The photodissociation dynamics of OCIO between 306 and 370 nm: Fragment translational energy release and recoil anisotropy

Article abstract of DOI:10.1063/1.473652

The photodissociation OClO(? ²A₂)→ClO(X? 2∏)+O(³P) was studied at wavelengths between 306 and 370 nm using photofragment translational energy spectroscopy. The flight time distributions and anisotropies of the recoiling fragments were measured with the photolysis wavelength tuned to 10 maxima of the structured absorption spectrum, corresponding to a vibronic excitation of the parent molecule with 9-18 quanta in the symmetric stretching coordinate on the ? ²A₂ surface. The translational energy distributions show that the ClO fragments are created in highly inverted vibrational state distributions which become extremely broad [v (Cl-O)～1-15] with increasing excitation energy. The large fraction of vibrationally hot ClO fragments produced-particularly at λ<325 nm-could enhance various thermodynamically unfavorable atmospheric reactions in connection with ozone depletion. The main mechanistic features of the dissociation process, which account for the almost constant average translational energy and linearly increasing vibrational energy of ClO as a function of the excitation energy, can be interpreted, to a first approximation, as vibrational predissociation on the A ²A₂ potential energy surface involving a relatively late exit barrier. From the measured translational energies the barrier height is estimated to be about 48 kJ/mol. 1997 American Institute of Physics.

Full text of DOI:10.1063/1.473652

The photodissociation dynamics of OClO between 306 and 370 nm: Fragment
translational energy release and recoil anisotropy
Alan Furlan, Heiner A. Scheld, and J. Robert Huber
Citation: The Journal of Chemical Physics 106, 6538 (1997); doi: 10.1063/1.473652
View online: http://dx.doi.org/10.1063/1.473652
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/106/16?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Overtone vibrational spectroscopy in H2-H2O complexes: A combined high level theoretical ab initio, dynamical
and experimental study
J. Chem. Phys. 137, 084301 (2012); 10.1063/1.4732581
Dynamics of OH radical generation in laser-induced photodissociation of tetrahydropyran at 193 nm
J. Chem. Phys. 124, 024305 (2006); 10.1063/1.2149378
The dissociation dynamics of HeI 35 Cl (B,v ′ =2,3) complexes with varying amounts of internal energy
J. Chem. Phys. 122, 044318 (2005); 10.1063/1.1829971
Molecular dynamics study of the photodissociation of OClO in bulk liquids
J. Chem. Phys. 116, 8930 (2002); 10.1063/1.1471557
Photodissociation dynamics of OClO at 157 nm
J. Chem. Phys. 108, 10061 (1998); 10.1063/1.476466
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

The photodissociation dynamics of OClO between 306 and 370 nm:
Fragment translational energy release and recoil anisotropy
Alan Furlan, Heiner A. Scheld, and J. Robert Hubera)
¨
¨
Physikalisch-Chemisches Institut der Universitat Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland
¨
͑Received 12 November 1996; accepted 22 January 1997͒
2
2
3
˜
˜
The photodissociation OClO(A A2)→ClO(X ⌸)ϩO( P) was studied at wavelengths between
306 and 370 nm using photofragment translational energy spectroscopy. The flight time
distributions and anisotropies of the recoiling fragments were measured with the photolysis
wavelength tuned to 10 maxima of the structured absorption spectrum, corresponding to a vibronic
excitation of the parent molecule with 9–18 quanta in the symmetric stretching coordinate on the
2
˜
A A2 surface. The translational energy distributions show that the ClO fragments are created in
highly inverted vibrational state distributions which become extremely broad ͓ ͑Cl–O͒ϳ1–15͔
v
with increasing excitation energy. The large fraction of vibrationally hot ClO fragments produced–
particularly at ␭Ͻ325 nm–could enhance various thermodynamically unfavorable atmospheric
reactions in connection with ozone depletion. The main mechanistic features of the dissociation
process, which account for the almost constant average translational energy and linearly increasing
vibrational energy of ClO as a function of the excitation energy, can be interpreted, to a first
2
˜
approximation, as vibrational predissociation on the A A2 potential energy surface involving a
relatively late exit barrier. From the measured translational energies the barrier height is estimated
to be about 48 kJ/mol. © 1997 American Institute of Physics. ͓S0021-9606͑97͒01716-9͔
I. INTRODUCTION
bending mode ␯2 enhances the efficiency of channel ͑2͒, rela-
tive to excitation of modes involving only the symmetric
Chlorine atoms and chlorine monoxide radicals ͑ClO͒
are known to play a major role in reactions responsible for
atmospheric ozone depletion.1–5 In the cold Antarctic lower
stratosphere, where heterogeneous reactions on stratospheric
clouds can occur, additional ozone-depleting reaction cycles
are potentially important involving, among other species,
ClOOCl and OClO.3,5,6 This possible role in the ozone deple-
tion has stimulated extensive studies, both experimental and
theoretical, on OClO and its photodissociation in the near
UV region.
stretching vibration ␯1.7,17
The formation of ClOO, created by the photoisomeriza-
tion OClO→ClOO or by recombination of the primary prod-
ucts ClOϩO→ClOO from Eq. ͑1͒, has been observed in
noble gas matrices.18–21 Photoisomerization and the subse-
quent decay of ClOO to ClϩO2 is however not important in
the gas phase. The high fragment translational energy release
found in the photofragment translational energy ͑PTS͒ study
by Davis and Lee7 is an indication that the ClOO intermedi-
ate is an unlikely precursor.
The reaction dynamics of OClO has also been studied on
a femtosecond time scale using multiphoton ionization in
combination with mass spectrometry.12 On tuning the exci-
tation laser from the vibronic band ␯1ϭ11–17 ͑352–308
nm͒, the bond breakage time of reaction ͑1͒ was found to
change from ϳ500 to 300 fs, which is consistent with the
increasing linewidths of these vibronic bands with increasing
excitation energy.16 In a recent study Delmdahl et al.13 have
probed the ClO photofragment from reaction ͑1͒ after exci-
tation at 351 nm ͑␯1ϭ11͒ and 308 nm ͑␯1ϭ18͒ by a two-
photon laser-induced fluorescence technique. At 351 nm they
found the ClO fragments to be created in the vibrational
2
˜
Following excitation into the A A2 state, the predomi-
nant dissociation channel of OClO between 275 and 475 nm
was found to be6–13
2
2
3
˜
˜
OClO A A ͒→ClO X ⌸͒ϩO P͒.
