State-resolved rotational energy transfer in open shell collisions:
Cl(
2
P
3/2
)؉HCl
Zhong-Quan Zhao, William B. Chapman, and David J. Nesbitt
Joint Institute for Laboratory Astrophysics, National Institute for Standards and Technology, and
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440
͑Received 9 December 1994; accepted 1 February 1995͒
Time- and frequency-resolved infrared ͑IR͒ laser absorption methods are used to probe hot atom
energy transfer in open shell interactions of Cl(
2
P
3/2
)ϩHCl(J) in the single collision regime. The
Cl(
2
P
3/2
) atoms are prepared by 308 nm laser photolysis of Cl
2
, and suffer collisions at E
rel
¯
ϳ 3500 cm
Ϫ1
with a room temperature HCl distribution in a fast flow cell. Selective collisional
excitation of final HCl(J
f
) states is monitored by transient IR absorption on R(Jу4) branch lines
in the HCl(
v
ϭ1←0) band, while depletion of the initial HCl(J
i
) states is monitored by transient
bleaching of the room temperature Doppler profiles. Analysis of the J dependent Doppler profiles
permits extraction of rotational loss ͓
loss
(J
i
)ϭ
͚
f
P(J
i
)•
f ←i
͔ and gain ͓
gain
(J
f
)ϭ
͚
i
P(J
i
)•
f ←i
͔
cross sections, as a function of initial and final J states, respectively. Absolute transient
concentrations of the HCl(J
i
) and HCl(J
f
) are measured directly from absorbances via Beer’s Law,
and used to extract absolute collisional cross sections. The results are compared with quasiclassical
trajectory ͑QCT͒ calculations on a hybrid ab initio/LEPS surface of Schatz and Gordon, which
prove remarkably successful in reproducing both the J dependent trends and absolute values of the
state-resolved gain and loss collision cross sections. © 1995 American Institute of Physics.
I. INTRODUCTION
A detailed understanding of the state-to-state quantum
dynamics of both reactive and inelastic collision phenomena
has long been a focus of the chemical physics community.
1–7
With recent advances in experimental methods for state
preparation and interrogation, there is now a wealth of stud-
ies which permit the monitoring of ‘‘full collision’’ dynamics
at a completely state-resolved level of detail. High resolution
supersonic jet spectroscopies have been successfully ex-
ploited to extend these energy transfer studies into the ‘‘half-
collision’’ regime by probing weakly bound complexes
trapped in potential wells due to van der Waals and/or hy-
drogen bond interactions.
8–11
Reactive H-atom transfer
events from this half-collision perspective have been studied
in detail by Neumark and co-workers via photodetachment
of corresponding anionic complexes.
12–16
There have also been ongoing studies of half-collision
reaction dynamics of ‘‘oriented’’ reaction intermediates in
neutral clusters, stimulated by the early work of Soep, Wittig,
and co-workers,
17,18
and now pursued directly in the time
domain by recent ultrafast experiments of Zewail, Wittig,
and co-workers.
19,20
In conjunction with initial state selection
provided by infrared-ultraviolet ͑IR-UV͒ double resonance
schemes pioneered by Crim and co-workers,
21
these methods
have been recently used by Plusquellic et al.
22
to study pho-
tolysis reaction dynamics in size- and quantum state-selected
clusters. Significant advances in theoretical methods
23–29
have also been demonstrated for calculating cross sections
for quantum and semiclassical reactive/inelastic scattering
processes from a trial potential surface for Eտ0, as well as
for calculating bound quantum states in the attractive region
for E Շ0. As a result of the combined successes in these
many areas, the experimental and theoretical tools have rap-
idly evolved for exploring the topology of reactive and non-
reactive potential surfaces.
Collisions of open shell species play a particularly im-
portant role in the detailed dynamics both of chemical reac-
tion and energy transfer phenomena. The product state dis-
tributions from prototypical A ϩBC radical reactions have
provided considerable insight into the qualitative nature of
the chemical reaction potential surface, as first demonstrated
in the elegant ‘‘arrested relaxation’’ chemiluminescence
studies
30,31
of Polanyi and co-workers. The experimental ad-
vances in recent years have made possible many further stud-
ies of chemical reaction dynamics in simple AϩBC open
shell systems at a quantum state-resolved level of descrip-
tion. For example, coherent Raman methods
32,33
have been
used by Aker, Valentini, and co-workers for a series of reac-
tive and inelastic studies of HϩHX and X
2
, where XϭCl,
Br, and I. More recently, multiple resonance techniques have
been implemented by the Zare group to preselect specific
v
,J
states in reagents, and thereby probe reactive scattering in
systems such as ClϩHCl(
v
) and CH
4
with full state-to-state
resolution.
34
Schemes for UV photolysis in tetratomic
HX–HX clusters by Wittig and co-workers
35
hold promise
for new insights into energy transfer and chemical reaction
dynamics in both X–H–X and H–X–H triatomic sys-
tems.
36,37
High resolution diode laser absorption methods
have been used by Flynn and co-workers
38–40
to study colli-
sional excitation of CO
2
, CO by hyperthermal H atoms, as
well as time resolved Fourier-transform infrared ͑FTIR͒
studies by Leone
41,42
and co-workers on vibrationally inelas-
tic and H atom transfer collisions in HϩDF and HϩH
2
O. In
each of these areas, detailed comparison of high level theory
and experiment provides the crucial forum for probing the
topology of a potential energy surface, both above and below
the barriers to chemical reaction.
One of the basic questions that such studies elucidate is
7046 J. Chem. Phys. 102 (18), 8 May 1995 0021-9606/95/102(18)/7046/13/$6.00 © 1995 American Institute of Physics