Photochemistry and dynamics of C6H6 –O2 clusters at 226 nm
Gary DeBoer and Mark A. Young
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
~Received 20 September 1996; accepted 24 December 1996!
The photochemistry and dynamics of small C6H6 –O2 clusters were studied in a supersonic
expansion using 226 nm laser excitation and multiphoton ionization probes. We were able to detect
a strong signal due to O( 3 P 2 ) when mixed clusters were present in the expansion but no O atom
fragments could be observed in the absence of benzene in the expansion mixture.
Photofragmentation of O2 in the unique environment of the cluster is enhanced by at least three
orders of magnitude compared to the isolated oxygen molecule. The kinetic energy release of the
O( 3 P 2 ) was determined with a time-of-flight method and found to be relatively small and
characterized by a completely isotropic spatial distribution. The fine structure population of the
O( 3 P j ) was also examined and the resultant branching fractions, P 2,1,0 5 0.68 6 0.03, 0.2660.06,
0.0660.01, are similar to those obtained for photodissociation of isolated O2 by other workers. We
also find that photochemical production of oxygen containing products, such as C6H6O, becomes
feasible in larger cluster species due to solvent cage effects which trap the recoiling O atom
fragments. The observed dynamics can be attributed to either excitation of the supramolecular
C6H6 –O2 charge-transfer state, or localized excitation of a perturbed transition in O2 . The net effect
of cluster absorption is to greatly enhance a chemical pathway that is only weakly observed in the
separated molecules, similar to the behavior that has recently been described for the C6H6 –I2
complex. © 1997 American Institute of Physics. @S0021-9606~97!01113-6#
I. INTRODUCTION
Recently, a great deal of attention has been devoted to
the study of unique photochemical reaction processes that
become active in the environment of a weakly bound molecular complex. A number of extremely intriguing chemical
pathways that require intimate contact between precursor
species may be observed. Of particular interest is cluster
chemistry that leads to the formation of reaction products at
wavelengths or energies that are otherwise ineffective in initiating reaction in the isolated cluster constituents. Amongst
a number of examples are studies of intermolecular chargetransfer ~CT! phenomena in C6H6 –I2 complexes,1–5 and the
production of I2 in ~CH3I!n and ~HI!n cluster species via a
concerted, single photon mechanism.6–8 Absorption into the
CT band of the C6H6 –I2 complex results in rapid cleavage of
the molecular iodine bond through two distinct pathways involving donor-acceptor electron transfer. In contrast, excitation of bare I2 at these wavelengths yields only relatively few
I atom products due to the very small molecular extinction
coefficient. Other interesting examples of cluster chemistry
involve systems that have significant relevance to a variety
of atmospheric processes.9–16 We are currently investigating
the role of cluster-induced photochemical pathways, such as
CT reactions, in oxygen and ozone based complexes, especially those that may be of importance to atmospheric chemistry. In the current article, we discuss some of our initial
work with the C6H6 –O2 complex using multiphoton ionization, coupled with mass-specific detection, to monitor photoproduct formation and to quantify the fine structure branching and kinetic energy release ~KER! of the O~3P j ! fragment.
Weakly bound species involving O2 or O3 complexed
with other molecular species, such as those that exist in sig5468
J. Chem. Phys. 106 (13), 1 April 1997
nificant abundance in the atmosphere, are likely to constitute
some of the most interesting and relevant types of cluster
systems. The ~O2!2 dimer complex has been observed to
yield O atoms upon excitation at energies well below the
threshold for dissociation in the bare oxygen molecule due to
the concerted formation of O3 .9 The chromophore is the
transition ~the
nominally forbidden A 8 3 D u ←X 3 S 2
g
Herzberg III band! in O2 which gains intensity through perturbations in the complex environment. The OH radical has
been detected as a photoproduct in the UV excitation of
O3 –H2O complexes.12,17,18 The absorption of O3 in the complex at 355 nm increases by approximately a factor of 100
relative to the bare molecule due to changes in the potential
energy surfaces caused by the solvent water molecule.18 The
concentration of the O3 –H2O complex in the atmosphere was
evaluated on the basis of a calculated equilibrium constant
and the ramification for tropospheric processes was found to
be relatively significant.10,11 Clusters of NO were efficiently
converted to cluster ions of the form NO1~N2O3!n and
15
NO1
2 ~N2O3!n by multiphoton excitation at 266 nm. Surprisingly, trace amounts of CH4 and H2O were necessary to facilitate the conversion even though they were not chemically
incorporated into the product. These results indicate that
more subtle effects such as cluster structure and temperature
may be critical elements in the dynamics of such processes.
Another interesting class of cluster species are those
composed of olefinic or aromatic partners which can form
relatively strongly bound complexes with O2 . For instance,
the dissociation energy of the C6H6 –O2 complex is D 0'560
cm21 16 as compared to an estimated binding energy of 180
cm21 for the ~O2!2 dimer.19 Perhaps of greater significance,
these complexes will, in general, exhibit a CT absorption
band at wavelengths dependent on the IP of the hydrocarbon
0021-9606/97/106(13)/5468/10/$10.00
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G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
and the structure of the complex. Due to the separation of
charge induced by excitation, the CT band can manifest a
large oscillator strength compared to the individual chromophores. The CT mechanism is truly a feature characteristic of the supramolecular complex and may be responsible
for new chemistry with no analogy in the photochemistry of
the isolated species. The C6H6 –O2 complex is known to exhibit a very broad CT absorption band20 with a maximum at
approximately 219 nm in the gas phase.21–23 The CT state
has been invoked to explain the efficient quenching of electronically excited arenes by O224,25 and condensed phase UV
photooxidation reactions.26–28 Long wavelength irradiation
into the tail of the CT band results in the production of
O2(a 1 D g ) in a variety of oxygenated solvent systems.29–31
Knee et al. investigated the photochemistry of benzene–
oxygen clusters formed in a supersonic expansion using nonresonant multiphoton ionization and time-of-flight mass
detection.13 The expansion conditions utilized resulted in the
formation of very large cluster species. Excitation at 248 nm
was found to be ineffective in inducing chemistry but 193
nm radiation led to the detection of a number of oxygencontaining mass fragments. Pump–probe experiments indicated that the chemistry was occurring in the neutral cluster
rather than resulting from ion–molecule processes. No attempt was made to determine the excess energy disposal into
the product photofragments. Information regarding the internal and translational energy distribution of product species
can be critical in evaluating the importance of subsequent
chemical reactions. A vivid example is the observation of
highly vibrationally excited O2 from 226 nm photodissociation of O3 , which can then react further with cold oxygen to
reform ozone.32,33 This reaction sequence is thought to make
a substantial contribution to the ozone balance in the atmosphere.
