The ultraviolet photodissociation dynamics of hydrogen bromide
Paul M. Regan, Stephen R. Langford, Andrew J. Orr-Ewing, and Michael N. R. Ashfold
Citation: The Journal of Chemical Physics 110, 281 (1999); doi: 10.1063/1.478063
View online: http://dx.doi.org/10.1063/1.478063
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 110, NUMBER 1
1 JANUARY 1999
The ultraviolet photodissociation dynamics of hydrogen bromide
Paul M. Regan, Stephen R. Langford, Andrew J. Orr-Ewing, and Michael N. R. Ashfolda)
School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
~Received 1 June 1998; accepted 25 September 1998!
The technique of H Rydberg atom photofragment translational spectroscopy has been applied to
investigate the ultraviolet photodissociation dynamics of hydrogen bromide. Branching fractions
between the channels forming ground Br( 2 P 3/2) and spin-orbit excited Br( 2 P 1/2) atoms have been
determined at 15 independent wavelengths in the range 201–253 nm, and photofragment recoil
anisotropies for these two channels have been characterized at six different wavelengths within the
same wavelength range. The channel forming ground state products, H1Br( 2 P 3/2), is observed to
arise solely from a perpendicular ~i.e., DV51! transition at all excitation energies, whereas the
channel to formation of excited state products, H1Br( 2 P 1/2), has a marked wavelength dependence:
at long wavelengths ~l5243 nm!, the photofragments are produced by a parallel ~i.e., DV50!
photodissociation mechanism, which becomes more perpendicular in character as the photolysis
energy is increased. Within the wavelength range studied, the branching fractions indicate that
Br( 2 P 3/2) products are formed in preference to Br( 2 P 1/2) products, with propensities that are
relatively invariant to excitation wavelength, although a small, yet pronounced, cusp appears at
l;235 nm. The observations are discussed with reference to the known behavior of the other
hydrogen halides and highlight the influence of spin-orbit interactions in the photofragmentation
dynamics of this series of molecules. © 1999 American Institute of Physics.
@S0021-9606~99!01601-3#
tion of ground state ( 2 S) hydrogen atoms and halogen atoms
in either of the two ground state ( 2 P) spin-orbit components.
Measurements of angular distributions of photofragment velocities and cross sections for formation of product quantum
states reflect the evolution of the system from the FC region
into the asymptotic channels. To this end we have studied the
UV photodissociation of HBr, as a continuation of the related work1 on the photodissociation of HI and in light of
recent studies of HCl photolysis.2–4
As for all HX molecules, the first UV absorption band of
HBr5 is broad and featureless, typical of prompt dissociation.
Figure 1 shows a schematic representation of some of the
PESs to which electric dipole-allowed excitation can occur
within this absorption band; all these low-lying electronic
excited states are repulsive and correlate to neutral atoms:
I. INTRODUCTION
Photon-induced dissociations reveal how atomic and
molecular chemical interactions change as a system evolves
from the vicinity of the equilibrium ~short range! internuclear distance to regions of large separation. An initial electronic transition from a bound state is subject to angular
momentum selection rules determined, in part, by the Hund’s
coupling case effective in the Franck–Condon ~FC! region;
at the asymptotic limit, the electronic quantum states of the
photofragments will reflect both the initial absorption and the
degree of mixing between different electronic energy states
at extended bond lengths. The progression of the system between these two extremes can be described in terms of ~wave
packet! motion on one or more potential energy surfaces
~PESs! and the aim of photodissociation studies is to correlate the observed product state distributions with the PESs of
the parent molecule that led to dissociation. This approach
can become intractable for polyatomic molecules, for which
many multidimensional PESs may be involved in the photodissociation and, even for diatomics, the effect of several
interacting electronic states greatly complicates the understanding of the dynamics.
