Concentration dependence of the spectroscopic and photochemical properties of atomic and molecular oxygen in argon matrices

Article abstract of DOI:10.1016/S1386-1425(00)00366-8

Vacuum ultraviolet (VUV) excitation (200-100 nm) and visible emission (300-650 nm) spectra of O₂ imbedded in Ar matrices at different concentrations are presented. At 0.1 and 0.2% concentrations a linear increase in the intensities of the excitation and emission spectra is observed. At these concentrations, photolysis of O₂ is found to be negligible. At higher concentrations (0.5, 1 and 2%) the normalized intensities of the excitation and emission spectra of O₂ decrease. With increasing concentration of O₂ the permanent photolysis of O₂ increases, which does not correlate, based on the excitation spectra of O in Ar, to the production of isolated O atoms. It has been shown that this anomalous behavior should be due to the formation of van der Waals dimers and oligomers at higher concentrations of O₂ that upon photolysis produce ozone.

Full text of DOI:10.1016/S1386-1425(00)00366-8

Spectrochimica Acta Part A 56 (2000) 2581–2588
www.elsevier.nl/locate/saa
Concentration dependence of the spectroscopic and
photochemical properties of atomic and molecular oxygen in
ꢀ
argon matrices
Murthy S. Gudipati *, Frank Schouren, Martin Kalb, Robert Wagner
Institut f u¨ r Physikalische Chemie, Uni6ersit a¨ t zu K o¨ ln, Luxemburger Strasse 116, D-50939 K o¨ ln, Germany
Received 22 February 2000; accepted 29 March 2000
Abstract
Vacuum ultraviolet (VUV) excitation (200–100 nm) and visible emission (300–650 nm) spectra of O imbedded in
2
Ar matrices at different concentrations are presented. At 0.1 and 0.2% concentrations a linear increase in the
intensities of the excitation and emission spectra is observed. At these concentrations, photolysis of O is found to be
2
negligible. At higher concentrations (0.5, 1 and 2%) the normalized intensities of the excitation and emission spectra
of O2 decrease. With increasing concentration of O2 the permanent photolysis of O2 increases, which does not
correlate, based on the excitation spectra of O in Ar, to the production of isolated O atoms. It has been shown that
this anomalous behavior should be due to the formation of van der Waals dimers and oligomers at higher
concentrations of O that upon photolysis produce ozone. © 2000 Elsevier Science B.V. All rights reserved.
2
Keywords: Vacuum UV spectroscopy; Matrix isolation; Molecular oxygen clusters; Atomic oxygen
1
. Introduction
matrices compared with the gas-phase [1–3]; (b)
nature of the electronic excitation of atomic oxy-
gen in Rg matrices [4,5]; and (c) long-range ther-
mal mobility of O atoms in Rg matrices [6,7]. Yet
another important aspect, that has so far not been
paid attention, is the dependence of the spectro-
The spectroscopic and photochemical proper-
ties of atomic and molecular oxygen in rare gas
Rg) matrices have been of significant interest in
(
the recent past due to several intriguing aspects.
Some of them are, (a) radiative deactivation of
excited O following different channels in the Rg
scopic and photochemical properties of O with
2
2
different dilutions in Rg matrices, that is, the
photo-induced behavior of O2 dimers and
oligomer clusters, whose probability of formation
ꢀ
This article is dedicated to Professor K Verkatesan, De-
partment of Organic Chemistry, Indian Institute of Science,
Bangalore 560 012, India.
in the matrices increases with increasing O con-
2
centration. Associative excitation and emission
between the ground and the first excited electronic
*
Corresponding author. Fax: +49-221-4705144.
E-mail address: murthy.gudipati@uni-koeln.de (M.S.
3
−
1
Gudipati).
states (X ꢀg and a D , respectively) of O dimer
g
2
1
386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00366-8