The photochemistry of C6H6 –O2 clusters formed in a
supersonic expansion was investigated via excitation and
multiphoton ionization in the vicinity of 226 nm. The presence of clusters in the expansion results in the photochemical
production of O~3P j ! fragments. In addition, photolysis of
larger cluster species of the form, ~C6H6!n –O2 , yields a number of oxygen-containing mass fragments, in accordance
with previous work. The KER of the O atom photofragment
was measured using a time-of-flight analysis and found to be
relatively small with an isotropic spatial distribution. The
final branching into the O~3P j ! spin–orbit states was also
determined. These cluster-based results can be compared to
direct excitation of the O2 molecule under isolated conditions.
II. EXPERIMENT
We have previously described the experimental methods
employed to obtain the KER data and the mass and wavelength resolved spectra.4,34,35 A pulsed, supersonic expansion
source is coupled to a time-of-flight mass spectrometer
~TOFMS! which can be operated in two modes; as a reflectron TOFMS to collect mass and wavelength resolved spectra or as a linear TOFMS using pulsed extraction of the pho-
5469
toions to measure translational spectra. The expansion was
obtained by passing a 2% O2/He mixture through a temperature controlled reservoir containing benzene. Typically, the
temperature was chosen to yield a benzene vapor pressure of
<0.8 Torr. The high purity He gas was further cleansed
through the use of a zeolite trap. For most of the results
described, a total stagnation pressure of '2300 Torr was
utilized.
An excimer-pumped dye laser, in conjunction with second harmonic generation, was used to generate the tunable
UV radiation. The UV light, with a bandwidth of approximately 0.4 cm21, was separated from the fundamental by a
Pellin–Brocca prism arrangement. The single probe laser,
tuned to approximately 226 nm, both excited the parent
C6H6 –O2 complex and detected ground state O~2p 3 P j 9!
atom products by 211 resonance enhanced multiphoton ionization ~REMPI! through the O~3 p 3 P j 8! state. Other product fragments were monitored by nonresonant MPI at these
wavelengths. Rotation of the linearly polarized laser light
with respect to the collection axis of the TOFMS was accomplished with a double Fresnel–Rhomb.
In the linear mode, any photoions produced by the probe
laser are allowed to drift under field-free conditions prior to
acceleration by a high-voltage pulse applied to the electrodes
of the spectrometer. The measured flight-times are then related to the nascent translational energy release by the position of the ions before they are extracted. The flight-time
spectra, f a~TOF!, are transformed to a laboratory frame velocity distribution, I a ( v z ), through
I a ~ v z ! 5 f a ~ TOF!
d ~ TOF!
,
d~ vz!
~1!
where v z is the projection of the fragment velocity onto the
collection axis of our instrument and a is the angle between
the polarization vector of the probe laser and the collection
axis of the TOFMS. Typically, the measured I a ( v z ) are fitted
by assuming an analytical form for the fragment recoil speed
distribution, g( v ), and integrating over the mass, velocity,
and angle dependent detection sensitivity of our instrument.
We have represented g( v ) either as a Gaussian function,
described by a peak velocity and a full width at half maximum ~FWHM!, or as a Maxwell–Boltzmann function, characterized by a rms velocity.
III. RESULTS
The mass resolved spectrum of a 2% O2/He expansion
was recorded with the probe laser tuned to 225.59 nm, corresponding to resonant excitation of O~3P 2!. The results are
displayed as the lower spectrum of Fig. 1. In the mass region
shown, no signals above the normal background level could
be observed and, specifically, no O atom fragments were
detected. The use of a richer oxygen mixture, approximately
10% O2/He, at a larger stagnation pressure of '5900 Torr
did elicit a strong O1 signal ~m/e516!. The likely source of
these O atom fragments is excitation of ~O2!n cluster species
produced in the richer expansion, in accordance with the
previous observations of Brown and Vaida using neat O2
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G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
FIG. 1. Mass spectra of a 2% O2/He mixture with ~top spectrum! and without ~bottom spectrum! C6H6 present in the expansion obtained by tuning the
probe laser to resonantly excite O~3P 2! fragments. A benzene vapor pressure
of 0.8 Torr ~0.03%! was used to obtain the top spectrum.
expansions.9 However, upon addition of modest amounts of
benzene vapor ~0.8 Torr, or '0.03%! to the 2% O2/He expansion an ion signal at m/e516 was readily discerned, as
shown in the upper spectrum of Fig. 1, and which we have
assigned to O1. Since a clear signal due to C1 and a weaker
CH1
n series was also observed, the identity of the m/e516
peak was confirmed by varying the probe laser wavelength.
Tuning the laser slightly away from the O~3P 2! resonance
caused a decrease in the m/e516 signal to background levels. The ratio of O1 to C1 was sensitive to the laser intensity
and focusing conditions, which is expected since the carbonbased fragments result from multiphoton dissociation of the
benzene parent.
Expansions produced by flowing pure He gas through
the benzene reservoir yielded a series of ~C6H6!1
n cluster
species along with characteristic benzene fragment ions. A
number of new mass peaks indicating incorporation of oxygen atoms were observed when the 2% O2/He mixture was
used as the carrier gas. These signals were more obvious
when higher benzene vapor pressures were utilized. In Fig. 2,
a portion of the mass spectrum obtained with 24 Torr of
benzene ~'1%! added to the expansion is shown, highlighting these new features. The largest peak at m/e594 ~C6H6O!
could be detected at benzene concentrations as low as approximately 0.1%. The richer benzene expansions reveal a
series of smaller peaks at m/e595 ~C6H7O!, 109 ~C6H5O2!,
110 ~C6H6O2!, 111 ~C6H7O2!, and 112 ~C6H8O2!. A similar
collection of features which include one more benzene fragment can also be seen in Fig. 2 at m/e5172, 173, 187, 188,
189, and 190. The remaining mass peaks recorded in the
spectrum are due to fragmentation of parent benzene clusters
and are observed even in the absence of molecular oxygen in
the expansion. In addition, we also observed most of the
smaller Cx Hy and Cx Hy O, x,y<6, fragments that were seen
to appear by Knee et al. upon 193 nm photolysis of mixed
oxygen/benzene supersonic expansions.13
The O1 signal could be easily detected even in the most
FIG. 2. A portion of the mass spectrum obtained with 24 Torr of benzene
~'1%! added to the 2% O2/He expansion showing new mass features that
include oxygen atoms.
dilute expansions studied, indicating that it originates from
very small cluster species consisting of only a few benzene
molecules. We could not accurately assess the dependence of
the heavier, oxygen containing mass signals on benzene vapor pressure but it appears that they most likely originate
from larger clusters of the form, ~C6H6!n –O2 , n>2. To provide further experimental verification, the dependence of the
O1 and C6H6O1 signals on stagnation pressure was determined and the results are shown in Fig. 3 as a log–log plot.