͑1͒
͑
͑
͑
2
The analysis of the highly structured absorption spectrum in
the near UV region has indicated that the decay is a predis-
sociative process with the bending mode ␯2, and at higher
energy also the asymmetric stretching mode ␯3, acting as
promoting modes.14–16 The second decay channel
Ϫ
g
ϩ
g
2
2
OClO A A ͒→Cl P͒ϩO 3⌺ ,1⌬g ,1⌺ ͒
͑2͒
˜
͑
͑
͑
2
2
was shown to be of minor importance. This channel has a
maximum yield of 3.9Ϯ0.8% at ␭excϭ404 nm which de-
creases to Ͻ0.2% at ␭excϽ370 nm.7 An interesting mode
selectivity was observed for the branching ratio of these two
decay channels.7,17 Excitation of the asymmetric stretching
states ϭ0–4 with a moderate rotational excitation. The
v
recoil anisotropy parameter was measured to be ␤ϭ1.14 and
the population ratio of the spin–orbit states ⌸3/2/⌸1/2
ϭ3.8Ϯ0.5. A much higher vibrational excitation of ClO was
observed at 308 nm but the rotational excitation remained
similar to that at 351 nm.
mode ␯ favors ClO formation, whereas excitation of the
3
From the theoretical efforts22,23 on the OClO photodis-
sociation the extensive calculations of Peterson and Werner23
a͒
Electronic mail: jrhuber@pci.unizh.ch
6538
J. Chem. Phys. 106 (16), 22 April 1997
0021-9606/97/106(16)/6538/10/$10.00
© 1997 American Institute of Physics
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
6539
are particularly pertinent to the present work. They calcu-
lated potential energy functions of the first four doublet elec-
tem and a specially designed Teflon™ molecular beam
valve, which has been described elsewhere.32 This enabled
us to maintain a constant OClO concentration of
4.5%Ϯ0.2% in He for 2–3 h. It should be noted that OClO is
explosive at concentrations in excess of 10% atmospheric
pressure.30
2
2
2
2
˜
˜
tronic states X B1 , B2 , A1 , and A A2 of OClO. Their
2
˜
results indicate that the predissociation of the A A2 state
involves a subtle interplay between the bending and asym-
metric stretching modes in the 2B2 and 2A1 states which
2
˜
couple via nonadiabatic interactions to the A A2 state. From
The recoil anisotropies ␤͑␭ ͒ were measured by means
exc
2
2
˜
the predicted surface crossings of the A1 with the A A2
of a half-wave plate ͑B. Halle, Berlin͒. The degree of the
laser polarization was found to be Ͼ97% over the wave-
length range used. Each anisotropy measurement was carried
out at six different polarization angles with 5000 laser shots
each. This procedure was repeated four times to minimize
the influence of long-term drifts and the runs were normal-
ized to the average laser power. The intensity I(⑀) measured
at the different polarization angles ⑀, where ⑀ is the angle
between the electric field vector E and the detector axis,
were fitted to the expression33,34
2
state, they concluded that it is the A1 state which predisso-
2
˜
ciates the A A2 state through spin–orbit coupling. Follow-
ing this initial crossing to the 2A1 potential surface, the mol-
ecule is predicted to dissociate to ClO and O ͑1͒ through the
2
linear configuration or cross to the B2 surface by vibronic
Herzberg–Teller interactions, where dissociation to ClOϩO
proceeds by the asymmetric stretching vibration.
The present study deals with the excitation wavelength
dependence of the kinetic energy ET of the primary frag-
ments emerging from reaction ͑1͒ and their recoil anisotropy
␤. In the wavelength range between ϳ370 and 306 nm the
excitation was tuned to the absorption bands of the symmet-
ric stretching mode progression from ͑9,0,0͒ to ͑18,0,0͒ and
ET(␭) and ␤͑␭͒ of the photofragments were measured by
using photofragment translational energy spectroscopy.24–27
I ⑀͒ϭA 2ϩ␤ 3 cos2 ⑀Ϫ⑀ ͒Ϫ1
͔
͖
͑3͒
͑
͓
͑
͕
0
and thus provided the anisotropy parameter ␤. Here A is a
normalization constant and ⑀0 is an offset angle, which ac-
counts for the difference in the direction of the fragments in
the lab-fixed system and the center-of-mass ͑c.m.͒ system.
The offset angle was measured at each chosen wavelength
separately, since it may change with the kinetic energy of the
fragments.33,34 Unpolarized time-of-flight ͑TOF͒ data were
obtained by adding two polarized spectra recorded at perpen-
dicular polarization angles, or alternatively by using a Cornu
pseudodepolarizer.
II. EXPERIMENT
The experiments were carried out with a photofragment
translational energy spectrometer which has been described
in detail elsewhere.28,29 It consists of a rotatable, pulsed mo-
lecular beam source, a 34.5-cm-long drift tube, and a quad-
rupole mass spectrometer for mass filtering and detection.
The photolysis laser beam crossed the molecular beam at a
distance of 65 mm from the piezoelectric pulsed nozzle. La-
To avoid saturation and multiphoton effects, the laser
pulse energy was kept at р1.0 mJ for all measurements. This
corresponds to a saturation parameter ͑absorption cross
sectionϫfluence͒ ␴Fϭ0.52 for the ͑11,0,0͒ level
͑␴ϭ1.2ϫ10Ϫ17 cm2͒35 and ␴Fϭ0.14 for the ͑18,0,0͒ level
͑␴ϭ3.6ϫ10Ϫ18 cm2͒.35 Within the pulse energy range 0.5–
3.0 mJ the signal intensity depended linearly on the laser
power, indicating that the observed photodissociation was a
single-photon process; multiphoton contributions were neg-
ligible under our experimental conditions.
ser pulses of 0.5–5 mJ ͑⌬␷ϭ0.15 cmϪ1, ␶ϳ6 ns͒ were ob-
˜
tained by frequency doubling the output of a Nd:YAG-
pumped dye laser ͑Scanmate 2E͒. The laser beam was
slightly focused to a 2ϫ2 mm2 spot at the intersection with
the molecular beam.
OClO was generated by passing Cl2 over NaClO2. The
NaClO2 contained 2 mbar of H2O to catalyze the reaction.30
After completion of the reaction, the yellow product was
passed through an additional U-tube filled with NaClO2 and
glass beads, and cooled with ice water to remove H2O. Ac-
curate determinations of flight times and fragment anisotro-
pies required a high stability of the molecular beam condi-
tions throughout the measurement period. For each measure-
ment 36 mbar of fresh OClO were produced and transferred
to a container, which was then filled with He to a total pres-
sure of 800 mbar and automatically held at constant pressure.
