less polar (CO) compared to the OH
functional group in vinyl alcohol. In order to verify the identification of acetaldehyde (CH CHO)
3
the irradiated methanol ices, calibration experiments were conducted by simply depositing a
sample containing 1.0 ± 0.2 % of acetaldehyde (CH CHO) in CH OH (Sample 1; SI Table S3).
3
3
The subsequent sublimation profile of m/z = 44 amu (Figure 3C) does indeed display two peaks
centered although slightly lower temperatures at 109 K and 134 K. Coincidentally, methanol
7
9
begins a phase transition from amorphous to crystalline at 100 – 125 K forcing the
8
0
acetaldehyde to escape via a molecular volcano type mechanism. Here, the first peak at 109 K
is due to sublimation of acetaldehyde induced from the phase change of methanol. However,
some acetaldehyde is trapped within the methanol matrix; hence the appearance of the second
peak at 134 K as it coꢀsublimates with methanol. Here, the sublimation of acetaldehyde is
observed until about 150 K, which clearly suggests that the trapped acetaldehyde molecules are
subliming together with the methanol matrix. A comparison of sublimation profiles from both
the calibration sample and the irradiated CH OH ices (Figure 3C) suggests that the first two
3
peaks at 122 K and 147 K in irradiated methanol ices are due to the sublimation of acetaldehyde.
16

Page 17 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
Although, the peak temperatures do not match perfectly, the overall trend does in that
acetaldehyde sublimates as methanol changes from amorphous to crystalline, followed by coꢀ
sublimation with methanol. To further enhance the latter point, the sublimation profile of
+
81, 82
photoionization fragment CH O (appearance energy = 10.4 eV)
of methanol at m/z = 31
3
amu are shown in Figure 3C which displays a sublimation peak at 147 K. Note that the
difference in peak temperatures between the irradiated methanol ice and the acetaldehyde
methanol doped calibration sample are 13 K warmer for both peaks; the shift towards a higher
temperature (higher binding energy) may be attributed to intermolecular interactions resulting
from the additional large complex organics that were synthesized in the irradiated ice than that of
the simple simulation sample. The peak sublimation temperatures at 170 and 203 K are discussed
below along with their tentative assignments. Recall that evidence of acetaldehyde was
ꢀ
1
confirmed via infrared absorption at 1724 cm (ν ) together with the isotopically labeled
4
counterparts as discussed in section 3.1.1. Finally, the sublimation peak at 238 K can be assigned
to fragmentation of C H O (glycerol) which shows a prominent fragmentation pattern at 10.49
3
8
3
8
3
eV. Note that due to the astrobiological importance pertaining to the prebiotic formation of
glycerol, we chose to focus on this specific molecule in a separate publication.
In the case of methanolꢀcarbon monoxide systems, the sublimation profiles of C H O
2
4
isotopomers are shown in Figure 3B and display four distinct sublimation peaks centered at 125
K, 147 K, 183 K, and 240 K. Similar calibration experiments were conducted with a sample
containing 0.5 ± 0.1% of acetaldehyde in a mixed CH OHꢀCO (4:5) ice system (Sample 2).
3
Here, the sublimation profile at m/z = 44 amu recorded during the calibration experiment (Figure
3C) depicts a sharp peak at 114 K with two shoulders at 105 K and 136 K. The shoulder at 105 K
can be correlated to sublimation of acetaldehyde induced via the phase change similar to that
observed in methanol/acetaldehyde simulation that peaked at 109 K; however, this observation is
the only similarity. A highly porous methanol ice results from carbon monoxide having already
sublimated at these temperatures allowing for more binding sites of the acetaldehyde within the
pores of the methanol matrix. The lack of two distinct peaks as observed in the methanol
calibration sample is most likely a reflection of the initial porosity of the methanol matrix. Here,
most of the acetaldehyde was allowed to sublimate in the mixed methanolꢀcarbon monoxide
from a combination of the high initial porosity followed with the phase transition, whereas the
pure methanol ice simulation sample trapped more of the acetaldehyde during the phase
17

Physical Chemistry Chemical Physics
Page 18 of 77
View Article Online
DOI: 10.1039/C4CP04149F
transition resulting in significant coꢀsublimation. However, some of the acetaldehyde did remain
trapped, thereby explaining the small shoulder at 136 K. Note that the difference of sublimation
temperatures between the observed peaks of the irradiated binary mixture and calibration sample
are 11 K warmer. Again, this shift towards higher temperatures and thus higher binding energies
is attributed to the intermolecular interactions with other complex organics left in the refractory
material. The drastic difference in sublimation profiles of the calibration and irradiated sample
may simply be due to the inherent effect that these large organics acting as ‘impurities’ have on
the phase change behavior of the ice sample.