The C6H6O1 fragment exhibits a steeper pressure dependence than the O1 species, as illustrated by the relative
slopes in Fig. 3, consistent with a parentage in larger clusters. The mass spectra described above were duplicated with
expansion mixtures obtained by passing the oxygen gas
through a cryogenic trap in order to rule out interference
from possible contaminants.
The probe laser intensity dependence of the O1 signal
was determined for a dilute, 0.03% benzene, expansion. The
FIG. 3. A log–log plot of the stagnation pressure dependence of the O1 and
C6H6O1 signals. The straight lines represent least-squares fit to the data,
yielding slopes of 1.460.1 and 2.360.2 for the O1 and C6H6O1 signals,
respectively.
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G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
5471
FIG. 4. The intensity dependence of the O1 signal measured with 0.03%
benzene vapor present in the 2% O2/He expansion. The solid line shows the
fit to all of the data points and has a slope of 3.360.1. Also shown is the
intensity dependence of the C6H6O1 ~m/e594! signal in a 1% benzene
expansion. The solid line indicates the fit to the data and has a slope of
2.760.1.
data, plotted in log–log form in Fig. 4, can be fit to a straight
line with a slope of 3.360.1. The formation of product O
atoms by single photon chemistry in the cluster and subsequent detection by 211 REMPI would be expected to result
in an observed four-photon dependence for the O1 signal.
The data of Fig. 4 may be indicative of partial saturation in
the multiphoton absorption process. A fit to the five lowest
energy data points yields a steeper slope of approximately
3.8, closer to the expected value. The laser intensity dependence of the C6H6O1 signal was similarly measured with a
1% benzene expansion. The results, also shown in Fig. 4, fit
to a line with a slope of 2.760.1, a noticeably reduced intensity dependence from that of the O1 signal. A partial fit to
the lowest energy points still results in a slope of less than
3.0.
An abbreviated search was conducted for common resonant features in the wavelength resolved MPI spectra of the
1
C6H1
6 ion and the C6H6O fragment. Spectra were collected
over the range 222–230 nm employing a variety of probe
laser energies and focusing conditions. All that could be observed in either mass channel was a nonresonant signal that
closely followed variations in the laser pulse energy. A corresponding wavelength scan of the O1 signal was not feasible due to the necessity for resonant excitation of O~3P j ! in
our one-color experimental arrangement.
Photofragment velocity spectra of the O~3P 2! species
were obtained with the laser oriented either parallel, a500,
or perpendicular, a5900, to the collection axis of the instrument. Representative results with a benzene concentration of
0.03% are contained in Fig. 5. At either polarization a broad
distribution peaked at zero-velocity in the laboratory frame
was observed, reflective of modest KER in these fragments.
Both distributions were fit to a single Maxwell–Boltzmann
function, g( v ), with a rms velocity of 997640 m/s, corresponding to a translational temperature of 651 K. Qualitatively similar results were obtained from expansions contain-
FIG. 5. The O~3P 2! photofragment velocity spectra with the laser oriented
parallel, a500 ~A!, and perpendicular, a5900 ~B!, to the TOFMS collection
axis. The benzene vapor concentration was 0.03%. The points are the experimental data and the solid line is a calculated fit to the distribution.
ing larger cluster species, produced using higher benzene
vapor pressures.
The angular distribution of product fragments due to dissociation induced by a single-photon electric dipole transition can be described by
I~ u !5
1
@ 11 b P 2 ~ cos u !# ,
4p
~2!
where b is the anisotropy parameter, u is the angle between
the electric polarization vector of the laser and the recoil
velocity vector, and P 2~cos u! is the second order Legendre
polynomial. In general, the angular and speed parts of the
photofragment distribution may not be exactly separable and
b will be a function of the fragment velocity, v z in the laboratory frame. The fact that a single Maxwell–Boltzmann
function can fit the data for both polarizations suggests that
the fragmentation process is largely isotropic and that the
anisotropy parameter is velocity independent, b( v z )50.
The final state branching into the different O atom spin–
orbit levels was determined from wavelength resolved scans
of the 3p 3 P j 8 ←2 p 3 P 2,1,0 transitions. Typical results are
depicted in Fig. 6. The limited resolution of the dye laser
permitted only partial resolution of the upper state fine structure. The integrated area of each measured peak was normalized by dividing by the cube of the applied probe laser intensity, consistent with the approximate intensity dependence
previously determined for the O1 signal ~vide supra!. The
average of several such spectra collected for each transition
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5472
G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
TABLE I. Fine structure branching ratios for O~3P i ! fragments of the
C6H6-O2 complex and isolated O2.
Isolatedb
a
Calculated
j9
Complexa
experimental
Experimental
Adiabatic
Statistical
2
1
0
.686.03
.266.06
.066.01
.66
.27
.07
1.0
0
0
.56
.33
.11
This work
Reference 38.
b
FIG. 6. Wavelength resolved REMPI spectra showing the relative intensity
of the 3p 3 P j 8 ←2 p 3 P j 9 transition for each value of j 9, as labeled in the
figure. The individual spectra are plotted in terms of relative frequency
~cm21! so that they may be viewed together on an expanded scale.
yielded a relative branching of P j 9 50.6860.03, 0.2660.06,
0.0660.01, for the j 952,1,0 states, respectively. The branching fractions are collected in Table I. These results may be
biased by alignment effects in the m j 9 distribution due to the
use of a single polarized laser for both the photodissociation
and probe steps. We did not attempt to obtain photofragment
velocity spectra for the other spin–orbit states in these experiments.
IV. DISCUSSION
Excitation of isolated O2 at wavelengths near 226 nm,
above the energetic threshold for oxygen dissociation, is into
the weak Herzberg continuum which is characterized by a
very small absorption cross section, s<10223 cm2.36,37
While photodissociation of O2 at these wavelengths is
possible,38 we observe no evidence for the production of
O~3P j ! when a 2% O2/He expansion is probed under our
experimental conditions ~Fig. 1!. However, when trace
amounts of benzene are added to the expansion mixture, a
strong signal due to O1 ~m/e516, Fig. 1! is easily detected.
