Dissociative photoionization of CH3SSCH3 in the region of ～8-25 eV

Article abstract of DOI:10.1063/1.478826

The dissociative photoionization of CH₃SSCH₃ has been investigated in the photon energy range of ～8-25 eV with a molecular beam/photoionization mass spectrometry/threshold photoelectron spectrometry system using synchrotron radiation as an ionization source. For dissociation above photon energy of 11.5 eV, six fragment ions of CH₃⁺, C₂H₃⁺, SH₃⁺, HCS⁺, S₂⁺, and CH₂S₂⁺ were reported for the first time. The photoionization efficiency spectra for the parent ion and for 12 observed fragment ions, CH₃⁺, C₂H₃⁺, SH₃⁺, HCS⁺, CH₂S⁺, CH₂SH⁺, CH₃SH⁺, CH₃SH₂⁺, CH₃SCH₂⁺, S₂⁺, CH₂S₂⁺, and CH₂S₂H⁺, were measured; their branching ratios as a function of photon energy were derived. Ionization energy of 8.20±0.04eV for CH₃SSCH₃ and the appearance energy for each fragment ion were determined from the onsets of the photoionization efficiency spectra. Based on the appearance energy and existing thermochemical data, plausible structures of the fragment ions and their neutral counterparts are proposed. Fragmentation mechanisms that involve H migration and structural rearrangement in the dissociative photoionization processes are discussed.

Full text of DOI:10.1063/1.478826

Dissociative photoionization of CH 3 SSCH 3 in the region of 8–25 eV
Su-Yu Chiang, Chien-I Ma, and Der-Jr Shr
Citation: The Journal of Chemical Physics 110, 9056 (1999); doi: 10.1063/1.478826
View online: http://dx.doi.org/10.1063/1.478826
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 110, NUMBER 18
8 MAY 1999
Dissociative photoionization of CH3SSCH3 in the region of ϳ8–25 eV
Su-Yu Chiang,a) Chien-I Ma, and Der-Jr Shr
Synchrotron Radiation Research Center, No. 1, R&D Road VI, Hsinchu Science-Based Industrial Park,
Hsinchu 300, Taiwan, Republic of China
͑Received 14 January 1999; accepted 16 February 1999͒
The dissociative photoionization of CH3SSCH3 has been investigated in the photon energy range of
ϳ8–25 eV with a molecular beam/photoionization mass spectrometry/threshold photoelectron
spectrometry system using synchrotron radiation as an ionization source. For dissociation above
photon energy of 11.5 eV, six fragment ions of CHϩ3 , C2H3ϩ, SHϩ3 , HCSϩ, S2ϩ, and CH2S2ϩ were
reported for the first time. The photoionization efficiency spectra for the parent ion and for 12
observed fragment ions, CHϩ3 , C2H3ϩ, SH3ϩ, HCSϩ, CH2Sϩ, CH2SHϩ, CH3SHϩ, CH3SHϩ2 ,
CH3SCHϩ2 , Sϩ2 , CH2S2ϩ, and CH2S2Hϩ, were measured; their branching ratios as a function of
photon energy were derived. Ionization energy of 8.20Ϯ0.04 eV for CH3SSCH3 and the appearance
energy for each fragment ion were determined from the onsets of the photoionization efficiency
spectra. Based on the appearance energy and existing thermochemical data, plausible structures of
the fragment ions and their neutral counterparts are proposed. Fragmentation mechanisms that
involve H migration and structural rearrangement in the dissociative photoionization processes are
discussed. © 1999 American Institute of Physics. ͓S0021-9606͑99͒01418-X͔
10
¨
I. INTRODUCTION
energy ion–molecule reactions, Leeck and Kenttamaa as-
signed an IE of 8.0Ϯ0.2 eV, close to the 8.18 eV reported by
Li et al.8 The adiabatic IE was also corrected from 7.4
Ϯ0.3 to 8.18Ϯ0.03 eV following new RRKM calculations.9
Nevertheless, in a more recent work with a discharge flow
coupled to a quadrupole mass spectrometer ͑QMS͒ system,
no sign of a small step was found near the onset of the PIE
curve of CH3SSCH3, thus yielding IEϭ8.34Ϯ0.03 eV
again.30
Dimethyl disulfide (CH3SSCH3) is an important precur-
sor in the atmospheric sulfur chemistry cycles that contribute
to formation of acid rain.1–6 Hence it is of fundamental im-
portance to understand its photochemistry through study of
energetics and structures of its photoionization products, and
branching ratios as a function of photon energy in various
dissociation channels. Numerous experimental measure-
ments and theoretical calculations in the literature establish a
reliable thermochemical database of organosulfur molecules,
radicals, and ions.7–25 However, accurate determination of
the adiabatic ionization energy ͑IE͒ of CH3SSCH3 is hin-
dered because of alteration of its geometry upon ionization,
and the unimolecular decomposition properties of
CH3SSCHϩ3 were studied only in the photon energy range of
8–12 eV.7–10,26–30
A value of ϳ8.3 eV for the IE of CH3SSCH3 was deter-
mined by photoionization ͑PI͒ and photoelectron spectros-
copy ͑PES͒ in conventional gaseous experiments.7,27 How-
ever, using threshold photoelectron–photoion coincidence
͑TPEPICO͒, Butler et al.7 measured the dissociation rates of
energy-selected parent ions in channels of formation of
C2H5Sϩ and CH2Sϩ, and found IEϭ7.4Ϯ0.3 eV in order to
fit the ion dissociation rates with Rice–Ramsperger–Kassel–
Marcus ͑RRKM͒ theory.31,32 Li et al.8 detected a small step
occurring at 8.18Ϯ0.03 eV besides a much stronger step at
8.33 eV in the threshold region of the PI efficiency ͑PIE͒
curve of CH3SSCH3 seeded in Ar. This new value, 8.18 eV,
in fair agreement with a theoretical prediction at 8.15 eV in
the same paper,8 was assigned as the IE of
trans-CH3SSCHϩ3 , whereas the step at 8.33 eV was attrib-
uted to formation of cis-CH3SSCHϩ3 . Later, using low-
Dissociation products of CH3SSCHϩ3 were identified by
Butler et al.7 in the photon energy range of 8–12 eV, and the
appearance energies ͑AEs͒ of six major fragment ions,
CH3SSϩ, C2H5Sϩ, CH3SH2ϩ, CH3SHϩ, CH3Sϩ, and CH2Sϩ,
at 298 K were obtained from the onsets of their respective
PIE curves. Through appropriate thermochemical cycles and
measured AEs, heats of formation of observed fragment ions
were derived; thus their most probable structures were pro-
posed. Among the derived heats of formation, some are sub-
stantially smaller than values reported in other experiments.