C H O Isomers: Identification of Ethanol (C H OH) and Dimethyl ether (CH OCH )
2
6
2
5
3
3
Integrated ion counts at massꢀtoꢀcharge ratios corresponding to isotopomers of C H O products
2
6
were observed in both irradiated methanol and methanolꢀcarbon monoxide isotopologue ices and
are shown in Figures 3A and 3B, respectively. In irradiated methanol ices, sublimation of C H O
2
6
depicts two distinct sublimation peaks centered at 122 K and 144 K as shown in Figure 3A. The
sublimation profile of the product C H O from irradiated methanolꢀcarbon monoxide ices,
2
6
depicts a pronounced peak at 124 K with a shoulder at 143 K (Figure 3B). Note that both C H O
2
6
isomers, ethanol (C H OH; 10.48 eV) and dimethyl ether (CH OCH ; 10.0 eV), can be ionized
2
5
3
3
with the 10.49 eV VUV photons. In order to ensure the assignment of these isomers, calibration
experiments are performed with ethanol and dimethyl ether in both methanol (Samples 3 and 5)
and methanolꢀcarbon monoxide (Samples 4 and 6) ices are compared with sublimation profiles
of the C H O irradiation product obtained from both irradiated methanol and methanolꢀcarbon
2
6
monoxide ices shown in Figure 3D.
In the calibration experiments with methanol ices, the sublimation of dimethyl ether started at
102 K and the sublimation profile shows two peaks positioned at 112 K and 128 K with a
shoulder at 142 K. The lower sublimation temperature of dimethyl ether is reasonable due to its
lower polarity compared to methanol. Here, the peak at 112 K can be correlated to the
sublimation of dimethyl ether (CH OCH ) from methanol ices whereas the peak at 128 K follows
3
3
the same pattern as observed with the acetaldehyde (HCOCH ), i.e. methanol phase change from
3
amorphous to crystalline resulting in a molecular volcano. Finally, the shoulder at 145 K is the
result of coꢀsublimation of trapped dimethyl ether (CH OCH ) within the methanol matrix. Note
3
3
that the temperature difference between the irradiated methanol ice sample and the dimethyl
18

Page 19 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
ether calibration sample is similar to that observed for the acetaldehyde sample (about 12 K);
again this shift in temperature is attributed to intermolecular forces of the large refractory
organics formed in the process of irradiation. In case of methanolꢀcarbon monoxide ices, the
calibration experiment with dimethyl ether displays two peaks at 108 K and 119 K with a small
shoulder at 145 K. In a similar way, the first peak can be assigned to the sublimation of dimethyl
ether (CH OCH ) molecules followed with sublimation of trapped dimethyl ether (CH OCH )
3
3
3
3
molecules during the phase change, and finally coꢀsublimation with methanol at 145 K; as
discussed above, the residual methanol matrix at these temperatures is most likely of higher
porosity compared to the pure methanol samples due to the prior sublimation of carbon
monoxide. As such, allowing most of the dimethyl ether to leave. We are again attributing the
differences in sublimations profiles of the irradiated and calibration sample to the intrinsic
effects of what could be considered contamination from the remaining organics on the phase
change behavior of methanol.
The calibration experiments (Figure 3D) also depict that the sublimation of ethanol (C H OH)
2
5
with both peaks positioned at 145 K in the methanol and methanolꢀcarbon monoxide ices,
matching well the observed profile of the irradiated samples. This observation is not unexpected
1
6
8
+
as ethanol is completely miscible with methanol. In addition, QMS traces of C H O (m/z = 48
2
18
+
amu) and C H O (m/z = 47 amu) ions show two peaks at temperatures of 122 K and 144 K in
2
5
1
8
18
18
irradiated CH OH ice and at 124 K and 146 K in irradiated CH OHꢀC O ice (SI Figure S6).
3
3
18
18
Note that, both ethanol (CH CH OH) and dimethyl ether (CH OCH ) show prominent
3
2
3
3
1
8
+
81
fragmentation at C H O during the electron impact ionization. Giving further evidence
2
5
supporting our assignment of dimethyl ether and ethanol formed in irradiated methanol and
methanolꢀcarbon monoxide ices.
3.2.1.2. Complex Organic Molecules with Two Oxygen Atoms
By comparing the sublimation profiles of the integrated ion counts observed from the
isotopically labeled ices of both methanol and methanolꢀcarbon monoxide ices, products with
molecular formula C H O , C H O , and C H O are identified. It is worth mentioning that the
2
2
2
2
4
2
2
6
2
sublimation profiles of the products in the irradiated methanol ices are relatively narrow with
sharp peaks compared to the sublimation profiles observed in irradiated methanolꢀcarbon
monoxide ices as can be seen in Figures 4ꢀ8 and SI Figures S7, S8. The broad sublimation
19

Physical Chemistry Chemical Physics
Page 20 of 77
View Article Online
DOI: 10.1039/C4CP04149F
features observed in the latter mixed ices are most likely due to the intermolecular interactions
with the residual high mass organics (up to 150 amu) formed within the processed mixed ices
(
Figure 2B). In comparison, the radiation induced chemical processing of methanol isotopologue
ices resulted identification of fewer products with massꢀtoꢀcharge ratios only up to 90 amu
Figure 2A); as such, diminished dipoleꢀdipole interactions of the subliming molecules with the
(
residual organic matrix resulting in the relatively narrow sublimation profiles observed in the
case of methanol systems.