The velocity distribution of the molecular beam pulse
A time offset of 3.9 ␮s(e/m)1/2 has been subtracted in
all of the TOF spectra shown in order to correct for the
transit time of the ions through the mass filter.28,29
III. RESULTS
A. Measurements with unpolarized light
Figures 1͑a͒ and 2͑a͒ show the TOF distributions re-
corded with the mass spectrometer set to the fragment mass
m/eϭ51 using different scattering angles ⌰. The signal at
this mass setting can be unambiguously assigned to the
35ClO fragment formed in process ͑1͒. The solid line repre-
sents the best fit calculated by a forward convolution
procedure,36 and the corresponding translational energy dis-
tributions P(ET) of the fragment pairs of channel ͑1͒ are
displayed in Figs. 1͑b͒ and 2͑b͒. Comparison of Fig. 1 with
Fig. 2 reveals the high sensitivity of the translational energy
2
f( )ϳ exp[Ϫ( Ϫ s)2/␣2] was determined before and
v
v
v
v
after each measurement, using a chopper wheel synchronized
with the pulsed valve. The chopper data for f( ) were
v
checked by laser-induced hole burning.31 The stream veloc-
ity
and the width ␣ of the OClO/He mixture were mea-
vs
sured to be 1240Ϯ35 and 70Ϯ4 m/s, respectively. Because
OClO is subject to decomposition upon contact with metal,
the experiments were performed with a metal-free inlet sys-
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

6540
Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
FIG. 1. ͑a͒ Unpolarized TOF distributions of ClO fragments measured at
m/eϭ51 with ⌰ϭ18°, 24°, and 30° from OClO excited to the ͑11,0,0͒
vibronic level in the A 2A2 state. ͑b͒ Translational energy distribution
FIG. 2. ͑a͒ Unpolarized TOF distributions of ClO fragments measured at
m/eϭ51 with ⌰ϭ9°, 18°, and 30° from OClO excited to the ͑16,0,0͒ level.
͑b͒ Translational energy distribution P(ET) obtained from the three TOF
spectra shown above.
˜
P(ET) obtained from a forward convolution fitted to the three TOF spectra
shown above.
distribution with respect to the excitation energy or the ex-
n
2
˜
cited overtone ␷ in the A A2 state, here as examples
length three or four TOF spectra at different ⌰ ͑9°, 18°, 24°,
and/or 30°͒ were recorded. These extended sets of TOF spec-
tra allowed an accurate determination of P(ET). Figure 3
shows the TOF spectra after excitation into the vibronic
bands ͑14,0,0͒ to ͑18,0,0͒, illustrating the change from a
single peak spectrum centered at about 200 ␮s flight time to
a considerably broadened spectrum with the fastest peak re-
maining at ϳ200 ␮s. These TOF spectra correspond to the
center-of-mass ͑c.m.͒ translational energy distributions
P(ET) given in Fig. 4. The roughly Gaussian shaped distri-
bution obtained with ͑9,0,0͒ excitation broadens with in-
creasing excitation energy, an effect which becomes clearly
apparent between the ͑13,0,0͒ and ͑15,0,0͒ excitation. In this
context it is noted that Davis and Lee17 measured corre-
sponding TOF spectra at lower excitation energies except
those with excitation into the ͑9,0,0͒ and ͑10,0,0͒ band. The
P(ET) distributions from these latter measurements agree
1
͑11,0,0͒ and ͑16,0,0͒.
The threshold scattering angle ⌰thr is determined by the
fastest and hence vibrationally and rotationally cold ClO
fragments. According to the Newton diagram, ⌰thr is 46° at
360 nm and 70° at 308 nm but in our experiment the TOF
signal was found to be very weak at ⌰Ͼ35°. The fraction of
vibrationally cold ClO products is obviously very low ͑see
below͒. Close to the molecular beam at ⌰Ͼ20° the TOF
spectra exhibited contributions from parent molecules. This
background could however be subtracted on a shot to shot
basis by leaving every second molecular beam pulse unpho-
tolyzed.
We performed TOF measurements at ten different exci-
tation wavelengths from 368.63 to 306.93 nm which corre-
spond to excitation into the symmetric stretching mode
2
˜
͑9,0,0͒ to ͑18,0,0͒ in the A A2 state of OClO. At each wave-
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
6541
FIG. 3. Translational energy distributions P(ET) obtained from excitation
to the vibronic levels ͑9,0,0͒–͑13,0,0͒ in the A 2A2 state of OClO. Each of
˜
the curves was fitted to TOF spectra measured at ⌰ϭ18°, 24°, and 30°.
very well with our findings of Fig. 4 ͑bottom͒.
The intensity of the TOF spectra in Figs. 1–3 reflects the
population of the rovibrational states of the OCl fragment but
also depends on the electron impact ionization cross section
␴
and the fragmentation pattern. We have assumed that
ion
these parameters remain constant for the different vibrational
excitation of the ClO fragment. For modest vibrational exci-
tation this has been confirmed in the work of Davis and
Lee.17 TOF spectra of the counterfragments O, which carry
the same information as those of ClO due to momentum
conservation, would obviously not require such an assump-
tion. The poor signal-to-noise ratio of the TOF spectra at
m/eϭ16 prevented us from definitively confirming the inde-
pendence of these parameters also for higher excited vibra-
tional states of ClO. We are, however, confident that this is
the case since ␴ion is relatively insensitive to fragment vibra-
tional excitation at high electron impact energies ͑160 eV͒
and the dissociation pattern is not expected to change for the
strongly bound ClOϩ ion which has a dissociation energy of
about 470 kJ/mol.37
FIG. 4. Translational energy distributions P(ET) for the levels ͑14,0,0͒–
͑18,0,0͒. Each of the curves was fitted to TOF spectra measured at ⌰ϭ9°,
18°, and 30°.
with the highest translational energy EmT ax are those which
possess no internal energy, i.e., they are formed in the vibra-
tional, rotational, and spin–orbit ground state. Based on the
previous results we therefore expect, at least for the lowest
applied excitation energies, the fastest fragments of the
P(ET) distribution to obey the relationship EavlϵEmT axϭh␯
Ϫ D0, which provides the dissociation energy. From the dis-
tributions ͑9,0,0͒ to ͑12,0,0͒ in Fig. 4 we thus find
D0ϭ248Ϯ2 kJ/mol. This is in excellent agreement with the
value of 246.9Ϯ0.8 kJ/mol recently reported by Davis and
Lee.17
From the P(ET) distribution of Fig. 4 the dissociation
energy D0 of reaction ͑1͒ and the internal energy distribution
of the OCl fragment can be extracted. Energy conservation
requires the available energy to be
E
avlϭh␯ϩEint͑OClO)ϪD0 ,
͑4͒
where h␯ is the photon energy and Eint͑OClO͒ the internal
energy of the parent molecule. The latter can be neglected
under our supersonic expansion conditions. The fragments
For the product pair ClO ͑2⌸3/2͒ and O (3P2) in their
electronic ground states Eint is comprised of the vibrational
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

6542
Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
FIG. 6. Internal energy distributions P(Eint͒ for the vibronic levels
͑14,0,0͒–͑18,0,0͒. See the caption of Fig. 5 and the text.