2
582
M.S. Gudipati et al. / Spectrochimica Acta Part A 56 (2000) 2581–2588
in Rg matrices at higher O concentrations are
ing the emission of O at 448 nm and the emission
2
2
known in the literature [8]. UV-photolysis [9,10]
or electron bombardment [11] of solid O2 is
known to generate ozone. Thus, one would expect
of O at 562 nm, respectively, were recorded on
freshly prepared matrices. Then the emission spec-
tra of O between 300 and 650 nm by exciting O2
2
the same reaction in O dimers and oligomers.
However, to the best of our knowledge, so far
there has been no evidence in the literature, where
at 160 nm were recorded. Thereafter, several exci-
tation and emission spectra were recorded to fol-
2
low the photolysis of O and the production of O
2
direct photolysis of O clusters isolated in Rg
matrices yields ozone . Our results presented in
atoms. The measurements were ended by record-
2
1
ing the former set of excitation spectra of O and
2
this article show that indeed ozone is generated
during the UV/vacuum ultraviolet (VUV) photol-
ysis of O2 in Ar matrices. Hence, the present
studies are of relevance to understand photochem-
ical and spectroscopic behavior of aggregates of
O. Laser photolysis of 1% O in Ar matrices was
2
carried out with
a
Nd-YAG (Continuum,
Surelite-II-10, frequency tripled to 355 nm, 10 Hz,
5 ns pulses with a pulse energy of 150 mJ)
pumped dye laser (Radiant Dyes, Narrow-Scan)
at 219 nm after frequency doubling with a BBO
crystal, with a pulse energy of ꢀ1 mJ. Infrared
spectra were recorded on a Nicolet Impact-410
O in Rg matrices.
2
−
1
FTIR spectrometer with 1 cm
resolution.
2. Experimental
The experiments have been carried out at the
3
. Results
In order to quantitatively compare the spectro-
synchrotron radiation facility BESSY-I in Berlin
using a three meter normal incidence (3m-NIM-1)
and one meter Seya-Namioka (1m-SEYA)
monochromator. Emission spectra were measured
using a 0.5-m monochromator (Acton Research,
SpectraPro-500). All the experimental details are
given in earlier publications [4,12]. Photolysis of
scopic data, the same sequence of measurements
were carried out for approximately the same
amount of irradiation time with all the matrices
containing different O concentrations. Emission
2
3
3 −
spectra due to the A% D “X ꢀ vibronic transi-
O was achieved by measuring several excitation
u
g
2
tions [2] by irradiating at 160 nm into the
spectra between 200 and 100 nm or by measuring
the emission spectra while irradiating the matrices
at 160 or 143 nm. The matrices were deposited on
a LiF window kept at 21 K and the spectra were
recorded at the same temperature. Ar matrices
with 0.1, 0.2, 0.5, 1 and 2% concentrations of O2
were used in the present study. All the other
experimental conditions were kept the same in-
cluding the deposition time. After the deposition,
the matrix window was turned in such a way that
the matrices could directly be irradiated with
monochromatized synchrotron light. Emission
from the matrices was collimated and monochro-
matized after passing through the LiF window. At
the beginning of each set of measurements, excita-
tion spectra between 200 and 100 nm by monitor-
3
−
u
3 −
B ꢀ X ꢀ transition (Schumann–Runge contin-
g
uum) of O are shown in Fig. 1. An additional
2
band at 562 nm, which gets relatively weaker
compared with the other bands upon increasing
O concentration, can also be seen in Fig. 1. This
band is due to the S“ D emission (SD-emission)
of atomic oxygen. Though O in Ar matrices can-
not directly be excited at 160 nm [4], significant
flux of 80 nm photons, that are present due to
higher order diffraction of the grating used to
disperse synchrotron radiation, can directly excite
O atoms in Ar matrices. At higher concentrations
2
1
1
of O , the absorption by O at 80 nm lessens the
probability of direct excitation of O atoms.
The integrated intensities after normalizing with
2
2
the concentration of O of the A%“X emission
2
1
between 350 and 510 nm are also given in paren-
theses in Fig. 1. These values indicate that at 0.1
Bruce Ault [28] notes weak formation of ozone upon 193
nm laser photolysis of simple alkynes and O2 or N O in Ar
2
matrices.
and 0.2% concentrations of O the emission inten-
2