Identification of Glyoxal (C H O )
2
2
2
A product with molecular formula C H O and the respective mass shifted isotopomers were
2
2
2
observed only in the irradiated mixed methanolꢀcarbon monoxide isotopologue ices with a
sublimation peak at 145 K as shown in Figure 4. We are assigning this sublimation of the
molecule to glyoxal (HCOCHO) for two reasons as follows. First, note that 2ꢀoxiranone
ꢀ1
[
Cyc(CH OC)O] (an isomer of glyoxal) is 38 kJ mol less stable than the former and holds an
2
8
4
ionization energy of 10.96 eV (calculated), and therefore not should attribute to any ion signal
collected at this particular photon energy whereas glyoxal can with its ionization energy at 10.2
ꢀ1
eV. Second, the third most stable isomer (63 kJ mol less stable than glyoxal) ethyneꢀ1,2ꢀdiol
8
4
(
HOCCOH) has an ionization energy 9.3 eV and can be ionized; however, this diol isomer is
expected to sublime at temperatures greater than the sublimation of ethylene glycol
HOCH CH OH) which displays a sublimation peak about 200 K (see below) due to their trends
(
2
2
in boiling points (ethylene glycol = 471 K, ethyneꢀ1,2ꢀdiol = 530 K), yet no detectable signal
was collected temperatures greater than 200 K. Therefore, we can conclude that in irradiated
methanolꢀcarbon monoxide ices that ethyneꢀ1,2ꢀdiol did not form or was below the detection
limit. Finally, please note the similar trend in sublimation peaks as discussed above; here, the
weak sublimation peak observed at 125 K is related to the phase change of methanol followed
with coꢀsublimation at 145 K.
C H O Isomers: Identification of Glycolaldehyde and Ethene-1,2-diol
2
4
2
Among the four stable isomers of C H O , glycolaldehyde (HOCH CHO; 10.2 eV), etheneꢀ1,2ꢀ
2
4
2
2
diol (HOCHCHOH; 9.62 eV), acetic acid (CH COOH; 10.65 eV) and methyl formate
3
(
HCOOCH ; 10.84 eV), only glycolaldehyde and etheneꢀ1,2ꢀdiol can be ionized with 10.49 eV
3
20

Page 21 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
VUV photons used in the present study. A significantly more detailed discussion related to
6
3
assignment of glycolaldehyde and etheneꢀ1,2ꢀdiol can be found in a previous publication. Very
briefly though, five sublimation peaks at 123 K ,166 K, 200 K, 210 K and 234 K in each
isotopically labeled methanol ice (Figure 5) were observed for the sublimation of C H O
2
4
2
isomers in the irradiated methanol ices. The first peak at 123 K is most likely the induced
sublimation of isomers C H O resulting from the amorphous to crystalline phase change
2
4
2
inducing a molecular cryoꢀvolcano. Following is the sublimation peak at 166 K which is
assigned to the sublimation of glycolaldehyde based on the coincidently decreasing ν band from
1
4
in situ FTIR observations. Next, the observed peak at 200 K is from coꢀsublimation with
6
3
ethylene glycol followed with the direct sublimation of etheneꢀ1,2ꢀdiol at 210 K. Finally, the
distinct peak around 234 K is assigned to the fragmentation from glycerol (C H O ) based on a
3
8
3
recent study on the photoionization of glycerol (C H O ) in which a prominent photo ion
3
8
3
8
3
fragment (C H O ) was identified with an appearance energy of 9.9 eV. In the case of
2
4
2
irradiated methanolꢀcarbon monoxide isotopologue ices, the C H O product is confirmed
2
4
2
sublimating at 125 K, 195 K and 218 K (Figure 5) with the latter two peaks assigned to
glycolaldehyde and etheneꢀ1,2ꢀdiol, respectively and the first peak related to the sublimation of
phase change of methanol at 125 K.
Identification of Ethylene Glycol (HOCH CH OH)
2
2
The sublimation profiles of C H O in both irradiated methanol and methanolꢀcarbon monoxide
2
6
2
ices show a prominent peak at 198 K along with a weak sublimation peak at 125 K as shown in
Figure 6. In the case of irradiated methanol systems, the sublimation profiles also display a slight
peak at 236 K. Here, the intense peak at 198 K suggests the sublimation of a single isomer of
with molecular formula C H O . In order to ensure the assignment of ethylene glycol
2
6
2
8
1
(
HOCH CH OH; I.E. = 10.16 eV) in both methanol and methanolꢀcarbon monoxide ices,
2 2
calibration experiments are performed with ethylene glycol in CH OH and CH OHꢀCO
3
3
displayed in Figure 7. The sublimation profiles of the ion counts at m/z = 62 amu recorded from
the calibration experiments are compared with the sublimation profiles of C H O observed in
2
6
2
both irradiated CH OH and CH OHꢀCO ices. The sublimation of ethylene glycol (Figure 7)
3
3
shows a single peak at 193 K in CH OH ice (Sample 11; SI Table S3) and at 194 K in CH OHꢀ
3
3
CO (Sample 12) ices, respectively. As expected and observed, ethylene glycol holds two
hydroxyl groups and will sublime at warmer temperatures higher than methanol. The sublimation
21

Physical Chemistry Chemical Physics
Page 22 of 77
View Article Online
DOI: 10.1039/C4CP04149F
profile of C H O product obtained in both irradiated ices of methanol and methanolꢀcarbon
2
6
2
monoxide ices are in reasonable agreement with the desorption profile of ethylene glycol
+
obtained from the calibration experiments. In addition, QMS traces corresponding to C H O are
2
6
also observed at about 198 K in both irradiated methanol and methanolꢀcarbon monoxide ices.