FIG. 5. Internal energy distributions P(Eint͒ for the vibronic levels ͑9,0,0͒–
͑13,0,0͒, obtained by using a dissociation energy D0͑O–ClO͒ϭ247 kJ/mol.
Since the contribution of rotational excitation to EintϭEavlϪET was found to
be small ͓͗Erot͘ϭ7.5 kJ/mol ͑Ref. 13͔͒ Eint is roughly equivalent to the
vibrational excitation of the ClO fragment. The vibrational energy spacing is
indicated by the grid ͑see the text͒.
ment velocity u ͑which defines ⑀0͒ is not much greater than
the threshold velocity required for detection, and as a conse-
quence ⑀0 is subject to large scatter. For these reasons we
restricted our anisotropy measurements to the fastest ClO
fragments at any given excitation wavelength, more specifi-
cally, only the fragments arriving within an interval of 20 ␮s
after the onset of the TOF signal were analyzed. The scatter-
ing angle ⌰ was kept constant at 18°. The ␤ values derived
from these measurements are given in Fig. 8, revealing the
dependence on the excitation wavelength. Between ϳ370
and rotational energy of the ClO fragment. A contribution
from electronic excitation may arise from ClO where the two
spin–orbit states ⌸3/2 and ⌸1/2 are separated by 318 cmϪ1 38
.
For the O atom the spin–orbit states lie 227 cmϪ1 (3P0) and
3
158 cmϪ1 ( P1) above the P2 ground state. Although we
are able to resolve only the vibrational structure, providing
the rotational distribution is not too broad, some of the finer
details are revealed at low translational energy ͑Fig. 4͒, or
correspondingly at high internal energy ͑Figs. 5 and 6͒,
where the resolution is greatest and the rotational excitation
is low. This is nicely demonstrated in Fig. 6 for the four
3
highest excited vibrational states ( ϭ12–15) of ClO the
v
structure of which appears as clearly distinguishable peaks
separated by ϳ650 cmϪ1
.
B. Polarized measurements and recoil anisotropy
Figure 7 displays the TOF signal intensity I measured as
a function of the laser polarization angle ⑀39–41 and expressed
by Eq. ͑3͒.33 To illustrate this dependence we selected the
data for excitation into the ͑10,0,0͒ and ͑16,0,0͒ vibronic
2
˜
absorption bands of OClO (A A2), which gives rise to a
strong fragment anisotropy change. Only the result for one
run is shown but every run was repeated three times. The
dependence of the anisotropy parameter ␤ on the flight time
was found to be small, ͉⌬␤(t)͉р0.15, but the variation of
the fitted offset angle ⑀0 with flight time was substantial.
Especially for flight times Ͼ300 ␮s an accurate determina-
tion of ⑀0 was difficult. Under these conditions the c.m. frag-
FIG. 7. Dependence of the ClO fragment signal on the polarization angle ⑀
for the OClO vibronic levels ͑10,0,0͒ and ͑16,0,0͒. The solid lines are fits
with ␤ϭ0.30 and ␤ϭ0.64, respectively.
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
6543
FIG. 9. The measured anisotropy ␤ as a function of the lifetime of the
excited OClO molecule. The curves were calculated assuming for the parent
molecule Trotϭ20, 40, and 80 K, and a moment of inertia Ieffϭ9.2ϫ10Ϫ46
kg m2, which corresponds to the average of Ia and Ia in the ground elec-
tronic state of OClO. The vertical lines indicate the lifetimes calculated for
␤͑10,0,0͒ϭ0.30 and ␤͑17,0,0͒ϭ0.67 and Trotϭ40 K.
FIG. 8. Anisotropy parameter ␤ measured at m/eϭ51 with the photolysis
wavelength tuned to the OClO vibronic levels ͑9,0,0͒–͑17,0,0͒. The lower
panel shows a part of the OClO absorption spectrum dominated by the
symmetric stretching mode ␯1 .
2
˜
the A A2 state, ␶ is found to be 2.5 and 0.45 ps, respec-
tively, for Trotϭ40 K. Since any state mixing in the absorp-
tion or during the dissociation process may lead to a depo-
larization and hence to a smaller ␤ value than without, these
lifetime estimates must be considered upper limits.
nm, with excitation into the symmetric stretching mode
͑9,0,0͒, and ϳ310 nm ͑17,0,0͒, ␤ increases from 0.3 to 0.6.
Two exploratory measurements including the asymmetric
stretching mode ͑10,0,2͒ and ͑11,0,2͒ exhibit a tendency to
slightly higher ␤ values.
IV. DISCUSSION
A. Energy distribution and alignment
The present investigation deals with the predissociation
A rough estimation of the dissociation lifetime can be
obtained from the anisotropy parameters.41–43 For dissocia-
tion occurring instantaneously after photon absorption,
␤ϭ2P2͑cos ␹͒, where P2 is the second-order Legendre
polynomial and ␹ the angle between the electronic transition
moment ␮ and the recoil direction. In the case of OClO the
process ͑1͒, where the applied excitation energies allow ac-
cess to the oxygen PJ state only. We first discuss the find-
3
ings on the translational energy distribution of the fragment
pair ClOϩO in comparison with the results previously
reported.12,13,17 Figure 4 presents a survey of the c.m. trans-
lational energy distributions obtained at ten different excita-
tion energies covering a range of 66.3 kJ/mol ͑5545 cmϪ1͒.