Hydrogen halides HX ~X5F, Cl, Br or I! are prototypical molecules that exemplify how the photochemistry
changes with atomic number of one atom: within this series
the spin-orbit interactions become stronger with increasing
nuclear charge. Near ultraviolet ~UV! photodissociation of
HX is known to involve several excited electronic states,
some of which are coupled. The PESs involved in the UV
photofragmentation of HX correlate adiabatically to forma-
HX1h n →H~ 2 S ! 1X~ 2 P 3/2!
→H~ 2 S ! 1X~ 2 P 1/2! .
~1b!
In keeping with tradition, we refer hereafter to the ground
state halogen atom X( 2 P 3/2) as X, and the excited state atom
X( 2 P 1/2) as X*. The branching fraction G between channels
1~a! and 1~b! is defined as:
G5
s @ X*#
,
s @ X# 1 s @ X*#
~2!
where s@X# and s@X*# are the cross sections for formation of
photofragment X and X*, respectively, at a particular wavelength.
The UV absorption continuum for the HX species is attributed, in varying degrees, to three transitions:6
a!
Electronic mail: [email protected]
0021-9606/99/110(1)/281/8/$15.00
~1a!
281
© 1999 American Institute of Physics
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J. Chem. Phys., Vol. 110, No. 1, 1 January 1999
FIG. 1. Schematic potential energy curves for the ground and first few
excited electronic states of HX plotted as a function of dissociation coordinate H-X. Two (0 2 ) states and a 3 P(2) state also correlate with the two
asymptotic channels, but are omitted from the diagram because they are all
electric-dipole forbidden from the X 1 S 1 (0 1 ) ground state.
3
1
P ~ 1 ! ←X 1 S 1 ~ 0 1 ! ,
~3!
3
P ~ 1 ! ←X 1 S 1 ~ 0 1 ! ,
~4!
P ~ 0 1 ! ←X 1 S 1 ~ 0 1 ! .
~5!
We choose to describe these transitions using both Hund’s
case ~a! and case ~c! notations in order to stress comparisons
as the coupling in HX develops predominantly from case
~a! for HF and HCl to case ~c! for HI.
Figure 1 also includes the 3 S 1 (1) state. Direct excitation to this state has been suggested7 as a contributor to the
UV absorption of HI, although recent experimental and theoretical photodissociation studies of HI,1 HBr,8 and HCl2,3
suggest that there is no direct population of the 3 S 1 (1) state
by absorption, but that it can be involved in the dissociation
process.
Adiabatically ~with the neglect of rotationally induced
couplings!, the perpendicular ~DV561! transitions ~3! and
~4! access states that correlate to H1X products, while the
3
P(0 1 ) state, excited via the parallel ~DV50! transition ~5!,
and the 3 S 1 (1) state correlate to H1X*.
In the case of HBr, Magnotta et al.9 measured the fractional yield of the Br and Br* spin-orbit state products following photolysis at 193 nm. The deduced branching fraction G was determined to be 0.15 but these workers could not
characterize the angular distributions of photofragment velocities. The first angular data were published by Wittig and
co-workers10,11 who studied the photodissociation at 193 nm
using velocity-aligned Doppler spectroscopy to interrogate
the H photofragment. They found that both spin-orbit states
of bromine were produced by predominantly perpendicular
transitions at this wavelength, with Br* forming 14% of the
total bromine atom yield. From an analysis of Doppler profiles of the H photofragments formed by photolysis at 157
and 243 nm, Kawasaki and co-workers12 concluded that excitation at both wavelengths had predominantly perpendicular character. However, the resolution of the data was insufficient to distinguish signal from different spin-orbit channels
and no attempt could be made to quantify the angular nature
of the two dissociation paths. Kinugawa and Arikawa13 combined ion imaging with Doppler spectroscopy of the H atom
fragments at 243 nm to obtain a value of G50.11 and angular distributions for both channels: as at 193 nm, a purely
perpendicular recoil velocity distribution was observed for
H1Br products but, in contrast to the shorter wavelength
data, the H1Br* channel exhibited a purely parallel distribution. Recently, Péoux et al.8 applied time-independent,
time-dependent, and semiclassical methods, based on their
ab initio calculated electronic potential curves, transition
moments, spin-orbit constants and coupling matrix elements
@also including the 3 S 1 (1) state# to calculate the excitation
energy dependence of the product spin-orbit branching ratios
over the UV absorption band of HBr.