Photoprocesses in benzene–oxygen cluster species formed in
the expansion must be responsible for the greatly enhanced
yield of O atom fragments relative to excitation of bare O2 .
We can formulate a lower limit for the enhancement factor in
our experiment based on signal-to-noise considerations. Assuming that the quantum yield for oxygen photodissociation
is unity for 226 nm absorption in both the cluster and the O2
monomer, and estimating that roughly 1% of the O2 is bound
in benzene complexes, we calculate that the cluster absorption is enhanced by at least a factor of 103 relative to isolated
O2 , corresponding to a cross section of greater than 10220
cm2.
The O1 signal is observed in highly rarefied sample mixtures with benzene concentrations of '0.03%, and there is
no evidence for ~O2!n formation. Under comparable expansion conditions, the more strongly bound benzene–iodine
system is thought to primarily form the C6H6 –I2 dimer
species.1,4 We did not perform detailed concentration studies
to ascertain with certainty whether the C6H6 –O2 complex is
the dominant species in the dilute expansions used in the
current investigation. However, the experimental photofragment spectra are found to be relatively insensitive to the
concentration of benzene vapor pressure in the expansion ~up
to concentrations of a few percent!, indicating that the
mechanism for O atom production is probably identical in
small species, such as the complex, and in larger ~C6H6!n –O2
clusters.
The laser intensity dependence data for O1, shown in the
log–log plot of Fig. 4, has a slope significantly greater than 3
and approaches a limiting value of 4 at low intensities. These
observations are consistent with single photon initiation of
photochemistry in the neutral cluster, followed by 211
REMPI detection of the O atom fragment. Knee et al. also
determined that single photon absorption of 193 nm radiation
by neutral benzene–oxygen clusters led to similar
photochemistry.13 The small KER we measure for O~3P j !
appears incompatible with multiphoton excitation and fragmentation given the much larger excess energy that would be
available from such processes. A diagram depicting the relative energies of a number of different product channels for
the C6H6 –O2 system is shown in Fig. 7. Reactions in ionized
clusters are probably not active in the formation of the O
atom product due to fast chemistry occurring at the single
photon level and the relatively low intensity nanosecond laser probe used, similar to the results found for other reactive
cluster systems.4,6,34,39 For instance, two photon ionization of
complexed benzene followed by reaction with oxygen
1
3
C6H1
6 1O2→C6H6OH 1O~ P j ! ,
~3!
is energetically feasible ~Fig. 7! but we detect no m/e594
~corresponding to the C6H5OH1 product! under lean expansion conditions which yield a strong O atom signal. Also, an
O1 signal from reaction ~3! would require 5 photons rather
than the 4 photon dependence actually observed. The stagnation pressure and probe laser intensity dependence of the
larger oxygen-containing mass fragments, specifically the
m/e594 ~C6H6O! signal, are clearly different and are further
discussed below.
Photodissociation of the C6H6 –O2 complex can lead to
three-body fragmentation
hn
C6H6 –O2→ C6H61O~ 3 P 2 ! 1O~ 3 P 2 ! .
~4!
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G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
5473
FIG. 8. Fragment translational energy distribution, P(E t ), for the O~3P 2!
atom determined from the data of Fig. 5. The dotted line shows the distribution obtained by assuming that dissociation takes place in the O2 c.m.
frame, with no recoil imparted to the benzene fragment. Both distributions
are normalized to unity area. The arrow represents the excess energy available at the laser probe wavelength less the binding energy of the complex.
FIG. 7. An energy level diagram for the C6H6 –O2 system showing different
available product channels. The O~3P 2! probe excitation energy used in
these experiments is indicated by the arrow. The binding energy of the
ground state C6H6 –O2 complex is taken to be '560 cm21.
Independent measurement of an individual fragment speed
distribution cannot, then, uniquely determine the center of
mass ~c.m.! translational energy. We have transformed the
fitted speed distribution, g( v ), for the O~3P 2! data of Fig. 5
to a fragment translational energy distribution, P(E t ), according to
P~ Et!5
g~ v !
,
mv
~5!
where m, in this case, is the O atom mass. The result, contained in Fig. 8, yields a mean translational energy for the O
atom fragment of, E t 5665653 cm21. Assuming that the O
atom recoils against a C6H6 –O fragment gives a mean translational energy release of 778 cm21, still quite small. The
excess energy available for reaction ~4! at the probe laser
wavelength is about 2500 cm21, corrected for the binding
energy of the ground-state complex, and is indicated by the
arrow in Fig. 8. An upper bound to the c.m. translational
energy can be obtained by supposing that, in the limit of a
very weakly interacting complex, dissociation of the molecular oxygen imparts no recoil to the benzene and all of the
translational energy is carried away by the O atom fragments. The resulting distribution, shown as the dotted line
plot in Fig. 8, extends slightly beyond the available energy
and has E t 51330 cm21. The O atom spin–orbit energy, determined from the measured fine structure branching, is
about 55 cm21. The effect of photodissociation in the
C6H6 –O2 complex, then, is to partition approximately 1100–
1800 cm21 of excess energy into internal excitation of the
aromatic fragment.
The C6H6 –O2 complex is a weakly bound species with a
binding energy of about 560 cm21.16 While weakly interacting in the ground state, the complex exhibits a relatively
intense absorption to a CT state with much stronger, ionictype intermolecular interactions.20,40 The gas phase C6H6 –O2
CT band peaks at approximately 219 nm and appears to be
much stronger than the unperturbed benzene chromophore at
these wavelengths. Goodling et al. relied on concentration
studies to determine a CT absorption cross section of
~2.360.7!310220 cm2 at a wavelength of 218 nm.23 The excitation wavelength we have employed is near the peak of
the measured CT absorption band, which is quite broad. Our
estimate for the cluster absorption cross section is of the
same order of magnitude as the experimental value of Goodling et al. It is possible, then, that excitation of the
C6H6 –O2 CT state is responsible for the production of O
atom fragments in our experiment.