In order to establish independently the heat of formation of
CH2Sϩ, Ruscic and Berkowitz20 reexamined the dissociation
onsets of CH3SSCH3ϩ on four fragment ions, CH2Sϩ,
CH2SHϩ, CH3SHϩ, and CH3SHϩ2 , but no absolute AEs of
these four fragment ions, merely relative shifts between their
respective AEs, were determined by matching curvatures of
their respective photoion yield curves.
Not long ago, Ma et al.9 reexamined the unimolecular
dissociation onset of jet-cooled CH3SSCHϩ3 in the channel
forming CH3Sϩ2 . A clear threshold at 11.07Ϯ0.05 eV is sig-
nificantly greater than 10.15 eV reported by Butler et al.,7
but agrees satisfactorily with Butler’s onset at 11.10 eV.
Moreover, Ma et al. recalculated the RRKM dissociation
rates for channels forming C2H5Sϩ2 and CH3S2ϩ using
IEϭ8.18 eV and rates of decay of C2H5Sϩ2 measured by But-
ler et al. According to Ma’s RRKM result, a kinetic shift at
a͒
Corresponding author; electronic mail: schiang@srrc.gov.tw
0021-9606/99/110(18)/9056/8/$15.00
9056
© 1999 American Institute of Physics
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J. Chem. Phys., Vol. 110, No. 18, 8 May 1999
Chiang, Ma, and Shr
9057
0.2 eV for the CH3Sϩ2 channel was estimated. Based on their
onset, kinetic shift, and ab initio calculations,33 they pro-
posed that the most likely isomer of the ion CH3Sϩ2 formed at
11.07 eV was CH2SSHϩ, rather than CH3SSϩ suggested by
Butler et al.
densation of CH3SSCH3. To maintain the vacuum of the
beamline, a differential pumping system was installed be-
tween the beamline and the ionization chamber; we kept the
pressure in the ionization chamber less than 5ϫ10Ϫ7 Torr
when the sample beam was introduced.
The sample in the molecular beam was ionized about
105 mm downstream of the nozzle with the monochromatic
synchrotron radiation that intersects the molecular beam at a
right angle. The ions and electrons produced were extracted
in opposite directions and toward their respective detection
axes perpendicular to the plane defined by molecular and
photon beams. The ions were mass analyzed with a quadru-
pole mass spectrometer ͑Extrel, C50͒ and detected with an
electron multiplier ͑channeltron͒ operated in pulse-counting
mode. A threshold photoelectron spectrometer with two mi-
crochannel plates as detectors served for threshold electron
collection. Both ion and electron signals were amplified and
counted with a dual-photon counter before being transferred
to a computer for further processing. To normalize intensities
of the ions and electrons to photon intensity, we placed Ni
meshes ͑90% transmission͒ at the entrance and exit of the
ionization chamber to monitor the variation of the photon
flux. The mesh currents were converted to frequencies that
were then fed into a second dual-photon counter. All data
acquisition processes were controlled with a computer via an
IEEE-488 interface.
The mass spectra were measured at wavelengths that
correspond to various electronic states of CH3SSCHϩ3 . The
mass resolution (m/⌬m) was about 150 at m/zϭ94. As no
signal at mass greater than that of CH3SSCHϩ3 was detected,
all observed fragment ions were considered to originate from
dissociative processes of the parent ion. The PIE curves of
the parent ion and various fragment ions resulted from mea-
surement of the signal at the respective m/z channels as a
function of the wavelength with an increment 4 Å, and were
normalized to the photon flux. No attempt was made to ac-
cumulate the ion signals of 34S-containing species since these
were not abundant. To determine the ionization energy of
CH3SSCH3 and the appearance energies of all observed frag-
ment ions precisely, we measured the PIE curves near the
threshold regions with a wavelength increment of 0.2–1 Å
and an accumulation period of 10–45 s/point depending on
the abundance of the ions.
No data are available on dissociative properties of
CH3SSCHϩ3 beyond a photon energy of 12 eV, nor are
branching ratios of various dissociation channels reported
previously. With this concern and with the discrepancies
among heats of formation and structures in the literature for
fragment ions of CH3SSϩ and C2H5Sϩ produced in the pho-
ton energy region of 8–12 eV, and among IE values of
CH3SSCH3, an extensive study of the dissociative photoion-
ization of CH3SSCH3 is deemed necessary. In this work, we
investigated the dissociative photoionization of CH3SSCH3
in the photon energy region of ϳ8–25 eV using a molecular
beam/QMS/TPES setup and synchrotron radiation as an ion-
ization source. The branching ratios of the parent ion and the
12 observed fragment ions as a function of photon energy in
the range of ϳ8–25 eV were measured for the first time. The
IE of CH3SSCH3 and the AE for each fragment ion were
determined from the onsets of the photoionization efficiency
spectra. From the determined AEs of these observed frag-
ment ions, new information on the likely structures of the
fragment ions and their neutral counterparts is provided. Fur-
thermore, we discuss fragmentation mechanisms involving H
migration and structural rearrangement.