Also observed were ion fragments of typical of ethylene glycol due to electron impact ionization
+
+
+
+
at C H O , C H O , C H O , C H O , as shown in SI Figure S6. Therefore, the sublimation
2
3
2
4
2
5
2
5
2
peak at 198 K recorded using ReTOF mass spectrometry in irradiated methanol and methanolꢀ
carbon monoxide ices is assigned to the ethylene glycol (HOCH CH OH). Also recall that, we
2
2
ꢀ1
have identified ethylene glycol via the observation of ν band of ethylene glycol at 1094 cm
9
using in situ infrared spectroscopy.
As mentioned, a weak peak is also observed at about 125 K (Figure 6) in both irradiated
methanol and methanolꢀcarbon monoxide ices. As this temperature is well below the sublimation
of methanol (~ 145 K) and is correlated with the amorphous to cystralline phase change, the peak
observed at this temperature could either indicate a ‘cyroꢀvolcano’ of ethylene glycol or possibly
the sublimation of a less polar C H O isomer. Among the isomers are: ethylene glycol (470 K),
2
6
2
ethylhydroperoxide (368 K), methoxymethanol (356 K), and dimethyl peroxide (239 K) with the
boiling points given in parentheses. Of these isomers, dimethyl peroxide is expected to sublime
at temperatures below the sublimation of methanol (338 K). Consequently, the weak sublimation
peak at 125 K may be related to dimethyl peroxide (CH OOCH ; 9.1 eV). Finally, the
3
3
sublimation peak at 236 K (Figure 7) in the irradiated methanol isotopologue ices is once again
attributed to glycerol (C H O ) in which a prominent photo fragment at m/z = 62 amu (C H O )
3
8
3
2
6
2
8
3
has been identified with an appearance energy of 9.9 eV.
Identification of methoxymethanol (CH OCH OH)
3
2
Methoxymethanol is a rather interesting radiation byproduct as it has been previously identified
5
7, 85
only in the low energy electron radiolysis of methanol ices.
Based on the functional groups
present in methoxymethanol (CH OCH OH) and a comparison of the boiling point trends,
3
2
methoxymethanol (356 K) is expected to sublime at a temperature higher than methanol (338 K)
but at a lower temperature compared to ethylene glycol (470 K). However, there are no
significant ion counts of C H O (Figure 8) at temperatures between the sublimation of methanol
2
6
2
and ethylene glycol (140 – 170 K) that would normally indicate the sublimation of a different
22

Page 23 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
molecular isomer. Previously, methoxymethanol was assigned as a low energy electron radioꢀ
lysis product of methanol ice from the combination of a unique sublimation profile and fragꢀ
mentation pattern. The justification was based heavily on a previous mass spectroscopic analysis
of synthesized methoxymethanol which identified a prominent ion fragment at m/z = 61 amu
+
corresponding to C H O ; whereas the expected parent molecular ion peak at m/z = 62 amu
2
5
2
+
86
(
C H O ) was not observed or determined to be within the noise level. Following a similar
2 6 2
approach here, we concur with the previous identification of methoxymethanol in energetically
processed methanol ices and present the novel identification of this product in irradiated binary
ice mixture of methanol and carbon monoxide. Evidence supporting the identification methoxyꢀ
methanol is twofold as presented here. First, upon examination of the QMS data, isotopomers
+
2
connected with C H O were observed subliming with peak ion counts at 170 K, with no
2
5
detectable signal contributing from the parent (C H O ) at these temperatures, in agreement to
2
6
2
5
7, 85
+
the previous studies.