M.S. Gudipati et al. / Spectrochimica Acta Part A 56 (2000) 2581–2588
2583
sity is proportional to the concentration. At 0.5
and 1% concentrations, the observed emission
intensity is 60 and 40%, respectively, of the ex-
pected intensity indicating the involvement of
nonradiative channels, which may be photophysi-
cal or photochemical in nature. At 2% concentra-
tion, the nonradiative channels dominate (81%)
resulting in only 19% of the expected emission.
The nature of the nonradiative channels of deacti-
vation of the excited O will be discussed in the
2
next section.
Excitation spectra recorded by monitoring the
A%(%=0)“X(¦=8) emission band at 448 nm
from freshly prepared matrices (solid lines) and
after photolysis (dotted lines) are shown in Fig. 2.
In agreement with the emission intensities shown
in Fig. 1, we observe the same trend in the
intensities of the excitation spectra before photol-
Fig. 2. Excitation spectra of O2 at different concentrations in
Ar matrices in the Schumann-Runge continuum region. The
3
emission was monitored at 448 nm due to the A% D (%=)0“
u
3
−
g
X ꢀ (¦=8) transition. Solid line spectra were recorded from
freshly deposited matrices and dotted line spectra after photol-
ysis between 200 and 100 nm. Numbers in parentheses are
percentage observed integrated intensities between 185 and 110
nm (expected 100%) after normalizing with O2 concentration.
Permanent photolysis, 0 (0.1% O ); 11% (0.2% O ); 25% (0.5%
2
2
O ); 30% (1% O ); and 53% (2% O ).
2
2
2
ysis. Additionally, permanent photolysis of O2
increases with increasing concentration of O2.
Excitation spectra of atomic oxygen recorded
by monitoring the SD-emission at 562 nm from
Ar matrices containing different initial concentra-
tions of O are shown in Fig. 3. The solid line
2
spectra were recorded before photolysis and the
dotted line spectra after photolysis (after record-
ing the excitation spectra of O shown in Fig. 2).
2
The excitation spectra of O in Ar (and Kr) ma-
trices have been fully assigned by us recently [4].
In Ar matrices, the spectrum contains three re-
gions, the longer wavelength region between 125
3
3 −
Fig. 1. A% D (%=)0“X S (¦) emission spectra of O2 at
u
g
different concentrations in Ar matrices. Excitation wavelength
was 160 nm. Numbers in parentheses are the integrated inten-
sities between 350 and 510 nm relative to the integrated
intensity of 0.1% O2 in Ar after normalizing with O2 concen-
tration.
+
−
and 150 nm due to Ar O ’ArO charge-transfer
3
3
transitions; the 121.7-nm band due to the S’ P
Rydberg transition and between 100 and 110 nm

2
584
M.S. Gudipati et al. / Spectrochimica Acta Part A 56 (2000) 2581–2588
due to Ar excitonic transitions [13]. The excitonic
energy of Ar lattice is trapped efficiently by the
impurity O atoms. Thus, the intensity of the
excitonic bands of Ar contains information on the
local crystallinity of the Ar matrices. More the
crystallinity, more intense is the excitonic band
due to long range exciton migration and eventual
trapping by O.
At the outset, we find increase in the atomic
oxygen species that emits at 562 nm after photoly-
sis of O , except in the case of 2% O in Ar. At the
2
2
same time, we find that the Ar excitonic band also
grows in intensity when 0.1, 0.2 and 0.5% O in
2
Ar matrices were photolyzed indicating that the
local crystallinity is not significantly destroyed.
However, with 1 and 2% O in Ar matrices, the
2
Ar excitonic band decreases in intensity after the
photolysis, resulting in severe destruction of the
Ar crystal lattice. The normalized intensity of the
excitation band of O at 121.7 nm shows similar
trend as that of the A%“X emission bands of O2
with changing O concentration.
2
Fig. 3. Excitation spectra of O atoms in Ar matrices generated
at the beginning (solid line spectra) and at the end (dotted line
4. Discussion
spectra) of photolysis of O at different initial concentrations.
2
4.1. Nature of excitation and emission of O in
Ar matrices
2
The numbers in parentheses are the observed integrated inten-
sities between 130 and 115 nm compared with the integrated
intensity from 0.1% O2 in Ar matrices (100%) after normaliz-
ing with the initial O2 concentration.
Some of the relevant electronic states of O that
2
3
3
3
1
correlate to O( P)+O( P) and O( P)+O( D)
upon dissociation [14] in the gas-phase are shown
3
1
in Fig. 4. When excited below the O( P)+O( D)
dissociation threshold (7.2 eV) and above the
3
3
O( P)+O( P) dissociation threshold (5.1 eV), an
3
3
O( P) atom-pair is generated. This O( P) atom-
1
pair, due to its repulsive interaction with Ar( S)
[5,15], is efficiently trapped in the mother-cage
and recombines to O2 in its excited electronic
states (A%, A, c, b and a, as seen in Fig. 4).
Consequently, A%“X, c“a, b“X and a“X
emission bands ranging between 300 and 1300 nm
are observed routinely by exciting O below 7.2
2
3
1
eV [2,3,8,16]. Above the O( P)+O( D) dissocia-
tion threshold, the excitation is predominantly
Fig. 4. Potential energy curves of some of the relevant valence
into the B state of O . Our experimental observa-
2
electronic states of O in the gas-phase [14]. The Franck–Con-
2
tions presented in Fig. 2 reveal that in spite of the
don region for vertical excitation from the ground-state is
marked with shaded box.
1
nonrepulsive interactions between O( D) and Ar