II. EXPERIMENT
Dissociative photoionization of CH3SSCH3 was per-
formed with a molecular beam/QMS/TPES system, that will
be described in detail in a forthcoming publication,34 using
synchrotron radiation as an ionization source. Synchrotron
radiation from the 1.5 GeV electron storage ring of the Syn-
chrotron Radiation Research Center ͑SRRC͒ in Taiwan is
dispersed by a 1 m Seya-Namioka monochromator, which is
equipped with three gratings with groove densities of 2400,
1200, and 600 lines/mm to cover a spectral range of 300–
3000 Å.35 Typically, gratings with 600 and 1200 lines/mm
and a slit width 0.1–0.2 mm were used in the wavelength
region ϳ500–1550 Å, which provides a wavelength resolu-
tion of 1.25–2.5 Å ͑full width at half maximum͒ and a pho-
ton flux Ͼ109 photons/s. The wavelength of the monochro-
mator was calibrated absolutely on recording photoionization
and threshold photoelectron spectra of Ar, and found to be
better than 0.2 Å.36,37 For wavelengths longer than 1050 Å, a
LiF window served to eliminate high-order harmonic con-
tamination from the grating. With a grating at 1200 lines/
mm, the high-order contribution is about 1%–3% below
1050 Å.
The CH3SSCH3 sample was obtained from a commercial
source ͑Merck͒ with a stated purity of 99%; no further puri-
fication was made except for freeze–pump–thaw degassing.
Noble gases, He and Ar, with purities Ͼ99.9995% were used
without purification.
III. RESULTS AND DISCUSSION
A. Threshold photoelectron spectrum of CH3SSCH3
In this experiment, CH3SSCH3 vapor carried by He was
expanded through a nozzle with a diameter of 0.125 mm and
skimmed with two conical skimmers with apertures of 1 and
2 mm to form a cooled sample beam. The total stagnation
pressure monitored ͑MKS baratron͒ was ϳ330 Torr ͑pres-
sure ratio, CH3SSCH3:HeϷ1:10͒. With a resistive heater,
the temperature of the nozzle tip monitored with a Chromel–
Alumel thermocouple was kept at 313Ϯ1 K to avoid con-
The TPE spectrum of CH3SSCH3 was measured with a
wavelength increment of 2 Å in the region of 300–1500 Å
͑ϳ41–8.3 eV͒, as shown in Fig. 1. Gratings with groove
densities of 2400, 1200, and 600 lines/mm were used in re-
gions of 300–750, 750–1040, and 1050–1500 Å, respec-
tively. The sharp feature at 1080 Å originates from the TPE
of He because for this spectrum no LiF filter was installed to
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9058
J. Chem. Phys., Vol. 110, No. 18, 8 May 1999
Chiang, Ma, and Shr
FIG. 1. TPE spectrum of CH3SSCH3 in the wavelength region of 300–1500
Å ͑ϳ41–8.3 eV͒. This spectrum was measured with a wavelength increment
of 2 Å, three gratings with groove densities of 2400, 1200, and 600
lines/mm were used in regions of 300–750, 750–1040, and 1050–1500 Å,
*
respectively. The asterisk ͑ ͒ marks TPE signals of He originating from
excitations of the fundamental and high-order light from the grating, and
serves for wavelength calibration.
eliminate high-order light from the grating, but, together
with the peak at 504 Å, this feature served for wavelength
calibration. Table I lists band maxima in the wavelength and
energy in the range of ϳ8–23 eV. Also noted in Table I are
bands of the He I PE spectrum of CH3SSCH3 at vertical
ionization energies of 8.96, 9.26, 11.26, 12.31, 13.42, 14.35,
and 14.75 eV which, according to molecular–orbital calcu-
lations, correspond to removal of an electron from 12b, 13a,
12a, 11a, 11b, 10b, and 9b molecular orbitals, respec-
tively.29,36–38
The TPE spectrum in the region of 650–1550 Å exhibits
features similar to those in the PE spectrum, but with differ-
ent relative intensities, which could be attributed to different
Franck–Condon factors.29,38 In particular, the PE spectrum
shows negligible signals in the region of 1120–1240 Å. The
signals in the TPE spectrum in this Franck–Condon gap re-
gion might be due to autoionization processes that produce
low-energy electrons that are detected as threshold photo-
electrons.
FIG. 2. Fragmentation mass spectra of CH3SSCH3 excited at wavelengths of
1174 ͑10.56 eV͒, 1002 ͑12.37 eV͒, and 601 Å ͑20.63 eV͒. At an excitation
wavelength of 602 Å, 12 fragment ions of CHϩ3 , C2H3ϩ, SH3ϩ, HCSϩ,
CH2Sϩ, CH2SHϩ, CH3SHϩ, CH3SH2ϩ, CH3SCH2ϩ, Sϩ2 , CH2Sϩ2 , and
CH2S2Hϩ were observed. The Arϩ signal serves for wavelength calibration,
and magnified signals of fragment ions of CHϩ3 , C2H3ϩ, SH3ϩ, and Arϩ are in
the inset.
TABLE I. Band maxima in the threshold photoelectron spectrum of
CH3SSCH3 in the energy region of ϳ8–23 eV, compared with literature
results of the photoelectron spectrum.
The TPE spectrum in the region of 400–650 Å shows
three bands with maxima at 680, 599, and 540 Å, unreported
for the previous PE experiments. To ascertain these assign-
ments and to determine vertical energies of these electronic
bands, a molecular–orbital calculation is needed. In sum-
mary, our TPE spectrum agrees satisfactorily with the previ-
ous PE spectrum reported in the literature, except for the
newly observed bands at 1226.6, 1183.5, 680.0, 599.4, and
540.0 Å.
Peak Wavelength
Energy
Energya Difference
no.
͑Å͒
͑eV͒
͑eV͒
͑eV͒
Character
nSϪ
1
2
3
4
5
6
7
8
9
1380.6
1339.6
1226.6
1183.5
1100.4
1002.4
922.4
870.3
838.3
680.0
599.4
8.98Ϯ0.02
9.26Ϯ0.02
10.11Ϯ0.02
10.48Ϯ0.03
11.27Ϯ0.02 11.26
12.37Ϯ0.03 12.31
13.44Ϯ0.03 13.42
14.25Ϯ0.04 14.35
14.79Ϯ0.07 14.75
8.96
9.26
¯
0.02
0.00
¯
nSϩ
Autoionizationb
Autoionizationb
¯
¯
0.01
0.06
0.02
Ϫ0.1
0.04
¯
␴
SS
␴
CS
␴
CS
␲
CH
3
␲
CH
3
¯
¯
¯
B. Primary fragment ions observed
10
11
12
18.23Ϯ0.09
20.68Ϯ0.09
22.96Ϯ0.2
¯
¯
¯
The fragmentation mass spectra of CH3SSCH3 excited at
wavelengths of 1174 ͑10.56 eV͒, 1002 ͑12.37 eV͒, and 601
Å ͑20.63 eV͒, which correspond to excitation of CH3SSCH3ϩ
to various electronic excited states, are shown in Fig. 2. The
spectra were scanned with a step of 0.2 amu and a slit width
¯
540.0
¯
aReference 29.
bAssigned in this work.