The analogous C6H6 –I2 CT complex, isolated in a supersonic expansion, was earlier examined in our laboratory4,5
and by Zewail and co-workers.1–3 Direct excitation of the
well-known C6H6 –I2 CT band results in either facile dissociation of the iodine bond to form I atom fragments or dissociation of the complex to yield molecular products.5 The
absorption cross section for isolated I2 at the same wavelengths is small and only about 10% of the measured I atom
signal comes from bare molecule fragmentation even though
the fraction of complexed I2 in the expansion is small.4 The
enhancement of the O atom signal from excitation of
C6H6 –O2 complexes is even larger, by a factor of '4, than
the corresponding increase in the benzene–iodine system. A
comparable CT excitation mechanism, and similar reaction
dynamics, could be active in the C6H6 –O2 complex resulting
in molecular oxygen dissociation.
The individual components of the C6H6 –O2 complex
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5474
G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
also constitute potential chromophores. The à 1 B 2u 2X̃ 1 A 1g
transition in C6H6 is accessible at the probe wavelengths
used to excite the cluster species. The excited state potentials
of benzene are not expected to be greatly perturbed in the
weakly bound oxygen complex, in analogy with aromaticrare gas van der Waals complexes.41 For instance, the
à 1 B 2u state band origin is found at the same wavelength in
argon or oxygen matrices.42 Excitation at 226 nm would be
to the high energy tail of the à 1 B 2u 2X̃ 1 A 1g band, approximately 6200 cm21 above the à 1 B 2u state origin, where only
weak features have been observed in absorption.43 The
C6H6 –O2 CT absorption cross section is much larger than
that of the benzene monomer at these wavelengths.21–23,42
Molecular oxygen is known to efficiently quench electronically excited benzene in both the gas and condensed
phase.24,25,44 To precipitate oxygen dissociation in the cluster, the quenching process would have to yield O2 in a high
lying electronic state and ground state benzene. In general,
however, quenching is found to produce benzene in the triplet excited state, which would leave insufficient energy to
break the O2 bond at 226 nm excitation. Absorption into the
CT band of arene–O2 complexes in nonpolar solvents has
been found to generate the triplet hydrocarbon with unit
quantum yield.31 A quenching mechanism would also have
to compete with internal conversion in benzene which becomes very rapid for excitation above 3000 cm21 in the
à 1 B 2u state.45,46 We note that no resonant features could be
detected in the MPI wavelength spectrum of C6H6 in the
region of 226 nm ~vide supra!. It appears highly unlikely that
localized excitation of the benzene chromophore in the
C6H6 –O2 complex is the source of the photochemistry we
have described.
The O2 molecule also absorbs weakly throughout the UV
via the Herzberg continuum which is comprised of three
3
3 1
states; c 1 S 2
u , A 8 D u , and A S u . Transitions from the
ground state to the first state are forbidden on the basis of
spin selection rules while the latter two states are orbitally
forbidden. The spin selection rule is the most rigorous but
intermolecular interactions can lead to sufficient relaxation
of symmetry to allow the 3Du and 3S1
u states to gain inten3 2
sity. The A 3 S 1
u ←X S g transition dominates absorption in
isolated O2 at the wavelengths utilized in the current study.
However, the cross section for the A 8 3 D u ←X 3 S 1
g transition in gas phase ~O2!2 dimers is thought to increase by 1–3
orders of magnitude due to perturbations in the complex
environment.9,47 Goodman and Brus have reported fluorescence excitation spectra for O2 isolated in Ar and N2 matrices that could be assigned to absorption by the A 8 3 D u
state.48 Matrix effects can apparently relax the DL50,61
selection rule while, in contrast, direct absorption to the
1
2
A 3S 1
u state ~forbidden by S }S ! was barely observable.
On the basis of perturbations in the spectra, Goodman and
Brus estimated that the A 3 S 1
u potential was shifted from the
gas phase by a small amount, 40 cm21, in a N2 matrix. A
correspondingly small shift was deduced for the A 8 3 D u
state, as well. In studies of CT phenomena in arenes isolated
in oxygen matrices, absorption due to the nominally forbid-
den O2 transitions is still observed to be much weaker than
the CT band.42
Besides relaxation of selection rules, solvent induced
shifts in electronic states may also influence cluster
absorption.49 Stabilization of the 1B 2 state in O3 , which has
ionic character, due to complexation with a polar H2O molecule was thought to be responsible for the 100-fold increase
in absorption strength observed in photochemical studies of
O3 –H2O species.18 A similar mechanism in the C6H6 –O2
complex is not likely to be operative since the Herzberg continuum arises from valence states and the benzene complex
partner is nonpolar. The small electronic shifts detected by
Goodman and Brus for O2 in weakly interacting cryogenic
matrices are also consistent with such an interpretation.
Population of the CT state by the probe laser could initiate O2 bond dissociation through processes similar to those
elucidated for the C6H6 –I2 system. In the harpoon mechanism, fragmentation in the CT state proceeds along an ionic
2
potential surface to yield an I atom and the C6H1
6 •••I
1–3
complex.
Only a fraction of the available excess energy
appears as translational excitation of the products. However,
0
2
the O2
2 anion, D 0~O2 !54.078 eV, is much more strongly
2
0 2
bound than I2 , D 0~I2 !51.05 eV, and the harpoon mechanism may not become viable until much shorter wavelength
excitation. A useful qualitative picture can be obtained from
a consideration of the relevant neutral O2 potential curves50
and the anionic O2
2 potential shifted by the CT excitation
energy, E CT , which is given by
E CT5IP2EA2
e2
.
R DA
~6!
The IP of the benzene donor is 9.25 eV and the oxygen
acceptor has EA50.45 eV. The distance between the charge
centers in the complex, R DA , is assumed to be '3.3 Å, in
accordance with an ab initio calculation.51 The resultant potential curves are depicted in Fig. 9. The anion potential will
certainly be distorted in the electric field of the benzene
cation,52 perhaps facilitating dissociation.
Reverse electron-transfer in the initially accessed
C6H6 –I2 CT state is thought to produce either electronically
excited I2 , leading to iodine dissociation, or electronically
excited C6H6 , causing fragmentation of the complex. In the
former process, a similar analysis of the neutral I2 potential
curves and the appropriately shifted I2
2 potential indicates
that there may be an efficient crossing between the ionic and
neutral potentials near the Franck–Condon excitation region.
The excited I2 state is purely repulsive resulting in prompt
fragmentation to fast I atom products with a significant spatial anisotropy. The analogous mechanism in C6H6 –O2 may
not be as facile ~Fig. 9! and a relatively long-lived CT state
might redistribute energy into benzene internal modes before
charge-recombination takes place, yielding slower atomic
fragments and an isotropic spatial distribution. Clearly, these
notions are speculative and assessing the viability of any of
these mechanisms would necessitate a more rigorous examination of the relevant potentials.