Second, C H O isotopomers that were detected utilizing single
2 5 2
photoionization ReTOF mass spectrometry and have identical sublimation profiles to that
derived from the QMS data. Sublimation profiles observed at these massꢀtoꢀcharge ratios are
shown in Figure 8 for irradiated methanol and methanolꢀcarbon monoxide ices. Note that
possibility of radicals sublimating is discounted as the applied thermal energy would allow for
any trapped radicals to diffuse and easily react at these temperatures. Also, previous attempts at
dosing the irradiated methanol ice with oxygen atoms did not decrease the overall TPD signal of
+
57
C H O further implying that C H O radicals are not directly sublimating. From our
2
5
2
2
5
2
observations, i.e. the direct correlation between two different gas phase detection methods;
methoxymethanol is suggested to be either an extremely thermally liable compound or
photoionization at 10.49 eV leads to fragmentation thereby explaining the lack of parent ion
signal in the TPD analysis. In both irradiated methanol and methanolꢀcarbon monoxide ices, the
+
2
sublimation of C H O started at 143 K with a prominent peak positioned at 170 K and 184 K,
2
5
+
respectively. Additional sublimation peaks observed at this m/z linked to C H O in irradiated
2
5
2
CH OH ices (Figure 8) at 202 K and 238 K are attributed to the coꢀsublimation of methoxyꢀ
3
methanol (CH OCH OH) with ethylene glycol (HOCH CH OH) and photofragmentation of
3
2
2
2
83
glycerol (C H O ), respectively. The sublimation peak at 202 K is excluded as a fragmentation
3
8
3
of ethylene glycol (HOCH CH OH) as the calibration experiments with ethylene glycol did not
2
2
exhibit any fragmentation at m/z = 61 amu (C H O ).
2
5
2
23

Physical Chemistry Chemical Physics
Page 24 of 77
View Article Online
DOI: 10.1039/C4CP04149F
3.2.2. Molecules with tentative assignments
Vinyl Alcohol (CH CHOH): Vinyl alcohol (CH CHOH; 9.3 eV) is tentatively assigned to
2
2
account for the ion signal (m/z = 44 amu) observed at the sublimation temperature (peak) of 170
2 K in the irradiated methanol ice systems and at 185 ± 2 K in the processed methanolꢀcarbon
±
monoxide systems (Figures 3A and 3B). Here, the sublimation of vinyl alcohol at temperatures
higher than acetaldehyde is reasonable since the molecule is overall more polar, and thus will
have a higher energy of desorption. The sublimation peak at 203 ± 2 K observed in the TPD
spectra of the processed methanol ices coincides with the sublimation of ethylene glycol and thus
may possibly be attributed to coꢀsublimation of vinyl alcohol with ethylene glycol due to the
similar polarity. Photoꢀfragmentation from ethylene glycol is excluded since photoionization of
the ethylene glycol calibration sample did not show any fragmentation. Unfortunately, similar
calibration experiments with vinyl alcohol could not be conducted due to the inherent instability
of the molecule (and commercially unavailability) as it will tautomerize to acetaldehyde.
C H O Isomers: A radiolysis by product with molecular formula C H O was identified only in
3
4
3
4
the irradiated methanolꢀcarbon monoxide ices. In Figure 3A, the sublimation profile of C H O
3
4
isotopomers shows two distinct peaks centered at 127 K and 144 K which again is attributed to
the phase change induced molecular volcano followed with coꢀsublimation of methanol.
However, a broad sublimation profile that extends up to 300 K is observed and again is most
likely a combination of different isomers sublimating and photofragmentation of high mass
organics. Molecular isomers that may contribute to the ion counts at the derived molecular
formula are: propenal (CH CHCHO; 10.1 eV), cyclopropanone (CH (CO)CH ; 9.1 eV), methyl
2
2
2
ketene (CH CHCO; 8.95 eV). Of these, only propenal (CH CHCHO) is readily available and
3
2
was used as a calibration with 1.5 ± 0.5% in methanolꢀcarbon monoxide ice in order to help with
the possible identification. The sublimation profile at m/z = 56 amu of propenal recorded from
calibration sample together with the sublimation profile of C H O isomers observed in irradiated
3
4
CH OHꢀCO ices are shown in SI Figure 9. The sublimation profile at m/z = 56 amu shows two
3
peaks at 128 K and 138 K and these temperatures can be correlated to the sublimation of
propenal during the phase change and sublimation of methanol matrix as discussed above. The
correlation of the TPD spectra would suggest that sublimation of propenal; however, we again
cannot eliminated the possibility of cyclopropanone (CH (CO)CH ) and methyl ketene
2
2
(
CH CHCO) isomers and therefore can only present a tentative assignment here.
3
24

Page 25 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
C H O Isomers: Products with the molecular formula C H O were identified in both irradiated
3
6
3
6
methanol and methanolꢀcarbon monoxide ices. The sublimation profiles at m/z corresponding to
isotopologues of C H O isomers are shown in Figures 3A and 3B. Of the molecular isomers,
3
6
acetone (CH COCH ; 9.7 eV), propanal (C H CHO; 10.0 eV), and allyl alcohol
3
3
2
5
(
CH CHCH OH; 9.7 eV) can account for the observed ion signal as these have ionization
2 2
energies below the 10.49 eV threshold. The TPD spectra of C H O molecular product depicts
3
6
four distinct peaks centered at 125 K, 145 K, 172 K, and 203 K in irradiated methanol ices.