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J. Chem. Phys., Vol. 110, No. 18, 8 May 1999
Chiang, Ma, and Shr
9059
of 100 ␮m. As can be discerned in Fig. 2, signals due to
several new fragment ions appear as the photon energy in-
creases. At an excitation wavelength of 601 Å, 12 fragment
ions, separate from sulfur isotopic species, were observed at
m/zϭ15, 27, 35, 45, 46, 47, 48, 49, 61, 64, 78, and 79,
corresponding to isomeric structures of CHϩ3 , C2H3ϩ, SH3ϩ ,
HCSϩ, CH2Sϩ, CH3Sϩ, CH4Sϩ, CH5Sϩ, C2H5Sϩ, S2ϩ ,
CH2Sϩ2 , and CH3Sϩ2 , respectively. The fragment ions, CHϩ3 ,
C2Hϩ3 , SHϩ3 , HCSϩ, S2ϩ , and CH2Sϩ2 , are reported for the
first time. The production of fragment ions at m/zϭ27 and
61 implies that the parent ion undergoes isomerization before
fragmentation. We discuss in Sec. III D isomeric structures
of these fragment ions based on our determined AEs and
heats of formation of fragment ions and their neutral coun-
terparts reported in the literature.
C. Photoionization efficiency curves and relative
branching ratios
Figure 3͑a͒ shows the PIE curves of the parent ion and
all observed fragment ions in the wavelength region of 550–
1400 Å ͑22.5–8.9 eV͒. The PIE curves are normalized with
respect to photon flux and scanned linearly in wavelength.
The corresponding scale of energy in electron volts is also
shown at the top. Since the natural abundance of 34S is sub-
stantial, the PIE curves at m/zϭ47, 48, and 49 were cor-
rected for contributions from ion signals at m/zϭ45, 46, and
47, respectively. But, no attempt was made to correct the
high-order light contribution from the grating, as it is less
than 3% and difficult to estimate. Based on the normalized
PIE curves, the branching ratios of parent and fragment ions
as a function of wavelength are derived and they are depicted
in Fig. 3͑b͒, in which the total ion intensity is scaled to 100
for each data point.
To show the correlation between electronic bands and
intensity variations of parent and fragment ions, dashed lines
indicating vertical transition energies of 11 bands in the TPE
spectrum are also depicted in Figs. 3͑a͒ and 3͑b͒. Although
the PIE curves fail to exhibit fine feature like those in the
TPE spectrum, the variation of intensity among the curves
shows phenomena worth noting. In Fig. 3͑a͒, the intensity
due to the parent ion increases from threshold to a maximum
at ϳ1180 Å, and then decreases abruptly until ϳ922 Å, after
which it attains a plateau and declines monotonically from
ϳ770 Å toward shorter wavelengths. The stepwise increase
of intensity up to 1180 Å follows the onsets of electronic
states, and the decrease of intensity from ϳ1180 Å reflects
formation of fragment ions with m/zϭ46, 47, 48, 49, and 61
from their respective thresholds in the region of 1120–1200
Å, seen in Fig. 3͑b͒. As mentioned before, threshold electron
signals were observed in this Franck–Condon gap region;
such a phenomenon indicates that autoionization states play
an important role in the formation of fragment ions with
m/zϭ46, 47, 48, 49, and 61 at threshold.
FIG. 3. ͑a͒ Normalized PIE curves of CH3SSCHϩ3 and all observed fragment
ions in the wavelength region of 550–1400 Å ͑22.5–8.9 eV͒. ͑b͒ Relative
branching ratios of CH3SSCHϩ3 and all observed fragment ions in the wave-
length region of 550–1200 Å ͑22.5–10.3 eV͒. The m/z ratios of each frag-
ment ion are indicated in parentheses. The total ion intensity is scaled to 100
for the entire wavelength region. Dashed lines, marked at vertical energies
of the 11 bands observed in the TPE spectrum, indicate the correlation
between these electronic bands and the intensity variation of parent and
fragment ions.
Also seen in Fig. 3͑a͒ is that fragment ions with m/z
ϭ46, 48, 49, and 61 show similar behavior. Beginning at
threshold, the ion signals at m/zϭ46, 48, 49, and 61 increase
and reach a common maximum at ϳ1090 Å, and then remain
nearly constant until ϳ770 Å, except for the signal at m/z
ϭ61 which declines steadily from its maximum toward
shorter wavelengths. The nearly constant intensities of frag-
ment ions at m/zϭ46, 48, and 49 suggest that they are
formed through similar dissociation paths, and that their dis-
sociation rates are greater than that of the ion with m/z
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9060
J. Chem. Phys., Vol. 110, No. 18, 8 May 1999
Chiang, Ma, and Shr
FIG. 4. ͑a͒–͑d͒ Photoion yields of CH3SSCHϩ3 and
fragment ions observed at m/zϭ79, 61, and 46, and
their respective linearly fitted lines near the threshold
region. A LiF window served to eliminate high-order
contamination of light from the grating for the fragment
ions.