While the distribution broadens with increasing excitation
2
2
9
˜ ˜
A A2←X B1 transition is induced by ␮ lying in the mo-
lecular plane and parallel to the line connecting the two O
atoms ͑C2v molecular symmetry͒. The equilibrium bond
energy, the average translational energy E appears to re-
͗
͘
T
2
23
˜
angle in the A A2 state is 107°, so that the ejection of the
O atom along the breaking Cl–O bond yields ␹ϭ͑180
Ϫ107͒/2. Under these conditions the maximum value of ␤,
which corresponds to a bond rupture proceeding much faster
than a molecular rotation, is ␤maxϭ0.93. Providing ␮ is well
defined as assumed above, a reduction of ␤max can be as-
cribed to rotation of the parent molecule prior to dissocia-
tion. Following the treatment of Wilson and Busch41 we cal-
culated the dissociation lifetime ␶ from the ␤ values given in
Fig. 8 by assuming a rotational temperature of the molecule
in the beam of Trotϭ40 K ͓Eq. ͑5͒ in Ref. 42͔. Because the
uncertainty in Trot introduces a relatively large error in ␶, we
performed the calculation also for Trotϭ20 and 80 K. The
result is shown in Fig. 9. For two widely different ␤ values,
measured for excitation into the ͑9,0,0͒ and ͑17,0,0͒ band of
main roughly constant at a value of 50–55 kJ/mol ͓see also
Figs. 1͑b͒ and 2͑b͒ as well as Table I͔. The analysis confirms
this expectation as demonstrated with the plot
E
vs
͘
͗
T
E
excϭh␯ in Fig. 10. Since Eavlϭ E ϩ E ͘, the almost
͗ ͘ ͗
T int
constant translational energy release implies that the average
internal energy ͗Eint͘ increases approximately linearly with
E
avlϭh␯ϪD0 . The increase in excitation energy is therefore
deposited into internal energy of the fragments. From their
measurements of the state populations of the ClO fragment
after excitation at 351 nm, Delmdahl et al.13 found a popu-
lation ratio for the spin–orbit states of P(2⌸3/2):
P(2⌸1/2)ϳ4, which corresponds to an excitation energy of
Ͻ70 cmϪ1, and a Boltzmann distribution of 480 K for the
rotational states. No corresponding experimental data for the
3
spin–orbit states P0,1,2 of the O fragment are available but
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

6544
Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
TABLE II. Available energy and detailed fragment energy partitioning.
Eavl
E
͗Evib
͘
͗Erot͘
͗
͘
T
␯
kJ/mol
kJ/mol ͑%͒
kJ/mol ͑%͒
kJ/mol ͑%͒
1
a
11
18
92.9
144
52 ͑56͒
48 ͑33͒
ϳ33 ͑35͒
ϳ88 ͑61͒
ϳ8 ͑9͒
ϳ8 ͑6͒
a
aReference 13.
pected to increase in higher vibrational levels of ClO. Based
on these findings, we conclude that the internal energy is
mainly vibrational excitation Evib and that the increase in
photon energy is mainly channeled into the vibrational de-
gree of freedom of the ClO fragment. Thus after excitation of
͑11,0,0͒ and ͑17,0,0͒ the partitioning of Eavl into Evib in-
creases from 35% to 61% while ET decreases from 56% to
33% assuming Erot to be constant at ϳ7.8 kJ/mol. Table II
gives the detailed energy partitioning at these two excitation
energies. The present measurements yield a considerably
higher value for E at 351 nm ͑11,0,0͒ than the Doppler
͗
͘
T
profile LIF results13 ͑52 vs 23 kJ/mol͒. This is partly due to
the different choices of D0͑O–ClO͒.
The vibrational excitation of the ClO fragment with re-
spect to the reaction channel leading to ClO ͑2⌸3/2͒ and O
(3P2) in their spin–orbit ground states is exhibited in Figs. 5
FIG. 10. Average internal energy ͗Eint͘ and translational energy
fragments as a function of the excitation ͑photolysis͒ energy h␯. Eavl is
obtained with D0ϭ247 kJ/mol.
E
of the
͘
T
͗
and 6. The Cl–O vibrational quanta up to ϭ16 are indi-
v
cated on top of the figures and a grid complements this in-
formation. The vibrational energy spacings include the
anharmonicity.38 The rotational excitation of each vibrational
state, superposition with weaker P(Eint͒ distributions con-
tributed by ClO and O in excited spin–orbit states, and the
resolution of our apparatus29 prevent the vibrational structure
from becoming apparent except for excitation above
͑14,0,0͒. There the vibrational bands of the ClO stretch
their contribution to Eint will be very small since even for a
statistical distribution the excitation energy is Ͻ80 cmϪ1
.
The internal energy ͗Eint͘ vs Eexc displayed in Fig. 10 repre-
sents therefore ͗Eint͘ϭ͗Evib͘ϩ͗Erot͘ of the ClO fragment
where a small contribution of Ͻ200 cmϪ1 for spin–orbit
excitation has been neglected. The rotational energy at 351
ϭ12, 13, 14 and 15 are clearly discernible and manifest a
v
nm, measured for ClO( ϭ0), was reported to be ϳ7.8 kJ/
spacing of ϳ650 cmϪ1, which agrees with the expected vi-
brational frequencies of such high excitation.38 The appear-
ance of this structure, which is due to a decreased rotational
excitation of the ClO fragment, being now ErotϽ200 cmϪ1 as
v
mol ͑ϳ650 cmϪ1͒ and appeared to be similar at 308 nm.13
Compared to ͗Eint͘ given in Fig. 10 and Table I, this contri-
bution from the rotational excitation is small and is not ex-
TABLE I. Available energy, energy partitioning, and anisotropy.
E
intуEa,c
10%
a
a,b
a
a
͑␯1 ,0,0͒
␭
͑nm͒
Eexc
Eavl
E
͗Eint
͘
20%
␤
͗
͘
T
exc
9
10
11
12
13
14
15
16
17
18
368.84
360.1
324.3
332.1
339.7
347.5
354.9
362.4
369.9
377.0
383.7
390.6
77.4
85.2
92.4
100.6
108.0
115.5
123.0
130.1
136.8
144.0
52 ͑68͒
52 ͑61͒
52 ͑56͒
52 ͑52͒
54 ͑50͒
56 ͑49͒
56 ͑46͒
53 ͑41͒
50 ͑37͒
48 ͑33͒
25
34
40
49
57
58
67
77
87
96
36
42
46
57
67
70
82
104
120
128
43
47
52
63
74
82
99
116
128
137
0.31
0.30
0.39
0.43
0.44
0.50
0.64
0.65
0.67
352.04
344.13
337.01
329.99
323.37
317.23
311.67
306.21
aAll energies in kJ/mol; values in parenthesis are in percent of Eavl
.
bAvailable energy assuming a dissociation energy D0͑OClO͒ϭ247 kJ/mol.
c20% and 10% of the ClO fragments, respectively, have internal energies exceeding the values given in the
table.
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
6545
inferred from the bandwidth, also indicates that the contribu-
tion from spin–orbit excited fragments must be small.