Dissociation of electronically excited O2 at 226 nm from
one of the states that comprise the Herzberg continuum, ei-
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G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
FIG. 9. Theoretical potential energy curves for several neutral O2 states
~from Ref. 50! and a Morse potential for the O2
2 ground state. The anion
potential has been shifted by E CT , as explained in the text. The arrow
represents the energy of the O~3P j ! probe laser at the equilibrium bond
length of ground state O2 .
ther through direct excitation of a perturbed transition or via
prompt charge-recombination in the CT state, would produce
two O~3P j ! fragments. Energy conservation dictates that the
O~3P 2!1O~3P 2! product state would result in atomic fragments with a recoil velocity peaked at about 61350 m/s in
the laboratory frame. A study of the 226 nm photodissociation of isolated O2 also found an anisotropic fragment spatial
distribution with, b51.660.4, indicating that the transition
moment is largely parallel to the oxygen bond axis.38,53 The
velocity spectra we observe ~Fig. 5! are very much different
from these expectations, exhibiting a broad distribution
peaked at zero-velocity and a totally isotropic spatial distribution. The atomic fragments may experience strong cage
effects due to the close proximity of the solvent benzene
molecule, depending on the structure and rigidity of the complex. Ab initio calculations suggest that the C6H6 –O2 complex has a stable geometry with the oxygen bond axis oriented parallel to the aromatic ring,51 analogous to the resting
geometry discussed for the C6H6 –I2 complex.40 The
C6H6 –O2 complex is weakly bound and low frequency, large
amplitude intermolecular bending motions might enhance
cage interactions while reducing the measured anisotropy
parameter.54
The manifestation of solvent cage effects in terms of the
measured KER has been observed in a number of other cluster systems.4,35,55,56 A distinct velocity dependence of the b
parameter is often detected,4,55 as we demonstrated for the
caged I atom fragments produced from CT excitation of the
C6H6 –I2 complex.4 Within the limits of our signal-to-noise,
we detect no residual anisotropy for the O atom fragments of
5475
the C6H6 –O2 complex and the anisotropy parameter appears
to be independent of velocity, b( v z )50. However, the average excess energy available in the C6H6 –I2 experiments,
'16 000 cm21, is much larger than that available in the current studies of the C6H6 –O2 system, '2500 cm21. The general appearance of the O atom photofragment velocity spectra, and the relative strengths of the CT and oxygen localized
excitations observed in matrix experiments, might argue
against a perturbed O2 transition as the identity of the chromophore in the complex. A more definitive conclusion
awaits further experimentation. Photofragment excitation
spectra obtained by monitoring the wavelength dependence
of O atom production would be most useful in such an endeavor.
The fine structure branching fractions determined for the
O~3P j ! fragment of the C6H6 –O2 complex are listed in Table
I. The spin–orbit state populations roughly follow a Boltzmann distribution described by a temperature of 440 K, as
compared to the translational temperature of 651 K obtained
from fits to the O atom velocity spectra. A completely statistical distribution of spin–orbit states would yield branching fractions of P 2,1,050.56, 0.33, 0.11 ~Table I!. We have
also listed in Table I the resultant fine structure branching
measured for 226 nm photodissociation of isolated O2 from
the work of Tonokura et al.38 Surprisingly, the branching
fractions reported for the isolated molecule and our results
for complex fragmentation appear to be very similar, within
the experimental error. Neither result is in accordance with a
totally statistical distribution. In the adiabatic limit, dissociation on the A 3 S 1
u surface ~the Herzberg I band, which
dominates absorption in gas phase O2 at these wavelengths!
would correlate with the O~3P 2!1O~3P 2! product state and
P 251.0 ~Table I!.57 For the case of diabatic dissociation the
predicted fine structure branching is the same as in the statistical limit. Tonokura et al. argued that their findings,
which are in between the statistical and adiabatic limits, were
a consequence of nonadiabatic coupling in the asymptotic
region of the potential. Other investigations of O2 photodissociation have arrived at similar conclusions.57–59 Some aspects of the dissociation dynamics in the C6H6 –O2 complex
can manifest a correspondence with behavior characteristic
of the separated fragments, as we have also noted for the
case of the C6H6 –I2 complex.4
In addition to O~3P j !, we also detect a number of other
peaks in the mass spectrum ~Fig. 2! that indicate an oxygen
atom has been incorporated into the photoproduct. Knee
et al. reported a similar distribution of product fragments resulting from 193 nm excitation of oxygen–benzene
clusters.13 These studies utilized a large benzene vapor pressure, approximately 80 Torr, and a 10% O2/He carrier gas
mixture at stagnation pressures in excess of 40 atm, presumably resulting in the formation of much larger cluster species
than in the current experiments. Knee et al. also found that
248 nm excitation was ineffective in causing reaction. While
a wavelength of 248 nm is below the threshold for O2 dissociation, excitation at either 193 or 226 nm is above the dissociation limit, suggesting that molecular oxygen fragmentation is the initial step in the cluster photochemistry. The
J. Chem. Phys., Vol. 106, No. 13, 1 April 1997
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5476
G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
experiments of Knee et al. did not specifically search for
O~3P j ! fragments, although efficient detection probably necessitated a resonant photoionization pathway rather than the
nonresonant method that was employed.
The largest signal due to an oxygen containing product
that we detect is at m/e594 which corresponds to a fragment
species of the form, C6H6O. This feature has a steeper dependence on the stagnation pressure than the O atom signal,
as evidenced by the data in Fig. 3, and appears more prominently in the mass spectrum when a higher pressure of benzene vapor is present in the expansion. The evidence, then,
indicates that the C6H6O species originates from photochemical reactions in larger ~C6H6!n –O2 , n>2, parent clusters followed by fragmentation. In addition, the cubic laser
intensity dependence exhibited by the C6H6O peak ~Fig. 4! is
consistent with single photon absorption to initiate photochemistry and two photon ionization of the product. A two
photon ionization mechanism is different from that expected
for the O atom which would require three photons.