ϭ61 as the photon energy increases from their common
maximum.
ring above the intersection of two linearly fitted lines, one
the baseline and the other the onset. AE values estimated in
this way represent upper limits due to the possible presence
of reverse activation barriers and kinetic shifts, but data treat-
ment of this kind has proved to be useful in general.39,40
Table II lists the determined values, proposed dissocia-
tion channels, and AEs reported in the literature. As can be
seen in Table II, our IE at 1512Ϯ7 Å(8.20Ϯ0.04 eV͒ agrees
satisfactorily with values of 8.18Ϯ0.03 eV reported by Li
Competition of dissociation channels is observed for
pairs of fragments. For example, a decrease of signal for
m/zϭ61 is accompanied by an increase of signal for m/z
ϭ79, and a decrease of signal for m/zϭ79 is accompanied
by an increase for an ion with m/zϭ45. This phenomenon
could be due to dissociation channels opening as the internal
energy of the parent ion increases, or to the onsets of elec-
tronic states. However, it is difficult to distinguish these two
factors simply from the PIE curves as the photon energy
increases because the existence of many excited electronic
states complicates identification of formation sources of par-
ent and fragment ions. Measurements of photoelectron–
photoion coincidence spectra might reveal a correlation be-
tween formation of these fragment ions and ionic
electronically excited states.
Fragment ions with m/zϭ15, 27, 35, 45, 64, and 78, as
we know, have not been reported previously in dissociation
of CH3SSCHϩ3 . The proportions of fragment ions produced
at m/zϭ27 and 35 are small, less than a 1% contribution,
and are thus barely visible in Figs. 3͑a͒ and 3͑b͒. Fragment
ions of CHϩ3 , HCSϩ, S2ϩ , and CH2Sϩ2 are produced in sig-
nificant proportions above 800 Å; among them, HCSϩ is the
major fragment ion with more than a 20% contribution, seen
in Fig. 3͑b͒.
8
10
¨
et al. and 8.0Ϯ0.2 eV reported by Leeck and Kenttamaa.
The IE at 8.34Ϯ0.03 eV determined by discharge flow and a
QMS likely reflects their small detection sensitivity.30 Six
dissociation channels, ͑1͒, ͑2͒, ͑3͒, ͑4͒, ͑10͒, and ͑11͒, are
observed for the first time, and for other channels, ͑5͒, ͑6͒,
͑7͒, ͑8͒, ͑9͒, and ͑12͒, our AE values significantly exceed
literature values.7 The proposed dissociation products and
the reason for the large AE differences are discussed based
on our AEs and heats of formation taken from the literature.
2. Proposal of dissociation channels
In general, AEs and heats of formation of a fragment ion
and its neutral counterpart are equated as follows:
AE ͑fragment ion)у⌬H0f0͑fragment ion͒
ϩ⌬H0f0͑neutral partner͒
Ϫ⌬H0f0 CH SSCH ͒,
͑1͒
͑
3
3
D. Dissociation of CH3SSCH3؉
in which ⌬H0f0 is the heat of formation at 0 K. As the mo-
lecular beam cooled CH3SSCH3 in the present photoioniza-
tion experiment, the thermal internal energy of CH3SSCH3
was neglected in our calculations. For reference, heats of
formation of organosulfur species at 0 K taken from the lit-
erature are listed in Table III.9,14,20,23,41–43 In what follows
we discuss properties of these dissociation channels.
1. Ionization and appearance energies
The photoion yields and linearly fitted lines near the
threshold region for parent and fragment ions with m/z
ϭ79, 61, and 46 are shown in Figs. 4͑a͒–4͑d͒; a LiF window
served to eliminate high-order contamination from the grat-
ing. The observed onset rises gradually because of a small
Franck–Condon factor that reflects an altered conformation
upon ionization, the CSSC dihedral angle altering from ϳ85°
to 0° and 180°.11,26–28 The IE of CH3SSCH3 and the AE of
each fragment ion were thus determined from signals occur-
a. Dissociation channels of CH3SSCHϩ3 proposed for
wavelengths above 1100 Å. For the purpose of discussion,
we temporarily assign isomeric structures of CH3Sϩ2 ,
C2H5Sϩ, CH5Sϩ, CH4Sϩ, CH3Sϩ, and CH2Sϩ to the ob-
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J. Chem. Phys., Vol. 110, No. 18, 8 May 1999
Chiang, Ma, and Shr
9061
TABLE II. Ionization energy of CH3SSCH3, appearance energies of fragment ions, proposed structures of some
products, and AEs calculated from heats of formation listed in Table III and AEs in the literature.
Energy
Dissoc.
chn.
Energya
͑eV͒
Literatured
͑eV͒
b
c
Proposed ion
CH3SSCHϩ3
Proposed neutral
͑eV͒
͑eV͒
8.20Ϯ0.04
8.18Ϯ0.03e
8.33f
CHϩ3
1
2
3
4
5
6
7
8
9
¯
H2SϩSH
CH3ϩCS
¯
12.85Ϯ0.05
13.32Ϯ0.05
12.84Ϯ0.05
13.24Ϯ0.05
10.73Ϯ0.04
10.82Ϯ0.04
10.78Ϯ0.04
10.90Ϯ0.04
10.50Ϯ0.05
14.12Ϯ0.05
12.29Ϯ0.10
11.12Ϯ0.05
C2Hϩ3
12.92
12.58
͑0.40͒
͑0.26͒
SHϩ3
HCSϩ
CH2Sϩ
CH2SHϩ
CH3SHϩ
CH3SHϩ2
CH3SCHϩ2
S2ϩ
10.55
10.60
10.61
10.68
9.94
͑0.18͒
͑0.22͒
͑0.17͒
͑0.22͒
͑0.56͒
͑0.27͒
10.15Ϯ0.08
10.4 Ϯ0.1
10.4 Ϯ0.1
10.5 Ϯ0.1
10.08Ϯ0.08
CH3SHg
CH3S
CH2Sg
HCS
SH
10
11
12
13.85
2CH3
¯
CH2Sϩ2
CH2S2Hϩ
10.85
͑0.27͒
10.15Ϯ0.10
11.07Ϯ0.05h
CH3
aMeasured in this work.
bCalculated from heats of formation listed in Table III.
cDifference between the experimental values derived in this work and those calculated.
dReference 7, unless stated otherwise.
eAdiabatic IE for CH3SSCH3→trans-CH3SSCHϩ3 , Ref. 8.
fAdiabatic IE for CH3SSCH3→cis-CH3SSCHϩ3 , Ref. 8.
gViolation of Stevenson’s rule.
hReference 9.
and CH2Sϩ, with m/zϭ49–46, are observed with their re-
spective AEs, 10.90Ϯ0.04, 10.78Ϯ0.04, 10.82Ϯ0.04, and
10.73Ϯ0.04 eV. These values are significantly higher than
those of Butler et al., 10.5Ϯ0.1, 10.4Ϯ0.1, 10.4Ϯ0.1, and
10.15Ϯ0.08 eV,7 and the cause of this large discrepancy,
more than 0.3 eV, is unclear.
served fragment ions at m/zϭ79, 61, 49, 48, 47, and 46,
respectively, since several isomeric structures exist for each
observed fragment ion of a particular m/z ratio.