The lifetimes derived from the ␤ values which were
tion mechanism to be confined to the 2A2 PES. The influence
2
of the predicted spin–orbit coupling between the A2 and
2
2A1 states and the vibronic coupling of the latter to the B2
measured for excitation to states of the ␯ progression from
͑9,0,0͒ to ͑17,0,0͒ are 2.5–0.45 ps and they represent upper
state is thereby neglected. At lower excitation energy ͑Ͻ3.1
eV͒, where the atmospherically important reaction ͑2͒ lead-
ing to ClϩO2 reaches its maximum yield ͑ϳ4%17͒, the direct
1
limits. These values were obtained with the assumption that
2
2
˜
˜
the initial polarization alignment is given by the A A2
decay channel to ClOϩO on A A2 is predicted to be closed
2
←X B1 transition without admixture from the 2B2 and
˜
2
2
so that coupling to the lower lying A1 and B2 PES be-
2A1 neighboring excited states as inferred from an analysis
of the jet-cooled high-resolution absorption spectrum.44 Be-
cause these lifetimes ␶ are calculated with a simple impulsive
model in which the crucial bending angle in OClO(2A2) is
fixed during the bond-breaking process, they are only rough
estimates. Nevertheless the increase in ␤ with increasing Eexc
͑Fig. 8͒ is most likely due to a decreasing ␶. Femtosecond
real-time probing12 as well as high-resolution linewidth
measurements16 indicate the same effect. Baumert et al.12 re-
ported a biexponential decay of the OClOϩ signal. The faster
decay component was attributed to vibrational predissocia-
comes necessary for dissociation to occur. Under our condi-
2
tions, however, coupling to the B2 state appears minor be-
cause no ClϩO2 products were detected ͑Ͻ0.2%17͒, which is
predicted to be a characteristic feature of the 2B2 decay.
Furthermore the absence of a barrier on the 2B2 PES23 along
the ‘‘reaction coordinate’’ ͑the asymmetric stretch͒ is hard to
reconcile with our translational energy distributions ͑see be-
low͒ as well as the moderate rotational excitation of the ClO
fragment.13 The latter is expected to be high, as along the
2
bending coordinate of OClO the B2 PES shows the mini-
mum energy at a small angle of ϳ90° which gives rise to a
2
˜
2
tion on the A A2 potential energy surface ͑PES͒ and the
high torque. The neglect of an interaction with the A1 sur-
2
2
slower one to a decay on either the B2 or A1 PES, or the
face is probably more severe. The topography of this surface
along the asymmetric coordinate is similar to that of the
2
˜
ground state surface X B1 . Excitation of the ͑11,0,0͒ vi-
bronic level yielded a decay time being about a factor of 2
greater than excitation of the ͑17,0,0͒ level, i.e., ␶Ϸ500 and
300 fs, respectively. However, the parent-ion signal recorded
in these measurements does not distinguish between the par-
ent molecule decaying due to photodissociation and becom-
ing spectroscopically dark owing to, e.g., internal conver-
sion. The decay signal could therefore represent the sum of
various decay processes ͑including the decay to ClϩO2͒ so
that the measured lifetime would be a lower limit for the
OClO dissociation ͑1͒. Linewidth measurements of Richard
et al.16,44 provided lifetimes in the wavelength region Ͼ400
nm where single rotational lines are at least partly resolved.
The highest vibronic band with some resolved rotational
structure, the ͑9,0,0͒ band, showed widths of Ϸ1.2 cmϪ1 ͑full
width at half-maximum͒, which correspond to ␶Ϸ4 ps. Com-
pared with the results of these femtosecond and high-
resolution studies the lifetimes derived from the ␤ values are,
considering the uncertainties involved, in reasonable agree-
ment.
2
23
˜
A A2 PES but differs along the bending angle. This coor-
dinate is calculated to show only a small barrier ͑ϳ0.3 eV23͒
to dissociation so that the bending motion may act as a pro-
motor for dissociation at sufficient excitation energies. Cou-
2
2
˜
pling between the A A2 and A1 surfaces is however antici-
pated to introduce a depolarization of the recoil anisotropy.
Since the reduction of our measured ␤ values ͑Fig. 8͒ from
␤
maxϳ0.9 was found to be consistent with the dissociation
lifetime, evidence for a strong 2A2 –2A1 coupling has not
been found. Therefore, a discussion of our photodissociation
2
results confined to the initial A2 surface is attempted.
23
2
A schematic A2 PES based on the ab initio results is
depicted in Fig. 11. The wave packet, initially prepared in
the FC region of the excited PES, moves along the symmet-
ric stretching coordinate r and is reflected from the potential
back to the FC region.45 Within the potential well, defined by
the two symmetrically positioned barriers along r͑Cl–O͒, the
wave packet is then expected to move in elliptical cycles
mainly along r leaking over the barrier into the exit channels.
The residence time of the wave packet within the inner re-
gion of the PES around the FC point would thus determine
the dissociation time, which was measured to be in the range
B. Mechanistic aspects
In the present experiments, photoexcitation from the
ground state prepares OClO in the symmetric stretching
of 300–500 fs depending on the ␯ overtone. The wave
1
packet escapes preferentially with a large amplitude motion
into the exit channel giving rise to high vibrational excitation
of the ClO fragment. Moreover, the experiments show the
vibrational state distribution to be strongly inverted and ex-
tremely broad at high excitation energy ͑Fig. 6͒. The equi-
librium bond distance d͑Cl–O͒ in OClO (2A2) is 1.627 Å,23
2
˜
mode on the A A2 potential energy surface. According to
the most recent results of ab initio calculations by Peterson
and Werner,23 this PES is bound along the coordinates of the
symmetric stretching and the bending mode. Along the
asymmetric stretching coordinate a small barrier of about
3000 cmϪ1 separates the Franck–Condon ͑FC͒ or excitation
region from the repulsive surface of the exit channel. All the
excitation energies applied in the present work exceed the
threshold of ϳ25 000 cmϪ1 ͑3.1 eV͒ required to overcome
38
˜
the corresponding one in the ClO (X) fragment is 1.57 Å.
This fact leads to a minimum energy reaction path from the
FC point to the separated fragments which is almost parallel
to the r͑Cl–O͒ reaction coordinate. This implies that there
should be only a small vibrational–translational interaction45
with respect to the ClO vibration of the emerging fragment
and the translational motion along the reaction coordinate.
2
˜
this barrier on the A A2 PES and hence give access to a
direct decay to ClOϩO according to reaction ͑1͒. This exci-
tation condition may allow a discussion of the predissocia-
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

6546
Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
2
2
˜
˜
FIG. 12. Cut through the schematic X( B1) and A( A2) PESs of OClO
2
˜
FIG. 11. A schematic of the two-dimensional A( A2) PES of OClO. The
dot marks the Franck-Condon region and the arrow indicates the initial
motion of the localized wave packet along the symmetric stretching motion
͑see the text͒.
along the Cl–O bond coordinate. From the dissociation energy of reaction
˜ ˜
͑Ref. 17͒, and the measured translational energy
͑1͒, D0ϭ247 kJ/mol, the energy of the A←X origin E(0–0)ϭ251 kJ/mol
E ϳ52 kJ/mol, an inner
͘
T
͗
barrier height of ϳ48 kJ/mol is estimated. The range of excitation energies
͑324–391 kJ/mol͒ is indicated by the shadowed region.