Formation of the C6H6O product may necessitate the
more extensive solvent cage found in larger clusters, which
can trap the recoiling O~3P j ! fragments and lead to chemical
reaction. Grover et al. reached a similar conclusion based on
their observation that C6H6O1 appeared subsequent to VUV
photoionization of the ~C6H6!2 –O2 trimer species but not the
dimer complex.16 To corroborate the experimental results,
Grover et al. also determined a structure for the mixed trimer
using force field calculations. The calculated geometry
placed the second benzene molecule on the same side of the
aromatic plane as the oxygen molecule in a position that
could facilitate caging of O atom fragments. In their work,
Knee et al. contended that the oxygen-containing mass fragments were chemically bound species rather than van der
Waals ~vdW! complexes because the large amount of energy
deposited into the cluster by the multiphoton detection
method would result in rapid fragmentation.13 However,
evaporation of the large clusters produced in their expansion
may have stabilized weakly bound vdW species. The reaction of O~3P j ! with benzene was studied in a crossed beam
experiment60 and the major chemical pathways ~Fig. 7! were
determined to be O atom substitution
O~ 3 P j ! 1C6H6→C6H5O1H,
~7!
and O atom addition to possibly form internally excited
phenol
O~ 3 P j ! 1C6H6→C6H5OH.
~8!
The ground state phenol product would have a long lifetime
due to statistical considerations, .1 m sec, permitting detection in the crossed beam apparatus. Flight times in our experiment are on the order of tens of microseconds and so the
m/e594 feature we have observed may be due to C6H5OH
produced in reaction ~8!. Gas phase kinetics experiments
yielded an activation barrier of about 1400 cm21 for reaction
~8!.61 Photodissociation of isolated O2 at 226 nm would produce O atom fragments with about 1500 cm21 of translational energy, barely greater than the experimental barrier
without inclusion of cluster induced effects. A fragment at
m/e593 ~C6H5O! associated with photochemistry in clusters
was not identified in our studies, even thought reaction ~7!
was reported to be a major route in the crossed beam studies.
A sequence of other oxygen-containing fragments appears in the mass spectrum of Fig. 2, including small features
at m/e5110 ~C6H6 –O2! and 188 ~~C6H6!2 –O2! which might
be vdW complexes stabilized by evaporation of larger cluster
species. Mass fragments of the type, C6H6 –Ox Hy , such as
m/e595 ~C6H6 –OH!, 112 ~C6H6 –O2H2!, may be indicative
of abstraction reactions within the cluster. Abstraction of an
aromatic hydrogen by O~3P j !
O~ 3 P j ! 1C6H6→C6H51OH,
~9!
21
is endothermic by about 2940 cm in the gas phase and may
be slightly endoergic under our experimental conditions ~Fig.
7!. Reaction ~9! was not accessible in the crossed beam experiments due to the limited collision energies.60 Gas phase
reaction of oxygen atoms with toluene were found to produce H2O as a minor product, presumably due to an abstraction reaction.62
V. CONCLUSIONS
A laser probe in the vicinity of 226 nm was used to
excite small C6H6 –O2 cluster species generated in a supersonic expansion and to photoionize any resultant photofragments in a one color experimental arrangement. In the absence of benzene in the expansion, we were unable to detect
O~3P j ! fragments using a resonant 211 MPI process even
though isolated O2 absorbs weakly into the dissociative
Herzberg continuum at these wavelengths. The addition of
small amounts of benzene vapor, ,0.05%, to the expansion
resulted in a strong O1 signal in the mass spectrum. These
observations indicate that absorption by C6H6 –O2 complexes
is enhanced by over three orders of magnitude relative to the
bare oxygen molecule. The KER for the O~3P j ! product was
measured using a TOF method and found to be small, peaking at zero velocity in the laboratory frame, and result from
an isotropic fragmentation process. While the available excess energy is relatively small, the observed KER reflects the
dynamics of O2 dissociation and cage interactions with the
solvent benzene molecule. Excitation is near the peak of the
C6H6 –O2 CT band and oxygen fragmentation may proceed
via the charge-recombination or harpoon pathways previously elucidated for the C6H6 –I2 complex. Alternatively, the
reduced symmetry of the floppy, weakly bound complex may
enhance nominally forbidden transitions in O2 , permitting
direct excitation into the dissociative continuum. We also
determined the fine structure branching for the O~3P j ! product and found it to be similar to the results of isolated O2
photodissociation obtained by Tonokura et al.38 despite the
likely role of cluster induced effects.
Photochemistry to generate oxygen containing fragments
was observed to proceed in larger ~C6H6!n –O2 , n>2, species. Subsequent to oxygen dissociation, the more extensive
solvent cage present in the bigger clusters can effectively
trap recoiling O atoms and promote chemical reaction. The
largest signal ascribed to a photoproduct appears at m/e594
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G. DeBoer and M. A. Young: Photochemistry of C6H6 –O2 clusters
and may be due to phenol, C6H5OH, produced from O~3P j !
addition to benzene, in accordance with the conclusions of a
crossed beam experiment. Another major gas phase channel,
O atom substitution, was not observed in the cluster. Other
product fragments may be due to weakly bound vdW complexes or result from abstraction reactions in the cluster.
We intend to examine other oxygen based CT complexes
to determine whether the dynamics we have observed for
C6H6 –O2 and C6H6 –I2 constitutes a more general phenomena. We are especially interested in cluster systems, especially those of relevance to atmospheric chemistry, that can
enhance, or open, various photochemical channels relative to
the isolated species.
1
P. Y. Cheng, D. Zhong, and A. H. Zewail, Chem. Phys. Lett. 242, 369
~1995!.
2
P. Y. Cheng, D. Zhong, and A. H. Zewail, J. Chem. Phys. 103, 5153
~1995!.
3
P. Y. Cheng, D. Zhong, and A. H. Zewail, J. Chem. Phys. 105, 6216
~1996!.
4
G. Deboer, J. W. Burnett, A. Fujimoto, and M. A. Young, J. Phys. Chem.
100, 14882 ~1996!.
5
G. Deboer, J. W. Burnett, and M. A. Young, Chem. Phys. Lett. 259, 368
~1996!.
6
Y. B. Fan and D. J. Donaldson, J. Chem. Phys. 97, 189 ~1992!.
7
K. L. Randall and D. J. Donaldson, J. Phys. Chem. 99, 6763 ~1995!.
8
Y. B. Fan, K. L. Randall, and D. J. Donaldson, J. Chem. Phys. 98, 4700
~1993!.
9
L. Brown and V. Vaida, J. Phys. Chem. 100, 7849 ~1996!.
10
V. Vaida, G. J. Frost, L. A. Brown, R. Naaman, and Y. Hurwitz, Ber.
Bunsenges. Phys. Chem. 99, 371 ~1995!.