͑1͒ CH5Sϩ, CH4Sϩ, CH3Sϩ, and CH2Sϩ (m/z
ϭ49–46): Four fragment ions, CH5Sϩ, CH4Sϩ, CH3Sϩ,
Despite differences in the absolute AE, the differences in
our AE for adjacent masses, ⌬E(48, 49)ϭ0.12, ⌬E(47, 48)
ϭϪ0.04, and ⌬E(46, 47)ϭ0.09 eV, are close to the latter
two quantities determined by Butler et al., 0.0 and 0.1 eV.7
Our values are also in agreement with values of 0.108
Ϯ0.035, 0.000Ϯ0.030, and 0.068Ϯ0.030 eV obtained by
Ruscic and Berkowitz with a curve-matching method.20
Upper limits of heats of formation of these four fragment
ions are derived using Eq. ͑1͒ based on our AEs and well
known heats of formation of the most stable structures of
proposed neutral counterparts of HCS, CH2S, CH3S, and
CH3SH listed in Table III.14,20,42–43 The ⌬H0f0 of these four
fragment ions calculated in this work are 178.1, 218.7, 216.5,
and 248.7 kcal/mol, near the reported values of 173, 214.8,
211.5, and 244.5 kcal/mol, corresponding to CH3SHϩ2 ,
CH3SHϩ, CH2SHϩ, and CH2Sϩ, respectively.20,42 The dif-
ferences between our calculated value ⌬H0f0 and the litera-
ture value for each fragment ion are almost constant at 5.1,
3.9, 5.0, and 4.2 kcal/mol, and with the average being 4.6
Ϯ0.7 kcal/mol (0.2Ϯ0.03 eV). Our values, consistently
higher than literature values, are rationalized by the presence
of kinetic shifts and reverse activation energies for formation
of these fragment ions, as mentioned earlier. Thus, we pro-
pose CH3SHϩ2 , CH3SHϩ, CH2SHϩ, and CH2Sϩ as the most
likely structures of fragment ions observed at m/zϭ49, 48,
47, and 46, respectively.
TABLE III. Auxiliary heats of formation of selected organosulfur species
from the specified sources.
⌬H0f0
͑kcal/mol͒
a
Molecule
Ϫ1.6
262
CH3SSCH3
CHϩ3
CH3
35.6
CS
63
HCS
71.7b
243.9b
28.3b
244.5b
211.5b
31.44c
Ϫ2.9
214.8
173
HCSϩ
CH2S
CH2Sϩ
CH2SHϩ
CH3S
CH3SH
CH3SHϩ
CH3SHϩ2
CH3S2
CH2S2Hϩ
C2Hϩ3
17.8d
213e
267.9
195.1f
32.6
CH3SCHϩ2
SH
H2S
Ϫ4.2
190
246.4
Ϫ16.0
SHϩ3
S2ϩ
CH4
aReference 42, unless stated otherwise.
bReference 20.
dReference 41.
cReference 9.
fReference 23
cReferences 14 and 43.
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9062
J. Chem. Phys., Vol. 110, No. 18, 8 May 1999
Chiang, Ma, and Shr
As CH3SHϩ and CH2S are the only plausible structures
formation, too. The average value of the ⌬H0f0 difference
between our values and the literature values for CH3SHϩ2 ,
CH3SHϩ, and CH2SHϩ is 0.2 eV, close to the kinetic shift
for the channel to form CH2SSHϩ according to the RRKM
calculation. A perfect match of both values and the common
phenomenon involving H transfers mutually support assign-
ments to CH2SSHϩ, CH3SH2ϩ, CH3SHϩ, and CH2SHϩ.
b. Dissociation channels of CH3SSCHϩ3 proposed for
wavelengths below 1100 Å. Six fragment ions with m/z
ϭ15, 27, 35, 45, 64, and 78 were observed and assigned as
CHϩ3 , C2Hϩ3 , SH3ϩ, HCSϩ, S2ϩ, and CH2Sϩ2 , respectively.
These fragment ions are formed at dissociation energies
much higher than those for the dissociation channels dis-
cussed above. With more dissociation channels in competi-
tion, the kinetic shifts for channels producing these fragment
ions are expected to be large and more difficult to evaluate.
Moreover, as these fragment ions are observed for the first
time, no relevant datum such as ab initio molecular orbital
calculations and dissociation rates is available to aid in the
explanation of dissociation properties. It becomes difficult to
assign neutral partners solely from the determined AE, let
alone derivation of heats of formation of these fragment ions.
Therefore, we simply list these fragment ions and some neu-
tral fragments in Table II, and discuss them briefly in what
follows.
on the basis of energy considerations and the available ther-
mochemical data in the literature, the fragmentation channels
of CH3SHϩϩCH2S and CH2SϩϩCH3SH proposed here do
not follow Stevenson’s rule. Our assignments for these chan-
nels are consistent with those of Ruseic and Berkowitz that
were determined from the relative shifts between the respec-
tive AEs of fragmentation channels.20 Butler’s assignment
for CH2SHϩ is also consistent with ours, but not for CHSHϩ
and CH2SHϩ2 .7 A possible explanation is that their values at
170.0, 216.2, 208.0, and 234.1 kcal/mol derived from their
low AEs are lower than the literature values, except for 216.2
kcal/mol, which is probably due to an adopted value that is
too low, ⌬H0f0͑CH2S͒ϭ25.1 kcal/mol, for the neutral.