The independence of ET and Eint of the fragments found in
our experiments ͑see Fig. 10͒ is consistent with this expec-
tation.
mol and the ˜A←˜X(0Ϫ0) transition energy of 251 kJ/mol, a
barrier height of ϳ4000 cmϪ1 ͑48 kJ/mol͒ is obtained.
The rotational excitation of the ClO fragment has been
found to be moderate13 and no indication from the present
results points to an increase in Erot with increased excitation
energy; on the contrary, the P(Eint͒ distribution in Fig. 6
revealed a decrease at high fragment vibrational excitation.
The PES(␣,r) along the ␣ bending coordinate in the inner
region is therefore expected to show a small gradient giving
rise to only a small torque ץV(␣,r)/ץ␣. The narrow bands of
the ͑␯1,0,0͒ progression in the absorption spectrum reflect a
long-lived resonance structure which in the time-dependent
picture means a long residence time of the wave packet in
the inner PES region. Thus the measured lifetimes corre-
spond to more than ϳ10 vibrational periods of the excited
C. Atmospheric chemistry
Due to its catalytic destruction of ozone in the strato-
sphere, the ClO radical is an important species in strato-
spheric chemistry.3,4,6,46,47 It appears generally accepted that
the photolysis of the ClO dimer is the dominant chemical
reaction responsible for the loss of O3 in the antarctic. How-
ever, in regions of lower ClO concentrations this mechanism
is less important simply because the ͑ClO͒ concentration
2
depends on the square of the ClO concentration. In such
atmospheric regions other O3 loss pathways which involve,
e.g., vibrationally highly excited ClO may gain importance.
These hot radicals could provide the internal energy neces-
sary to overcome bimolecular reaction barriers not accessible
with cold radicals. According to our measurements, photoly-
sis of OClO at, e.g., the ͑18,0,0͒ level in A A2 state provides
50% of nascent ClO fragments with an internal energy Ͼ96
kJ/mol and 10%Ͼ137 kJ/mol ͑see Table I͒. Vaida and
Simon3,5,6 have suggested that the reaction of ClO with O3
can be strongly enhanced at high internal energies of ClO.
Other endothermic reactions pertinent to atmospheric chem-
istry such as48
state ␯ mode. Once the barrier is crossed the fragments
1
separate rapidly. Assuming a weak exit channel interaction
the fragment internal energy distribution is essentially com-
pleted after having crossed the top of the barrier, i.e., the
asymptotic or final rotational–vibrational state distribution
reflects the transition state wave function of these
coordinates.45 Consequently the barrier energy with respect
to the exit channel is deposited into translational motion of
the fragments. In view of our experimental findings that ET
remains almost constant while Eint increases considerably
with increasing excitation energy ͑Fig. 10͒, an exit channel
interaction between these degrees of freedom is inefficient.
Based on these considerations the barrier height can be esti-
mated as illustrated in Fig. 12. From the average translational
energy release, which was measured to be Ϸ52 kJ/mol at
lower excitation energy, the dissociation energy of 247 kJ/
2
˜
ClOϩO 3⌺Ϫ͒→ClO3 , ⌬H0ϭ52 kJ/mol,
͑
2
g
ClOϩO 1⌬g͒→O3ϩCl, ⌬H0ϭ67 kJ/mol,
͑
2
ClOϩH2O→HOϩHOCl, ⌬H0ϭ101 kJ/mol
might also become important at high internal energies.
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07

Furlan, Scheld, and Huber: Photodissociation dynamics of OClO
6547
5 J. D. Butler, Air Pollution Chemistry ͑Academic, London, 1979͒.
V. CONCLUSION
6 V. Vaida and J. D. Simon, Science 268, 1443 ͑1995͒.
7 H. F. Davis and Y. T. Lee, J. Phys. Chem. 96, 5681 ͑1992͒.
The photodissociation of OClO according to reaction ͑1͒
2
˜
8
is a predissociative process after excitation to the A A2
state. The previous and present experimental results can be
interpreted in a simplified manner in terms of a dissociation
confined to the initial A2 PES and thus neglecting interac-
tions with the neighboring states B2 and A1 .
to the two r͑O–Cl͒ coordinates, the schematic PES ͑Fig. 11͒
is characterized by a well in the Franck–Condon region,
which is separated by a small barrier estimated to be ϳ4000
cmϪ1 from the repulsive exit channel. The excitation ener-
gies Eexc used in the present study exceed this barrier and
give rise to a ClO vibrational energy distribution which be-
¨
V. Vaida, S. Solomon, E. C. Richard, E. Ruhl, and A. Jefferson, Nature
342, 405 ͑1989͒.
9 V. Vaida, E. C. Richard, A. Jefferson, L. A. Cooper, R. Flesch, and E.
¨
Ruhl, Ber. Bunsenges. Phys. Chem. 96, 391 ͑1992͒.
2
10 E. Bishenden and D. J. Donaldson, J. Chem. Phys. 99, 3129 ͑1993͒.
11 E. Bishenden and D. J. Donaldson, J. Chem. Phys. 101, 9565 ͑1994͒.
12 T. Baumert, J. L. Herek, and A. H. Zewail, J. Chem. Phys. 99, 4430
14,23
2
2
Reduced
͑1993͒.
13
¨
R. F. Delmdahl, S. Baumgartel, and K.-H. Gericke, J. Chem. Phys. 104,
2883 ͑1996͒.
14 S. Michielsen, A. J. Merer, S. A. Rice, F. A. Freed, and Y. J. Hamada, J.
Chem. Phys. 74, 3089 ͑1981͒.
15 D. A. McDonald and K. K. Innes, Chem. Phys. Lett. 59, 562 ͑1987͒.
16 E. C. Richard and V. Vaida, J. Chem. Phys. 94, 163 ͑1991͒.
17 H. F. Davis and Y. T. Lee, J. Chem. Phys. 105, 8142 ͑1996͒.
18 A. Arkell and I. Schwager, J. Am. Chem. Soc. 89, 5999 ͑1967͒.
19 F. J. Adrian, J. Bohandy, and B. F. Kim, J. Chem. Phys. 85, 2692 ͑1986͒.
20 K. Johnsson, A. Engdahl, P. Ouis, and B. Nelander, J. Mol Chem. 293,
comes extremely broad ͓ ͑ClO͒ϳ1–16͔ with increasing
v
Eexc . While the average translational energy of the fragment
remains almost constant, the increase in photoenergy is chan-
neled predominantly into Evib of ClO. This behavior is remi-
niscent of the S1 photodissociation of methylnitrite
CH3ONO→CH3OϩNO where a similar PES topology, al-
though not symmetric with respect to two equivalent reaction
coordinates, creates similar features of the fragment energy
137 ͑1993͒.