While in the irradiated mixed methanolꢀcarbon monoxide ices, displayed two sharp peaks at 126
K and 146 K together with a broad peak centered at about 187 K. Similar to that previously
discussed, the first sublimation peaks at about 125 K and 145 K are associated with the phase
transition of amorphous to crystalline (125 K) inducing a molecular volcano followed with coꢀ
sublimation of methanol near 145 K. To help identify which isomers were sublimating,
calibration experiments (SI Table S3) were performed with acetone (CH COCH ; Samples 1 and
3
3
2), propanal (CH CH CHO; Samples 3 and 4) and allyl alcohol (CH CHCH OH; Samples 5 and
3 2 2 2
6) in both methanol and methanolꢀcarbon monoxide ices. The sublimation profiles of ion counts
+
at m/z = 58 amu (C H O ) recorded from the calibration experiments are shown in SI Figure S10
3
6
with comparison of the sublimation profile at m/z = 58 amu recorded from both irradiated
CH OH and CH OHꢀCO ices. The sublimation temperatures of these molecules are lower than
3
3
the sublimation temperature of methanol as expected based on their polarity and subsequent
boiling point trends, whereas allyl alcohol (CH CHCH OH) is expected to sublime at a higher
2
2
temperature than methanol. Here, acetone and propanal both exhibited peak sublimation
temperatures of 137 K from methanol ice and 128 K and 126 K, respectively in the mixed
methanol carbon monoxide ice and the sublimation profiles of allyl alcohol from both methanol
and methanolꢀcarbon monoxide ices depict single sublimation peak at 154 K and 152 K,
respectively. Unfortunately, the TPD profiles of the calibration samples are not adequately
unique to disentangle for a definitive assignment. Moreover, in situ IR spectroscopy did not yield
strong evidence on the formation of these molecules to allow for certainty in the assignment
either in agreement with previous examinations on the energetic processing of methanol rich
5
3, 74, 79
ices.
However, ion counts were observed corresponding to molecular formula C H O and
3 6
thus we can only suggest that the possibility that acetone, propanal, and/or allyl alcohol were/was
formed. In addition possible contributions from photofragmentation of the higher mass products
25

Physical Chemistry Chemical Physics
Page 26 of 77
View Article Online
DOI: 10.1039/C4CP04149F
(
such as C H O ) may contribute to the ion counts observed at higher temperatures, namely the
4 8 2
broad sublimation profile extending from 170 – 250 K in the irradiate methanol carbon
monoxide ice along with the peaks at 172 K and 203 K in the TPD spectra of irradiated methanol
+
ices. For example, the appearance energies of C H O from C H O isomers, 3ꢀmethoxypropanal
3
6
4
8
2
and methoxymethyloxirane, are 8.5 eV and 10.2 eV, respectively. The sublimation of product
with the derived molecular formula C H O (discussed below) resulted in three peaks at 179 K,
4
8
2
208 K and 254 K in irradiated methanol ices and a broad sublimation feature within 175ꢀ275 K
temperature range in irradiated methanolꢀcarbon monoxide ices. Future experiments using
tunable vacuum ultraviolet light for selective photoionization will allow for us to make specific
assignments based upon their ionization energies.
C H O Isomers: Products were identified in the TPD spectra of the irradiated ice systems with
3
8
molecular formula C H O as shown in Figures 3A and 3B. The molecular isomers of this group
3
8
are either alcohols such as 1ꢀpropanol (CH CH CH OH; 10.2 eV), 2ꢀpropanol
3
2
2
(
CH CH(OH)CH ; 10.2 eV) or an ether molecule such as methoxyethane (CH OCH CH ; 9.7
3 3 3 2 3
eV). In irradiated methanol ices, the sublimation of C H O is identified via a distinct sublimation
3
8
peak at 124 K. In the case of methanolꢀcarbon monoxide ices, C H O isomers are observed
3
8
sublimating at 125 K and 142 K. In order to assist in the determination of which C H O isomers
3
8
were detected, calibration experiments were performed with only 1ꢀpropanol and 2ꢀpropanol in
both methanol and methanolꢀcarbon monoxide ices as methoxyethane is not commercially
available. The TPD profiles of 1ꢀpropanol and 2ꢀpropanol peak 155 K and 153 K, respectively in
CH OH ices and 152 K and 149 K, respectively, in CH OHꢀCO ices, as shown in SI Figure S11.
3
3
The above information clearly suggests that 1ꢀpropanol and 2ꢀpropanol isomers sublime at
higher temperatures than the methanol. Furthermore, the sublimation profiles of C H O
3
8
radiolytic byproduct isomers (m/z = 60 amu) do not agree with the alcohol calibration samples.