͑2͒ C2H5Sϩ (m/zϭ61): The AE of C2H5Sϩ at 10.50
Ϯ0.05 eV determined in this work is 0.42 eV higher than the
10.08 eV reported at 298 K, but near the value of 10.28 eV at
0 K derived from statistical fitting of decay rates by Butler
et al.7 Regarding the RRKM dissociation rates calculated by
Ma et al.,9 a large kinetic shift for the channel forming
C2H5Sϩ is expected due to the very tight transition structure
which leads to an energy dependence of the dissociation rate
rising very slowly with excess energy. A kinetic shift of 0.28
eV is derived if we take the difference between our AE at
10.50 eV and the IE at 8.20 eV of CH3SSCH3 and subtract
from it the activation energy at 2.02 eV obtained from the
RRKM calculation by Ma et al.9 This value agrees satisfac-
torily with the 0.28 eV derived from the RRKM/QET ͑quasi-
equilibrium theory͒ decay rates calculated by Butler et al.7
Using our new AE at 10.50 eV, the derived kinetic shift of
0.28 eV, and heats of formation of SH and CH3SSCH3 at 0 K
listed in Table III, we obtain ⌬H0f0͑C2H5Sϩ͒р201 kcal/mol
from Eq. ͑1͒. Compared with theoretical predictions
⌬H0f0ϭ236.5 (CH3CH2Sϩ͒, 192.6 (cis-CH3CHSHϩ), 192.6
(trans-CH3CHSHϩ), and 195.3 (CH3SCHϩ2 ) kcal/mol, our
value is nearer 195.3 (CH3SCHϩ2 ) kcal/mol.23 Thus, we as-
sign the isomer CH3SCHϩ2 as the most probable structure,
consistent with results from collisional activation, PD–PI,
and theoretical prediction.7,44,45
To form Sϩ2 , loss of two CH3 groups is the most direct
and simple way. If this is the case, these two CH3 losses
must be consecutive, which means that at least part of the
m/zϭ79 ion must be the unrearranged CH3Sϩ2 ion formed at
photon energies higher than the 11.12 eV onset. On the basis
of an energy consideration, two three-body formation pro-
cesses, SHϩ3 ϩCH3ϩCS and C2H3ϩϩH2SϩSH, are also as-
sumed. Formation of C2Hϩ3 likely occurs through a tight tran-
sition structure to form a C–C bond, while SHϩ3 may involve
H transfer.
IV. CONCLUSIONS
͑3͒ CH3Sϩ2 (m/zϭ79): The AE of CH3Sϩ2 at 11.12
Ϯ0.05 eV determined here is significantly higher than the
weak onset at 10.15 eV reported by Butler et al.,7 but agrees
with their strong onset at 11.10 eV. Our value also agrees
with 11.07 eV reported for photoionization of jet-cooled
CH3SSCH3 by Ma et al.9 Ma et al. estimated a large kinetic
shift of 0.2 eV for the channel forming CH3Sϩ2 , and
thus derived ⌬H0f0͑fragment ion͒ϭ213 kcal/mol, near
⌬H0f0͑CH2SSHϩ͒ϭ211 kcal/mol predicted from their ab ini-
tio calculation. With this result, they claim the structure of
CH2SSHϩ instead of CH3SSϩ proposed by Butler et al. to be
the most likely structure formed near the onset at 11.07 eV.
As the kinetic shift cannot be estimated from our measured
AE alone, the structure of CH2SSHϩ proposed by Ma et al.
was adopted in Table II.
Dissociative photoionization of CH3SSCH3 into chan-
nels to form CHϩ3 , C2Hϩ3 , CH3SCHϩ2 , HCSϩ, SH3ϩ, CH2Sϩ,
CH2SHϩ, CH3SHϩ, CH3SH2ϩ, Sϩ2 , CH2Sϩ2 , and CH2S2Hϩ is
effected with a molecular beam/QMS/TPES setup coupled to
a synchrotron light source. According to measured photoion-
ization efficiency spectra of the parent ion and 12 fragment
ions, the branching ratios of these ions as a function of pho-
ton energy were derived in the photon energy region at
ϳ8–25 eV. The IE of CH3SSCH3 and the AEs of observed
fragment ions were determined from signals occurring above
the intersection of the baseline and the rising edge, both fit-
ted to a line by least squares. The AEs thus derived for
formation of CH2Sϩ, CH2SHϩ, CH3SHϩ, CH3SHϩ2 , and
CH2S2Hϩ are more consistent with recent studies of
CH3SSCH3 and similar molecules. Based on the determined
AE and existing thermochemical data, plausible structures of
some fragment ions and their neutral counterparts were pro-
posed. Isomerization instead of simple cleavage of the C–S
bond or the S–S bond seems to be a common step in these
dissociative processes.
Formation of CH2SSHϩ implies that the dissociation
channel proceeds through a transition structure involving H
migration. We note that the proposed fragment ions of
CH3SHϩ2 , CH3SHϩ, and CH2SHϩ, from a comparison with
⌬H0f0 in the literature, seem to involve H transfers for their
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J. Chem. Phys., Vol. 110, No. 18, 8 May 1999
Chiang, Ma, and Shr
9063
24 J. E. Stevens, K. F. Freed, and M. F. Arendt, J. Chem. Phys. 101, 4832
͑1994͒.
ACKNOWLEDGMENTS
25 S. Nourbakhsh, K. Norwood, H.-M. Yin, C.-L. Liao, and C. Y. Ng, J.
Chem. Phys. 95, 5014 ͑1991͒.
One of the authors ͑S.-Y.͒ is grateful to Professor T.
Baer for reading their manuscript and for helpful comments.
This work was supported by Synchrotron Radiation Research
Center and National Science Council of the Republic of
China under Contract No. NSC87-2613-M-213-019.
26 R. J. Colton and J. W. Rabalais, J. Electron Spectrosc. Relat. Phenom. 3,
345 ͑1974͒.
27 A. D. Baker, M. Brisk, and M. Gellender, J. Electron Spectrosc. Relat.
Phenom. 3, 227 ͑1974͒.