Following our work on the UV photodissociation of HI1
we have applied the technique of H Rydberg atom photofragment translational spectroscopy ~PTS! to a study of the photolysis of jet-cooled HBr molecules. Measurements of
branching fractions and recoil anisotropies for photofragments formed in channels 1~a! and 1~b! are reported for 15
selected photolysis wavelengths within the first UV absorption band. The results illustrate significantly different photodissociation dynamics for HBr compared with both HI and
HCl, which can be rationalized in terms of mixing between
different electronic states within the FC region ~different
relative transition probabilities! and further mixing in the
exit channels ~nonadiabatic couplings!.
II. EXPERIMENT
Full details of the H Rydberg atom PTS experiment have
been presented in previous papers14–18 and we recount here
only the salient points. A molecular beam of HBr is created
in a differentially pumped chamber by skimming the gas
injected from a pulsed nozzle ~General Valve Series 9!
backed by a ~typically! 2% mixture of HBr in argon gas
~total pressure ;760 Torr!. The output of a pulsed Nd:YAG
pumped dye laser ~Spectra-Physics GCR-270 plus PDL 2!
which is either frequency tripled ~using KDP and BBO crystals, for l<220 nm! or doubled ~in BBO, l>220 nm!
crosses the molecular beam at right angles and photolyzes
HBr molecules. This light is linearly polarized with the electric field vector e aligned relative to the H atom time-offlight ~TOF! axis at an angle u that is set using a polarization
rotator ~Newport RFU 21 Fresnel rhomb!. To obtain photofragment angular distributions it is necessary to record the H
photofragment flux at values of u extending over ~at least!
one quadrant. After a delay of ;10 ns, two laser pulses,
counterpropagating to the photolysis beam, tag the resultant
H( 2 S) atom by exciting to a high-n Rydberg state via a
double resonance process ~l5121.57 nm and ;365 nm!.
These tagged H atoms are then allowed to travel field free
along a 425.1 mm long tube perpendicular to the plane defined by the molecular beam and laser pulses, at the end of
which they are field ionized and detected by a Johnston multiplier ~MM1-SG! mounted behind a 32 mm diameter aperture. The amplified output from the multiplier is displayed on
a digital oscilloscope ~LeCroy 9450, 350 MHz bandwidth! as
a TOF transient, which is transferred for collection to a personal computer via a general purpose interface bus ~GPIB!
interface. Measurements were made from ~at least! three data
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J. Chem. Phys., Vol. 110, No. 1, 1 January 1999
FIG. 2. H photofragment total kinetic energy release ~TKER! spectrum of
the 222 nm photodissociation of a jet-cooled sample of HBr, converted from
the raw time-of-flight ~TOF! data using Eq. ~6!, recorded with u m 554.7°.
sets, each of which typically required an accumulation of
;3000 laser shots to obtain a good signal-to-noise ratio.
The related study of HI photodissociation1 describes a
number of examinations of possible experimental artefacts
which may bias the detection sensitivity towards one or another product channel. These include: Doppler selectivity of
the H atom tagging which, even using the line center of the
Lyman-a resonance, might introduce a bias in favor of the
slower, H1X* channel; photofragment fly-out from the
probe laser volume during the time delay between photolysis
and probe laser pulses which might discriminate against the
faster, H1X channel, saturation of photolysis or Rydberg
tagging absorption steps, impure polarization of the photolysis laser or inaccurate setting of u, and photolysis of clusters
formed in the supersonic expansion of the 2% HX in Ar
mixture. The protocols developed to test for these sources of
potential error are described in detail elsewhere1 and these
effects were shown to be negligible in the studies of HI photodissociation. The smaller spin-orbit constant for Br, compared with I, means that the difference in the recoil velocities
for H1Br and H1Br* photofragments is smaller than for HI,
which would reduce further any potential velocity-dependent
bias in our H atom detection process. However, to confirm
this, similar experimental tests and computer simulations of
HBr photodissociation were performed at a number of the
excitation wavelengths employed in this study. Again, it was
concluded that there was no experimental bias in the detection of either of the photodissociation channels.