M.S. Gudipati et al. / Spectrochimica Acta Part A 56 (2000) 2581–2588
2585
Fig. 5. A sketch of probable photochemical and photophysical pathways in O2 clusters in Ar matrices.
1
(
S) [5,15], intracage recombination of the O
increase the probability of formation of aggre-
atom-pair is highly efficient. For this reason, we
gates (dimer and oligomer clusters), the observed
effect can safely be attributed to the photophysi-
cal and photochemical behavior of the aggregates
could measure excitation spectra of O up to 12
2
3
1
eV (100 nm), about 5 eV above the O( P)+O( D)
dissociation threshold. This should be either due
to the efficient deactivation of O( D) to O( P)
of O . If only photophysical nonradiative chan-
2
1
3
nels play a significant role in these aggregates,
then we should expect no further changes in spec-
tral intensities upon irradiation between 200 and
100 nm. The experimental observations clearly
indicate that this is not the case, namely, the
excitation spectra recorded at the end of each set
of measurements (dotted line spectra in Fig. 2)
reveal loss of intensity at higher concentrations.
Thus, permanent photolysis plays a significant
within the mother-cage [17] and/or due to an
3
1
efficient recombination of O( P)+O( D) to O in
2
the B state, which is predissociated by several
3
singlet and triplet repulsive states to O( P)+
3
O( P), as seen in Fig. 4. Thus, excitation of O2
between 200 and 100 nm leads to dissociation of
O followed by the intracage recombination of the
2
geminate O atoms, which in turn leads to the
A%“X emission of O . If the concentration of O
role in the O clusters. In fact, the emission spec-
2
2
2
has no effect on the kinetics of various photo-
physical (radiative and nonradiative deactivation)
and photochemical (permanent separation of O
tra shown in Fig. 1, that were measured after two
consecutive excitation spectra from freshly de-
posited matrices, reveal permanent photolysis of
atoms or reaction of O with O in the aggregates)
O . The normalized observed intensities decrease
2
2
channels, then one would expect a linear depen-
from 70 to 60% for 0.5% O , 70 to 40% for 1% O2
2
dence between the O concentration and excita-
and 25 to 19% for 2% O between the excitation
2
2
tion/emission intensities. However, the excitation
spectra shown in Fig. 2 from freshly deposited
matrices do not follow an expected linear depen-
spectra (Fig. 2) and emission spectra (Fig. 1),
respectively. Additionally, the normalized ob-
served intensities of the excitation spectra (Fig. 2)
from freshly deposited matrices deviate from the
expected linear dependence with concentration,
revealing that the emission quantum yields in
aggregates of O are lesser than in isolated O due
dence with concentration of O at higher concen-
2
trations (0.5, 1 and 2%). These results indicate
that with increasing O concentration, the nonra-
2
diative and photochemical channels gain signifi-
2
2
cance. As the increase in O concentration should
to the nonradiative photophysical processes.
2