28 K. Kimura and K. Osafune, Bull. Chem. Soc. Jpn. 48, 2421 ͑1975͒.
29 K. Kimura, S. Katsumata, Y. Achiba, Y. Yamazaki, and S. Iwata, Hand-
book of He I Photoelectron Spectra of Fundamental Organic Molecules
͑Japan Scientific Society, Tokyo, 1980͒.
1 R. P. Wayne, Chemistry of Atmospheres ͑Clarendon, Oxford, 1991͒.
2 M. O. Andreae and H. Raemdonck, Science 221, 744 ͑1983͒.
3 F. Yin, D. Grosjean, and J. H. Seinfeld, J. Geophys. Res. 91, 14417
͑1986͒.
30 W.-C. Hung, M.-Y. Shen, Y.-P. Lee, N.-S. Wang, and B.-M. Cheng, J.
Chem. Phys. 105, 7402 ͑1996͒.
4 J. G. Calvert and J. N. Pitts, Photochemistry ͑Wiley, New York 1996͒.
5 S. D. Thompson, D. G. Carrol, F. Watson, M. O’Donnell, and S. P. McG-
lynn, J. Chem. Phys. 45, 1367 ͑1966͒.
31 T. Baer and W. L. Hase, Unimolecular Reaction Dynamics: Theory and
Experiments ͑Oxford University Press, New York, 1996͒.
32 P. J. R and K. A. Holbrook, Unimolecular Reactions ͑Wiley, London,
1972͒.
6 P. M. Rao and A. R. Knight, Can. J. Chem. 46, 2462 ͑1968͒.
7 J. J. Butler, T. Baer, and S. A. Evans, J. Am. Chem. Soc. 105, 3451
͑1983͒.
33 Y.-S. Cheung, W.-K. Li, and C. Y. Ng, J. Mol. Struct. 339, 25 ͑1995͒.
34 S. Y. Chiang and C.-I Ma ͑unpublished͒.
8 W.-K. Li, Z.-X. Ma, C.-L. Liao, and C. Y. Ng, J. Chem. Phys. 99, 8440
͑1993͒.
35 P.-C. Tseng, T.-F. Hsieh, Y.-F. Song, K.-D. Lee, S.-C. Chung, C.-I. Chen,
H.-F. Lin, T.-E. Dann, L.-R. Huang, C.-C. Chen, J.-M. Chuang, K.-L.
Tsang, and C.-N. Chang, Rev. Sci. Instrum. 66, 1815 ͑1995͒.
36 K. Yoshino, J. Opt. Soc. Am. A 60, 1220 ͑1970͒.
37 R. E. Huffman, Y. Tanaka, and J. C. Larrabee, J. Chem. Phys. 39, 902
͑1963͒.
9 Z.-X. Ma, C.-L. Liao, C. Y. Ng, Y.-S. Cheung, W.-K. Li, and T. Baer, J.
Chem. Phys. 100, 4870 ͑1994͒.
10
¨
D. T. Leeck and H. I. Kenttamaa, Org. Mass Spectrom. 29, 106 ͑1994͒.
11 J. J. Butler and T. Baer, J. Am. Chem. Soc. 104, 5016 ͑1982͒.
12 J. M. Nicovich, K. D. Kreutter, C. A. van Dijk, and P. H. Wine, J. Phys.
Chem. 96, 2518 ͑1992͒.
13 J. J. Butler and T. Baer, Org. Mass Spectrom. 18, 248 ͑1983͒.
14 B. Ruscic and J. Berkowitz, J. Chem. Phys. 97, 1818 ͑1992͒.
15 S. Nourbakhsh, K. Norwood, H.-M. Yin, C.-L. Liao, and C. Y. Ng, J.
Chem. Phys. 95, 946 ͑1991͒.
38 A. Yokozeki and S. H. Bauer, J. Phys. Chem. 80, 618 ͑1976͒.
39 B. P. Tsai, T. Baer, A. S. Werner, and S. F. Lin, J. Phys. Chem. 79, 570
͑1975͒.
40 H. Shiromaru, Y. Achiba, K. Kimura, and Y. T. Lee, J. Phys. Chem. 91,
17 ͑1987͒.
16 S. Nourbakhsh, K. Norwood, G.-Z. He, and C. Y. Ng, J. Am. Chem. Soc.
113, 6311 ͑1991͒.
41 Z.-X. Ma, C.-L. Liao, C. Y. Ng, N. L. Ma, and W.-K. Li, J. Chem. Phys.
99, 6470 ͑1993͒.
17 R. H. Nobes and L. Radom, Chem. Phys. Lett. 189, 554 ͑1992͒.
18 S. Nourbakhsh, H.-M. Yin, C.-L. Liao, and C. Y. Ng, Chem. Phys. Lett.
183, 348 ͑1991͒.
42 G. Lias, J. E. Bartmess, J. L. Holmes, R. D. Levin, and W. G. Mallard, J.
Phys. Chem. Ref. Data 17, Suppl. No. 1 ͑1988͒.
43 J. M. Nicovich, K. D. Kreutter, C. A. van Dijk, and P. H. Wine, J. Phys.
Chem. 96, 2518 ͑1992͒.
19 C.-W. Hsu and C. Y. Ng, J. Chem. Phys. 101, 5596 ͑1994͒.
20 B. Ruscic and J. Berkowitz, J. Chem. Phys. 98, 2568 ͑1993͒.
21 S. Nourbakhsh, C.-L. Liao, and C. Y. Ng, J. Chem. Phys. 92, 6587 ͑1990͒.
22 H. Q. Zhao, Y.-S. Cheung, C.-L. Liao, and C. Y. Ng, J. Chem. Phys. 104,
130 ͑1996͒.
44 W. J. Borer, W. D. Weringa, and W. C. Nieuwpoort, Org. Mass Spectrom.
14, 543 ͑1979͒.
45 B. van der Graaf and F. W. Mc Lafferty, J. Am. Chem. Soc. 99, 6806
͑1977͒.
23 Z.-X. Ma, C.-L. Liao, H.-M. Yin, C. Y. Ng, S.-W. Chiu, N. L. Ma, and
W.-K. Li, Chem. Phys. Lett. 213, 250 ͑1993͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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