III. RESULTS
A. Bond dissociation energy of HBr
Conversion of a raw TOF spectrum into a total kinetic
energy release ~TKER! spectrum involves application of the
general relationship:
H
1
mH
TKER5KEH1KEX5 m H 11
2
mX
JH J
d 2
,
tH
~6!
where t H is the time of flight, and d5425.1 mm is the TOF
distance.
283
Figure 2 shows the H atom TKER spectrum transformed
from the TOF spectrum recorded following photodissociation at 222 nm of a molecular beam of 2% HBr in Ar, with
the electric field vector e of the photolysis laser set at 54.7°
~the magic angle! to the TOF axis. Two peaks are evident
indicating that only two product channels contribute at this
photolysis energy: the signal at higher TKER is attributed to
H1Br production, and the signal at lower TKER is assigned
to the H1Br* channel. The observed energy splitting is in
accord with the known separation of the spin-orbit components of the bromine photofragment ~3685 cm21!.19 Similar
data have been recorded at 15 excitation wavelengths between 201 and 253 nm for HBr molecules, using gas expansion conditions that ensured photolysis of predominantly
HBr (J50,1) molecules: in the companion paper on HI
photodissociation,1 it was demonstrated that the H atom PTS
technique is capable of resolving the rotational structure of
HI parent molecules (B;6.5 cm21 ), and hence a fortiori
HBr molecules (B;8.5 cm21 ). Therefore, the unresolved
peaks for each product channel in the TKER spectra are attributed to photofragments from low-J parent molecules. Energy conservation dictates that for a parent molecule with
v 50 and J50:
h n 5TKER1E ~ Br!int1D 00 ~ H–Br! ,
~7!
where E(Br!int is the internal energy of the bromine photofragment, and D 00 (H–Br) is the bond dissociation energy. By
this calculation we are able to provide a refined estimate for
D 00 (H–Br!530 210640 cm21 , which compares favorably
with the value quoted in Okabe,20 but is lower than the value
cited in Huber and Herzberg.21
B. Spin-orbit branching fractions
The photofragment recoil velocity distribution from a
one-photon electric dipole allowed photodissociation of randomly oriented molecules is given by:22
I ~ v̄ , u ! 5
1
f ~ v !@ 11 b P 2 ~ cos u !# ,
4p
~8!
where f ( v ) is the product speed distribution ~which for photolysis of a single quantum state of HX corresponds to a
single speed for each product channel!, b is the spatial anisotropy parameter, P 2 (cos u)5 21(3 cos2 u21) is the second
order Legendre polynomial and cos u5e•v̄.
For a prompt dissociation—as is the case for HBr subsequent to excitation in the UV absorption band—the anisotropy parameter has limiting values of b512 when dissociation follows excitation with a transition dipole moment
parallel to the internuclear axis ~DV50!, and b521
when fragmentation follows excitation involving a transition
dipole moment perpendicular to the internuclear axis ~DV
561!.