21
¨
H. S. P. Muller and H. Willner, J. Phys. Chem. 97, 10589 ͑1993͒.
22 J. L. Gole, J. Phys. Chem. 84, 1333 ͑1980͒.
23 K. A. Peterson and H.-J. Werner, J. Chem. Phys. 96, 8948 ͑1992͒.
24 G. E. Busch, R. T. Mahoney, R. I. Morse, and K. R. Wilson, J. Chem.
Phys. 51, 449 ͑1969͒.
partitioning.30 This is particularly striking when Etrans , Evib
,
25 A. M. Wodtke and Y. T. Lee, in Molecular Photodissociation Dynamics,
edited by M. N. R. Ashfold, J. E. Baggott ͑Royal Society of Chemistry,
London, 1987͒, pp. 31.
and Erot for the CH3ONO photodissociation given in Table 8
of Ref. 49 are compared to the corresponding data in Table I.
The decoupling between the translational and vibrational de-
grees of freedom of the fragments is also pronounced in
CH3ONO.
The reduced ␤ values relative to ␤max can be attributed
to a relatively long lifetime ␶ of the excited molecule prior to
dissociation. The observed increase of ␤ with increasing Eexc
is thus due to a shortening of ␶ which is estimated to be from
about 2.5 ps ͑9, 0, 0͒ to 0.45 ps ͑17, 0, 6͒. Within our sim-
plified picture, the relatively long lifetime reflects the resi-
26 M. N. R. Ashfold, I. R. Lambert, D. H. Mordaunt, G. P. Morley, and C.
M. Western, J. Phys. Chem. 96, 2938 ͑1992͒.
27 P. Felder, Chimia 48, 43 ͑1994͒.
28 P. Felder, Chem. Phys. 143, 141 ͑1990͒.
29
¨
P. Felder, Habilitation, University of Zurich, 1993.
30 R. I. Derby and W. S. Hutchinson, Inorg. Synth. 4, 152 ͑1953͒.
31 T. K. Minton, P. Felder, R. J. Brudzynski, and Y. T. Lee, J. Chem. Phys.
81, 1759 ͑1984͒.
32 M.-A. Thelen, P. Felder, and J. R. Huber, Chem. Phys. Lett. 213, 275
͑1993͒.
2
33 P. Felder, Chem. Phys. 155, 435 ͑1991͒.
dence time of the wave packet within the well of the A2
34 J. G. Frey and P. Felder, Mol. Phys. 75, 1419 ͑1992͒.
35 S. Hubinger and J. B. Nee, Chem. Phys. 181, 247 ͑1994͒.
36 R. K. Sparks, K. Shobatake, L. R. Carlson, and Y. T. Lee, Chem. Phys.
75, 3838 ͑1981͒.
PES around the excitation ͑FC͒ region. Its oscillations,
mainly along the symmetric stretching coordinate, give rise
to the well-structured absorption spectrum.
Finally, the surprisingly strong vibrational excitation of
the ClO fragment, which is manifested by a broad TOF spec-
tra ͑Figs. 5 and 6͒, can contribute a considerable amount of
internal energy to a bimolecular reaction. It is therefore con-
ceivable that atmospheric reactions involving ClO may
strongly be enhanced if the excitation conditions used in this
studied prevail in the atmosphere.
37 The dissociation energy of ClOϩ was calculated by subtracting the ioniza-
tion energy ͑10.85 eV͒ of ClO from the sum of the dissociation energy
͑2.75 eV͒ of ClO and the ionization energy of Cl ͑13.0 eV͒, yielding a
value of 4.9 eV ͑473 kJ/mol͒.
38 K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure
IV. Constants of Diatomic Molecules ͑Van Nostrand Reinhold, New York,
1979͒.
39 C. Jonah, J. Chem. Phys. 55, 1915 ͑1971͒.
40 R. N. Zare, Mol. Photochem. 4, 1 ͑1972͒.
41 G. Busch and K. R. Wilson, J. Chem. Phys. 56, 3626 ͑1972͒.
42 S. Yang and R. Bersohn, J. Chem. Phys. 61, 4400 ͑1974͒.
43 V. P. Hradil, T. Suzuki, S. A. Hewitt, P. L. Houston, and B. J. Whitaker,
J. Chem. Phys. 99, 4455 ͑1993͒.
ACKNOWLEDGMENTS
Support of this work by the Schweizerischer National-
¨
fonds zur Forderung der wissenschaftlichen Forschung is
44 E. C. Richard, C. T. Wickham-Jones, and V. Vaida, J. Phys. Chem. 93,
6347 ͑1989͒.
gratefully acknowledged. We thank Rolf Pfister for synthe-
sizing OClO, Dr. Gregory Hall for valuable discussions, and
Dr. Robert T. Carter for critically reading the manuscript.
45 R. Schinke, Photodissociation Dynamics ͑Cambridge University Press,
Cambridge, 1993͒.
M. Finkenbeiner, J. N. Crowley, O. Horie, R. Muller, G. K. Moortgat, and
46
¨
1 S. Solomon, R. R. Garcia, F. S. Rowland, and D. J. Wuebbels, Nature 321,
755 ͑1986͒.
P. J. Crutzen, J. Phys. Chem. 99, 16264 ͑1995͒.
47 R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson, J. A. Kerr, and J.
2 M. J. Molina and F. S. Rowland, Nature 249, 810 ͑1974͒.
3 L. T. Molina and M. J. Molina, J. Phys. Chem. 91, 433 ͑1987͒.
4 M. J. Molina, T.-L. Tso, L. T. Molina, and F. C.-Y. Wang, Science 238,
1253 ͑1987͒.
Troe, J. Phys. Chem. Ref. Data 18, 881 ͑1989͒.
48
¨
U. Bruhlmann, J. R. Huber, Z. Phys. D 7, 1 ͑1987͒.
49 A. Untch, R. Schinke, R. Cotting, and J. R. Huber, J. Chem. Phys. 99,
9553 ͑1993͒.
J. Chem. Phys., Vol. 106, No. 16, 22 April 1997
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:
130.113.111.210 On: Tue, 23 Dec 2014 19:08:07