As such we can only postulate the possible formation of methoxy ethane with the observed peaks
attributed to the molecular volcano induced from the amorphous to crystalline phase change at
1
25 K and coꢀsublimation of C H O isomers with methanol at 145 K.
3 8
C H O Isomers: Ion counts associated with the molecular formula C H O are observed in both
4
8
4
8
the TPD spectra of irradiated methanol and methanolꢀcarbon monoxide ices (Figures 3A and
B). Molecules such as butanal (C H CHO; 9.8 eV), isoꢀbutanal (CH CH(CH )CHO; 9.7 eV),
3
3
7
3
3
butanone (C H COCH ; 9.1 eV), and 2ꢀbutenꢀ1ꢀol (HOCH CHCHCH ; 9.1 eV) can contribute
2
5
3
2
3
26

Page 27 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
to the observed signal here. As mentioned previously, both irradiated ices exhibited peaks at the
sublimation temperature of methanol, and thus ion counts for these masses correspond again to
coꢀsublimation with methanol. However, the broad peak positioned at about 208 K is most likely
photofragmentation of higher mass organics, such as C H O (m/z = 72 amu) isomers discussed
3
4
2
below. Similarly in the irradiated methanolꢀcarbon monoxide ices (Figure 3B), the sublimation
profiles at C D O displays a broad sublimation feature centered at ~200 K which may be
4
8
attributed again to the photofragmentation of higher mass organics. Calibration experiments were
once again conducted in an effort to remove some of the ambiguity observed in the TPD spectra
of the potential carriers for the observed C H O isomers. Here, samples containing butanal
4
8
(
Sample 3 and 4), isoꢀbutanal (Sample 7 and 8) and butanone (Sample 5 and 6) in both CH OH
3
and CH OHꢀCO (4:5) ices. The sublimation profiles of the ion counts of C H O isomers at m/z =
3
4
8
72 amu are shown in SI Figure 12 and are compared with the sublimation profiles observed at
m/z = 72 amu (C H O) in irradiated CH OH ices and m/z = 80 amu (C D O) in irradiated
4
8
3
4
8
CD ODꢀCO ices. As shown in SI Figure 12, all TPD spectra are again not sufficiently unique to
3
make a definitive assignment and typically sublimate with methanol. Based on the above
information alone, we can only suggest that at least one of these molecules is formed within
irradiated methanol ices.
C H O Isomers: We have also identified C H O molecular isomers; however, only in
4
10
4
10
irradiated methanolꢀcarbon monoxide ices. Here, the sublimation profiles of integrated ion
counts corresponding to C H O isotopomers are shown in Figure 3B and are observed with
4
10
peaks at 128 K and 143 K. Alcohols and ethers are possible isomers and include: 1ꢀbutanol
(
CH CH CH CH OH; 10.0 eV), 2ꢀbutanol (CH CH CH(OH)CH ; 9.9 eV), iso-butanol
3 2 2 2 3 2 3
(
CH ) CHCH OH; 10.0 eV), tert-butanol (CH ) COH; 9.9 eV), methoxypropane (CH OC H ;
3 2 2 3 3 3 3 7
9
.41 eV) and ethoxyethane (C H OC H ; 9.51 eV). In order to quantify which isomers were
2 5 2 5
detected in the irradiated samples calibration experiments were performed with 1ꢀbutanol, 2ꢀ
butanol, isoꢀbutanol and tertꢀbutanol in CH OHꢀCO ices. The sublimation profile of these
3
C H O isomers at m/z = 74 amu are compared with the sublimation profile of the irradiation
4
10
product C H O (m/z = 74 amu) and shown in SI Figure S13. The TPD profile of 1ꢀbutanol, 2ꢀ
4
10
butanol, isoꢀbutanol and tertꢀbutanol peak at 159 K, 154 K, 159 K and 155 K, respectively, and
are not in agreement to the observed profile nor peak positions recorded in irradiated CH OHꢀ
3
CO ice (128 K and 143 K). Consequently, we once more can only suggest the possible
27

Physical Chemistry Chemical Physics
Page 28 of 77
View Article Online
DOI: 10.1039/C4CP04149F
identification of less polar ether molecules methoxypropane (CH OC H ) and ethoxyethane
3
3
7
(
C H OC H ) isomers; while again attributing the compaction of the methanol phase change at
2 5 2 5
~
125 K inducing a cryoꢀvolcano along coꢀsublimation with methanol at ~145 K of C H O
4 10
alcohols and ethers.
C H O Isomers: Ions counts attributed to C H O molecular formula have been detected in
3
4
2
3
4
2
both irradiated methanol and methanolꢀcarbon monoxide ices. The TPD spectra of irradiated
methanol ices show C H O isomers sublimating with peaks at 208 K and 236 K in irradiated
3
4
2
methanol ices (SI Figure S7) and 183 K and 236 K in the irradiated methanolꢀcarbon monoxide
ices (SI Figure S14). These observations suggest the formation isomers with at least two
different functional groups resulting in different polarities and subsequently different desorption
energies. Molecules such as methyl glyoxal (CH COCHO; 9.6 eV), 1,3ꢀpropanedial
3
(
HCOCH CHO) can account for the C H O isomers. In addition, more polar enꢀol isomers, such
2 3 4 2
as 2ꢀhydroxyacryldehyde (HCOC(OH)CH ) and 3ꢀhydroxyacryldehyde (HOCHCHCHO) can
2
also be correlated to this product as well. Here, the sublimation peaks at 203 K in irradiated
methanol ice and 183 K in irradiated methanolꢀcarbon monoxide ices, can be correlated to the
less polar isomers (possibly methyl glyoxal and 1,3ꢀpropanedial) and the sublimation peaks
centered at 236 K in both methanol and methanolꢀcarbon monoxide ices can be assigned to the
sublimation of glycerol, at photoionization of this molecule at 10.49 eV results in an ion
8
3
fragmentation.