11
G. J. Frost and V. Vaida, J. Geophys. Res. 100, 18803 ~1995!.
12
D. S. King, D. G. Sauder, and M. P. Casassa, J. Chem. Phys. 100, 4200
~1994!.
13
J. L. Knee, C. E. Otis, and P. M. Johnson, J. Phys. Chem. 86, 4467 ~1982!.
14
S. Hashimoto and H. Akimoto, J. Phys. Chem. 91, 1347 ~1987!.
15
M. Z. Martin, S. R. Desai, C. S. Feigerle, and J. C. Miller, J. Phys. Chem.
100, 8170 ~1996!.
16
J. R. Grover, G. Hagenow, and E. A. Walters, J. Chem. Phys. 97, 628
~1992!.
17
Y. Hurwitz, Y. Rudich, and R. Naaman, Isr. J. Chem. 34, 59 ~1994!.
18
Y. Hurwitz and R. Naaman, J. Chem. Phys. 102, 1941 ~1995!.
19
C. A. Long and G. E. Ewing, J. Chem. Phys. 58, 4824 ~1973!.
20
H. Tsubomura and R. S. Mulliken, J. Am. Chem. Soc. 82, 5966 ~1960!.
21
J. B. Birks, E. Pantos, and T. D. S. Hamilton, Chem. Phys. Lett. 20, 544
~1973!.
22
E. C. Lim and V. L. Kowalski, J. Chem. Phys. 36, 1729 ~1962!.
23
E. A. Goodling, K. R. Serak, and P. R. Ogilby, J. Phys. Chem. 95, 7868
~1991!.
24
D. R. Kearns, Chem. Rev. 71, 395 ~1971!.
25
R. G. Brown, and D. Phillips, J. Chem. Soc., Faraday Trans. 2 70, 630
~1974!.
5477
J. C. W. Chien, J. Phys. Chem. 69, 4317 ~1965!.
V. I. Stenberg, R. D. Olson, C. T. Wang, and N. Kulevsky, J. Org. Chem.
32, 3227 ~1967!.
28
K. Onodera, G. Furusawa, M. Kojima, M. Tsuchiya, S. Aihara, R. Akaba,
H. Sakuragi, and K. Tokumaru, Tetrahedron 41, 2215 ~1985!.
29
R. D. Scurlock and P. R. Ogilby, J. Am. Chem. Soc. 110, 640 ~1988!.
30
R. D. Scurlock and P. R. Ogilby, J. Phys. Chem. 93, 5493 ~1989!.
31
S. L. Logunov and M. A. J. Rodgers, J. Phys. Chem. 96, 2915 ~1992!.
32
R. L. Miller, A. G. Suits, P. L. Houston, R. Toumi, J. A. Mack, and A. M.
Wodkte, Science 265, 1831 ~1994!.
33
J. A. Syage, J. Phys. Chem. 99, 16530 ~1995!.
34
M. A. Young, J. Phys. Chem. 98, 7790 ~1994!.
35
M. A. Young, J. Chem. Phys. 102, 7925 ~1995!.
36
H. Okabe, Photochemistry of Small Molecules ~Wiley–Interscience, New
York, 1978!, p. 178.
37
B. J. Finlayson-Pitts and J. J. N. Pitts, Atmospheric Chemistry: Fundamentals and Experimental Techniques ~Wiley–Interscience, New York,
1986!, p. 137.
38
K. Tonokura, N. Shafer, Y. Matsumi, and M. Kawasaki, J. Chem. Phys.
95, 3394 ~1991!.
39
J. A. Syage and J. Steadman, Chem. Phys. Lett. 166, 159 ~1990!.
40
R. S. Mulliken and W. B. Person, Molecular Complexes ~Wiley–
Interscience, New York, 1969!, pp. 169–173.
41
M. Mons, J. L. Calve, F. Piuzzi, and I. Dimicoli, J. Chem. Phys. 92, 2155
~1990!.
42
A. J. Rest, K. Salisbury, and J. R. Sodeau, J. Chem. Soc., Faraday Trans.
2 73, 265 ~1977!.
43
G. H. Atkinson and C. S. Parmenter, J. Mol. Spectrosc. 73, 20 ~1978!.
44
A. Morikawa and R. J. Cvetanovic, J. Chem. Phys. 52, 3237 ~1970!.
45
C. E. Otis, J. L. Knee, and P. M. Johnson, J. Chem. Phys. 78, 2091 ~1983!.
46
N. Nakashima and K. Yoshihara, J. Chem. Phys. 77, 6040 ~1982!.
47
A. Horowitz, W. Schneider, and G. K. Moortgat, J. Phys. Chem. 93, 7859
~1989!.
48
J. Goodman and L. E. Brus, J. Chem. Phys. 67, 1482 ~1977!.
49
V. Vaida, D. J. Donaldson, S. P. Sapers, R. Naaman, and M. S. Child, J.
Phys. Chem. 93, 513 ~1989!.
50
R. P. Saxon, and B. Liu, J. Chem. Phys. 67, 5432 ~1977!.
51
G. Granucci and M. Persico, Chem. Phys. Lett. 205, 331 ~1993!.
52
P. E. Maslen, J. M. Papanikolas, J. Faeder, R. Parson, and S. V. ONeil, J.
Chem. Phys. 101, 5731 ~1994!.
53
R. Klotz and S. D. Peyerimhoff, Mol. Phys. 57, 573 ~1986!.
54
G. E. Busch and K. R. Wilson, J. Chem. Phys. 56, 3638 ~1972!.
55
J. A. Syage, Chem. Phys. Lett. 245, 605 ~1995!.
56
R. O. Loo, G. E. Hall, H.-P. Haerri, and P. L. Houston, J. Phys. Chem. 92,
5 ~1988!.
57
Y. L. Huang and R. J. Gordon, J. Chem. Phys. 94, 2640 ~1991!.
58
Y. Matsumi and M. Kawasaki, J. Chem. Phys. 93, 2481 ~1990!.
59
Y. Matsumi and M. Kawasaki, J. Chem. Phys. 94, 6338 ~1991!.
60
S. J. Sibener, R. J. Buss, P. Casavecchia, T. Hirooka, and Y. T. Lee, J.
Chem. Phys. 72, 4341 ~1980!.
61
R. Atkinson and J. N. Pitts, Chem. Phys. Lett. 63, 485 ~1979!.
62
G. R. H. Jones and R. J. Cvetanovic, Can. J. Chem. 39, 2444 ~1961!.
26
27
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