Measurements of branching fractions require elimination
of bias in the experimental detection caused by different photofragment anisotropies for different channels. This can be
achieved in two ways: ~i! setting the angle u m 554.7° makes
the term P 2 (cos u)50, thus removing any angular bias of the
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J. Chem. Phys., Vol. 110, No. 1, 1 January 1999
FIG. 3. Average Br* photofragment branching fractions G, resulting from
photolysis of jet-cooled HBr molecules measured in the present work ~d!
plotted as a function of excitation wavelength. The 2s error bar derived
from the spread of 30 measurements of G at 233 nm is applied to G at all
wavelengths. Also shown are previous experimental determinations of G, by
the groups of Leone ~Ref. 9! ~h!, Wittig ~Refs. 10 and 11! ~n! Kinugawa
~Ref. 13! ~s!, and Buck ~Ref. 23! ~,!, and the theoretical predictions of
Péoux et al. ~Ref. 8! ~- - -!.
signal; ~ii! taking a 1:2 weighted linear combination of signal
recorded at u50° and 90°, respectively, forms a sum which
is independent of u, since:
I i ~ u 50° ! }11 b ,
I' ~ u 590° ! }12
~9a!
b
.
2
~9b!
Method ~i! is practically more expedient than method ~ii!
because only one measurement is required, which reduces
the data collection, and hence is the preferred choice. However, as a procedural self-check, at wavelengths for which
recoil anisotropies were measured, the branching fractions
determined by method ~i! were compared with the values
obtained by method ~ii!, and were always found to be consistent.
Figure 3 shows average fractional branching yields of
Br*, G, for 15 photolysis wavelengths spanning 201–253
nm. The experimental reproducibility of G at 233 nm was
ascertained by making 30 independent measurements, and
the 2s error bar associated with G at this wavelength
FIG. 5. Best-fit anisotropy parameters b from the measured H1Br ~s! and
H1Br* ~d! product angular distributions resulting from photolysis of a
jet-cooled sample of HBr molecules, plotted as a function of excitation
wavelength. The error bars reflect the worst-fit values as described in the
text. Also shown are the previous measurements by Kinugawa ~h!, ~j!
~Ref. 13! and Wittig ~n!, ~m! ~Refs. 10 and 11!. Open symbols are for
H1Br; filled symbols are for H1Br*.
is applied to the values of G at all other reported wavelengths. Also included in Fig. 3 are other experimental
values,9–11,13,23 and the calculated wavelength dependence of
Péoux et al.8 Considering the expanded scale of the ordinate
in Fig. 3, there is a very good qualitative consensus between
experiment and theory: formation of Br is predominant over
Br* at all wavelengths studied, and the branching between
the two spin-orbit components is generally insensitive to
change in the excitation energy ~in stark contrast to HI photolysis, vide infra!. Quantitatively, there is a notable difference between the experimental values and theoretical predictions. The observations indicate a higher fractional yield of
Br* than the theoretical work suggests, and, particularly in
the range of 230–240 nm, there is a distinct cusp in the plot
of G against photolysis wavelength.
C. Photofragment angular distributions
Measuring the signal intensity as a function of u renders
the angular distribution of the H photofragment recoil velocities. Experimentally, the angular anisotropy is obtained by
recording three data sets in succession, the first and last at
u m 554.7° and the middle set at a selected angle between 0°
and 110°. Data sets for the selected angle are normalized, to
compensate for laser power drift, with reference to data recorded at the magic angle. This process is repeated for six
selected angles. A function was fitted to these normalized
plots using Eq. ~8! which yields a value for b, the anisotropy
parameter. Figure 4 shows sample normalized angular distributions and fits for the higher and lower energy components
of the TKER spectrum recorded for the photodissociation of
HBr at 222 nm. Note that all signal intensities, I( v , u ), are
normalized against the signal intensity at u590° of the
higher energy channel, i.e., I(H1Br, 90°!, and the error on
this signal is uniformly applied to all other data points at a
given wavelength. The error quoted on the anisotropy parameter represents the worst-fit value, which we define as the
anisotropy parameter that fits to the lowest ~highest! intensity
limit of u50° and the highest ~lowest! intensity limit of
u590°.