2
586
M.S. Gudipati et al. / Spectrochimica Acta Part A 56 (2000) 2581–2588
Thus, both nonradiative relaxation and perma-
ishes (Fig. 3). At the same time, we observe
nent photolysis of O increase with increasing O2
increasing permanent photolysis of O with in-
2
2
concentration in Ar matrices. The photophysical
and photochemical pathways that may be of im-
creasing O concentration. These observations re-
veal two important aspects of photochemistry of
2
portance in O aggregates are sketched in Fig. 5.
O aggregates in Rg matrices. Firstly, at higher
2
2
Due to the fact that we do not observe any
significant changes in the spectral profiles of the
excitation spectra or the line widths and positions
of the A%“X vibronic bands in the emission
concentrations, permanent photolysis of O oc-
curs not only in the defect sites, but also in perfect
sites of the Ar lattice. However, after photolysis
2
of O the crystallinity of the Ar lattice gets de-
2
spectra at different O concentrations, we con-
stroyed (diminution of the Ar exciton bands at
2
clude that the electronic interactions between O2
molecules both in the ground and electronically
excited states in the aggregates are weak, in agree-
ment with the experimental [18–20] and theoreti-
cal [21,22] data on (O ) .
105 nm after photolysis of O in Fig. 3). Secondly,
2
the photolysis of O does not give rise to O atoms
2
or even if generated, these O atoms undergo
nonradiative relaxation upon excitation due to the
presence of O in their vicinity, one of the possi-
2
2
3
bilities being the formation of O [24], as shown in
4
4.2. Excitation spectra of O atoms in Ar matrices
Fig. 5.
If the photolysis of O in the aggregates indeed
2
The excitation spectra of O shown in Fig. 2
produces ozone, then there remains an important
question, why ozone itself is not further pho-
tolyzed to O2 and O, subsequently generating
2
indicate that at lower concentrations (0.1 and
.2%) permanent photolysis of O is negligible.
0
2
Thus, the excitation spectra of O atoms in Ar
shown in Fig. 3 should be due to a minor fraction
back the O aggregates. In the VUV region be-
tween 6 and 9 eV (200–138 nm) the absorption
2
(
ꢀ10%) of photolyzed O at lower concentra-
cross-section of O is much weaker than the ab-
2
3
tions and we observe a linear increase in the
intensity of the excitation spectra indicating con-
centration dependent production of atomic oxy-
sorption cross-section of O between 180 and 138
2
nm (Schumann–Runge continuum) [25,26]. Thus,
when irradiated at longer wavelengths than 138
nm, though both O and O will be photolyzed,
gen at 0.1 and 0.2% initial concentrations of O in
2
3
2
Ar. In the literature, it is reported that the perma-
the equilibrium of photolysis lies more towards
the formation of ozone (Fig. 5). The spectral
region between 9 and 12 eV (138–100 nm) is
dominated by Rydberg transitions with significant
absorption cross-sections in both O and O in the
gas-phase [25]. However, in Rg matrices, it is well
known that the Rydberg transitions are energeti-
cally blue shifted compared with the gas-phase
[27] and sometimes even strongly suppressed. We
could not detect so far the Rydberg transitions in
nent photolysis of O occurs at defect sites of Rg
2
lattice and intracage recombination of geminate O
atom-pair occurs in perfect sites [23]. At 0.5% O2
concentration, however, we observe that only 47%
of the expected isolated O atoms are generated. It
should be kept in mind that in order to spectro-
scopically detect O atoms, photo-dissociation of
3
2
O must result in permanent cage exit of at least
2
one O atom (otherwise recombination of geminate
O atom-pair takes place). Accordingly, permanent
the excitation spectra of O by monitoring the
2
photo-dissociation of O followed by cage exit of
A%“X or a“X emission of O in Rg matrices,
2
2
at least one of the O atoms, results in the loss of
every second O atom generated in matrices con-
probably either due to the suppression of the
Rydberg transitions or due to other relaxation
taining 0.5% O initial concentration. This is only
pathways than dissociation of O after excitation
2
2
possible, if there exists another O molecule in the
into the Rydberg transitions. Hence, we assume
2
same Ar cage or in the neighboring unit cells of
the Ar lattice, which reacts with O generating O3.
that in O too, the Rydberg transitions do not
play significant role in the photodissociation of O3
in the Rg matrices. Further VUV spectroscopic
3
At higher O concentrations (1 and 2%) genera-
2
tion of the isolated atomic oxygen further dimin-
studies on O in Rg matrices are necessary to get
3