C H O Isomers: The sublimation profiles recorded connected to C H O isotopomers in
3
6
2
3
6
2
methanol ices (SI Figure S7) depict three prominent peaks at 173 K, 210 K and 236 K. The peak
83
at 236 K are most has been assigned to photofragmentation of glycerol (C H O ). In irradiated
3
8
3
methanolꢀcarbon monoxide ices, C H O were identified via a broad sublimation profile (150 ꢀ
3
6
2
300 K) with two peaks at 192 K and 244 K (SI Figure S14). Methyl sugar molecules such as
hydroxyacetone (CH COCH OH; 10.0 eV), 2ꢀhydroxypropanal (CH CH(OH)CHO) and 3ꢀ
3
2
3
hydroxypropanal (HOCH CH CHO) can be linked to the product C H O with one hydroxyl and
2
2
3
6
2
one carbonyl functional groups. In addition, isomers bearing acid and ester functional groups
such as propionic acid (C H COOH; 10.44 eV) and methyl acetate (CH COOCH ; 10.25 eV) can
2
5
3
3
also be linked to C H O isomers.
3
6
2
C H O Isomers: Isomers with molecular formula C H O have been identified in both
3
8
2
3
8
2
irradiated methanol and methanolꢀcarbon monoxide ices as shown in SI Figures S7 and S14,
28

Page 29 of 77
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C4CP04149F
respectively. In irradiated methanol ices, the sublimation profile shows two prominent peaks at
71 K and 203 K. Molecules such as methoxyethanol (CH OCH CH OH) and ethoxymethanol
1
3
2
2
(
CH CH OCH OH) may be responsible for the observed ion counts at 171 K as these molecules
3 2 2
are expected to sublime at temperatures similar to methoxymethanol. Similarly, the sublimation
of diꢀol isomers are expected to be at temperatures close to ethylene glycol. Hence, 1,2ꢀ
propanediol and/or 1,3ꢀpropanediol isomers may be responsible for the signal at 203 K. In the
case of irradiated methanolꢀcarbon monoxide ices, the sublimation of C H O (SI Figure S14)
3
8
2
shows a broad profile extends from 150 K to at least 270 K. The broad sublimation profile is
most likely due to possible intermolecular interactions with higher mass organics, as we have
mentioned before. Here, the peak at ~185 K may possibly be attributed to the sublimation of
methoxyethanol (CH OCH CH OH) or ethoxymethanol (CH CH OCH OH) isomers as the peak
3
2
2
3
2
2
position is close to the observed temperature for methoxymethanol (184 K). Whereas more polar
diꢀol isomers such as 1,2ꢀpropanediol and/or 1,3ꢀpropanediol may account for the extended
sublimation tail due to their higher desorption energies.
C H O Isomers: Product with formula C H O were only detected in the TPD spectra of
4
6
2
4
6
2
irradiated methanolꢀcarbon monoxide ices and is shown SI Figure S14 which shows a broad
sublimation profile with two subtle peaks at 192 K and 236 K which again suggest that isomers
of C H O with different polarity are sublimating. The weak features observed ~145 K can be
4
6
2
linked to the coꢀsublimation of C H O isomers with methanol matrix. Isomers of C H O can be
4
6
2
4
6
2
associated to dicarbonyl molecules such as butaneꢀ2,3ꢀdione (CH COCOCH ; 9.3 eV), 2ꢀ
3
3
oxobutanal (HCOCOCH CH ), and molecules bearing single carbonyl (CO) and single hydroxyl
2
3
(
OH) groups such as 2ꢀhydroxybutꢀ3ꢀenal (HCOCH(OH)CHCH ) and 4ꢀhydroxybutꢀ2ꢀenal
2
(
HCOCHCHCH OH). The dicarbonyl isomers are expected to sublime at lower temperatures (~
2
192 K) due to their less polar functional group compared to hydroxyꢀbearing groups isomers.
8
3
Fragmentation of glycerol at 10.49 eV is once again attributed to the peak at 236 K.
C H O Isomers: The sublimation profiles of due to C H O isomers are depicted in SI Figures
4
8
2
4
8
2
S15 and S16. In irradiated methanol ices, the sublimation of C H O displays three peaks at 179
4
8
2
K, 207 K and 254 K; however, in irradiated methanolꢀcarbon monoxide ices, the sublimation
profile of C H O shows a broad profile with a slight peak positioned at 236 K. These
4
8
2
observations again suggest the possible formation of multiple isomers of C H O with different
4
8
2
29