FIG. 4. Angular distributions of the H1Br ~s! ~right-hand axis! and
H1Br* ~d! ~left-hand axis! photofragments detected following 222 nm
photolysis of a jet-cooled sample of HBr molecules. Points are experimental
data; the curves are fits in terms of Eq. ~8!. The best-fit anisotropy parameters b are 21 and 11.4, respectively. The error bar associated with the
highest intensity signal, i.e., I(H1Br, 90°!, is attributed to all points.
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Regan et al.
J. Chem. Phys., Vol. 110, No. 1, 1 January 1999
A plot of the best-fit anisotropy parameters for H photofragments formed in conjunction with Br and Br* at a number of near UV excitation wavelengths is shown in Fig. 5.
Each anisotropy parameter and associated error bar is assessed as described above. The ground spin-orbit product Br
is seen to arise from a purely perpendicular photolysis
~b521! at all observed excitation energies. In contrast, the
angular distributions of the excited spin-orbit products Br*
indicate a dissociation mechanism that is very dependent on
the photolysis wavelength. At the longest wavelength employed in this study we observe formation of Br* via a parallel photodissociation ~b512!; as the excitation energy increases, the anisotropy parameter decreases monotonically.
This trend is in keeping with previous results at 243 nm13
and 193 nm10,11 which showed ~purely! parallel and ~predominantly! perpendicular character, respectively, for formation of Br* photofragments.
IV. DISCUSSION
To interpret these experimental results for HBr in terms
of dissociation on multiple PESs, it is valuable to draw comparisons with the known photodissociation dynamics of the
other HX molecules. First, the differences in behavior of the
various HX species upon optical excitation are described,
and then the different dynamics on coupled PESs are considered.
Formally in Hund’s case ~a! only transition ~3! to the
1
P(1) state is allowed: for example, the first absorption
band of the lightest hydrogen halide HF is considered ~see
Ref. 24 and references therein! to be carried solely by this
excitation. Similarly, calculations25 carried out on ab initio
PESs indicate that transition ~3! carries almost all ~.99.5%!
of the oscillator strength in the UV absorption by HCl. A
small contribution to the absorption arises from transition ~5!
to the 3 P(0 1 ) state, due to spin-orbit mixing ~of the e parity
rotational levels! of the nominal 3 P(0 1 ) state with the
X 1 S 1 (0 1 ) state, indicating a slight deviation from pure
case ~a! character.
Transition ~4! to the 3 P(1) state gains oscillator strength
by spin-orbit mixing of this state with the 1 P(1) state, and in
the case of the UV absorption of HI, transitions ~4! and ~5!
become comparable in importance to transition ~3!, consistent with case ~c! description. The relative probabilities of
transitions ~4! and ~5! in the equivalent spectrum of HBr
have been addressed theoretically by several groups:6,8,26
Briefly, transitions ~4! and ~5! are optically active @as well as
transition ~3!# and are significant in the long-wavelength tail
of the UV absorption, but decline in importance as the excitation energy increases.
As noted in Sec. III B, the symmetry of the absorption
step ~DV! leading to a prompt molecular dissociation is revealed by the anisotropy parameter ~b!. Once the nature of
the PES that is initially populated is established, it remains to
determine with which asymptotic product channel the PES
correlates. Two pictures are used commonly to illustrate correlations between separated photofragments and molecular
~excited! electronic states.27 These representations follow
naturally from the Born–Oppenheimer approximation, with
285
separation of the nuclear motion, described by the operator
T̂ nuc , from the electronic Hamiltonian Ĥ elec in the total molecular Hamiltonian Ĥ tot :
Ĥ tot5T̂ nuc1Ĥ elec .
~10!
Ĥ elec can be factorized further into an electrostatic and a
spin-orbit contribution. Within the diabatic picture, electronic wave functions can be chosen that diagonalize the
electrostatic term of the electronic Hamiltonian and approximately diagonalize the nuclear kinetic energy operator T̂ nuc .