M.S. Gudipati et al. / Spectrochimica Acta Part A 56 (2000) 2581–2588
2587
a detailed information on the photochemical reac-
tions of O3 from higher excited states in con-
densed phase.
lower energies with increasing concentration of O2
when O and O are co-deposited with Ar. Thus,
presence of O is known to effect the IR frequen-
cies of O in Rg matrices. Unlike in the above
mentioned experiments, in the present work O is
generated exclusively from aggregates of O iso-
lated in Ar. Hence, we believe that presence of at
least one O or O in the same Ar cage as O may
3
2
2
In order to find out whether O3 is indeed
formed in Ar matrices during the photolysis of
O , we recorded infrared spectra of 1% O in Ar
3
3
2
2
2
before and after laser photolysis at 219 nm. We
have chosen this wavelength because of two rea-
2
3
already be sufficient to shift the IR frequencies
observed from O isolated in Ar matrices to O in
sons. Firstly, excitation of O at this wavelength
2
into the Franck-Condon region of the Herzberg
3
3
3
3
solid O . The w band is very broad with several
continuum above the O( P)+O( P) dissociation
limit of 242 nm should generate ozone in the O2
aggregates. Secondly, due to less than 10% ab-
sorption cross-section of ozone at this wavelength
compared with the maximum at 253 nm of the
Hartley band [25], further photolysis of ozone can
be minimized. The w and w vibrational bands of
2 3
shoulders on both sides of the maximum indicat-
ing different environments around O . Unfortu-
3
nately, the resolution of our infrared spectra is
not sufficient to resolve these shoulders, that may
be due to different O /O or O/O complexes in
2
3
3
Ar. Further studies are necessary to understand
the nature of these complexes generated through
2
3
O could clearly be identified in the IR spectra
3
UV photolysis of O clusters in Rg matrices.
2
after photolysis, which were not present before
photolysis (Fig. 6). The observed energies 702.5
−
1
−1
cm
(w ) and 1037.5 cm
2
(w ) of O3 are in
3
5
. Conclusions
excellent agreement with the literature values for
O in solid oxygen matrices at 702.7 and 1037.6
3
UV and VUV irradiation of O in Ar matrices
−
1
2
cm , but not with the values for isolated O in
3
at different O concentrations reveal that the pho-
2
Ar matrices at 704.2, 703.6 and 1041.2, 1039.7
tophysical and photochemical behavior of O2
−
1
cm , the doublets being due to two different
sites [9,10]. However, Schriver-Mazzuoli et al. [10]
dimers and oligomer aggregates clearly deviate
from those of the isolated O molecules. In iso-
2
have noted that the IR frequencies of O shift to
3
lated molecules, photodissociation leads to in-
tracage recombination as the major channel and
permanent separation of O atoms as the minor
channel. In the aggregates, the major channel is
the production of O upon photolysis of O . The
3
2
minor channels being intracage recombination of
O atoms to O and cage exit of O atoms. Investi-
2
gations on mass selected clusters of O like (O )
2
2 2
and (O ) etc. in Rg matrices will be of significant
2
3
importance to understand the photophysical and
photochemical properties of the molecular oxygen
clusters.
Acknowledgements
Fig. 6. Infrared spectra of 1% O in Ar matrices before (dotted
2
lines) and after (solid lines) laser photolysis at 219 nm. The n2
region of O3 is shown as insert. Energies of both n2 and n3
normal modes of O3 are in excellent agreement with those of
O in solid O , but not with those of O isolated in Ar. Refer
This work is partly supported by the Deutsche
Forschungsgemeinschaft (Project GU 413/3).
Travel funds from DESY through BESSY and
continuing cooperation of Dr W. Braun and Dr
3
2
3
to text for further details.

2
588
M.S. Gudipati et al. / Spectrochimica Acta Part A 56 (2000) 2581–2588
[
13] N. Schwentner, E.-E. Koch, J. Jortner, Electronic Excita-
G. Reichardt of BESSY are acknowledged grate-
fully. We thank Metzner and his coworkers at our
workshop for excellent maintenance of the ap-
paratus. MSG wishes to thank his wife Dr Karin
Hassenr u¨ ck for financial assistance.
tions in Condensed Rare Gases, Springer, Berlin, 1985.
14] R.P. Saxon, B. Liu, J. Chem. Phys. 67 (1977) 5432.
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[
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[
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