The diabatic version of Fig. 1 would show all states correlating to a single @ H( 2 S)1X( 2 P) # limit: the spin-orbit interaction lifts the degeneracy of the asymptotic channels and
the off-diagonal elements in the spin-orbit operator cause a
mixing of diabatic states. The dissociation can be visualized
physically as a recoupling of angular momentum induced by
the spin-orbit interaction: at some internuclear separation,
the energy difference of the various diabatic states becomes
comparable to the spin-orbit splitting, and so there is a transformation from case ~a! or case ~c! to the case ~e! coupling
limit. This recoupling results in the formation of both
X( 2 P 3/2) and X( 2 P 1/2) products.
In the alternative, adiabatic representation, the electronic
Hamiltonian ~now including spin-orbit coupling! is diagonalised to give Born–Oppenheimer PESs. These PESs are now
coupled by the nuclear kinetic energy operator. Figure 1 is an
adiabatic representation of the PESs for the HX system. In
the limit of infinitely slow recoil of photofragments, the correlations of PESs with product channels shown in Fig. 1 are
rigorous. However, for separation at a finite velocity, adiabatic states become coupled and nonadiabatic transitions occur. Evidence of these transitions during dissociation is revealed in the branching fraction G. Consider, for example,
excitation to the 1 P(1) state. If the subsequent dissociation
proceeds adiabatically, then G50 ~adiabatic limit!. Conversely, G51/3 ~statistical limit!, as determined by the degeneracies of the spin-orbit states, if during the separation
there is complete nonadiabatic coupling between the 1 P(1)
and 3 S 1 (1) states ~since the selection rule DV50 applies in
the absence of rotationally induced coupling!.
As discussed below, the strong spin-orbit coupling in HI
means that adiabatic potentials have mixed singlet/triplet
character—case ~a! labels are inappropriate in this case, although it is useful to retain them for the purposes of
comparison—and these adiabats are likely to be widely separated. For HCl, however, the spin-orbit coupling is
much smaller than the electrostatic term in the electronic
Hamiltonian, and spin remains a good quantum number for
the adiabats. HBr is anticipated to have behavior somewhere
between these limits, hence, to put the present HBr results in
context, comparisons are drawn with recent studies of HI,1
HCl,2,3 and HF24 photolysis. The subsequent discussion summarizes the current understanding of the photodissociation
dynamics of HI and HCl before interpreting the results for
HBr.
Langford et al.1 measured H photofragment angular distributions ~b parameters! and branching fractions G following UV photolysis of HI at wavelengths in the range 201 and
303 nm. To supplement the following discussion, the branch-
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J. Chem. Phys., Vol. 110, No. 1, 1 January 1999
FIG. 6. Average I* photofragment branching fractions G, resulting from
photolysis of jet-cooled HI molecules, ~after Ref. 1!.
ing fractions from this study are presented in Fig. 6. Ground
state iodine photofragments I, form by initial perpendicular
excitation ( b I521) to, and subsequent separation on, the
1
P(1) and/or 3 P(1) surfaces, whereas spin-orbit excited
products I* are produced by a parallel ( b I* ;12) initial
excitation to, and dissociation on, the 3 P(0 1 ) surface, which
gains oscillator strength through spin-orbit mixing with the
ground state. These assignments are consistent with the expected behavior of the energy dependence of the partial absorption cross sections for transitions ~3!–~5!, based on the
energy ordering of states in the FC region: at the longest
wavelengths ~l.270 nm! G is small, reflecting the relative
strength of the 3 P(1)←X 1 S 1 (0 1 ) transition; excitation at
shorter wavelengths ~l,230 nm! results also in low branching fractions, signifying the prevalence of the 1 P(1)
←X 1 S 1 (0 1 ) transition; however, within the intermediate
wavelength range ~l5230–270 nm!, I* photofragments are
formed in excess of I, implying a dominance in this region of
the 3 P(0 1 )←X 1 S 1 (0 1 ) transition. The photochemistry
for this putative Hund’s case ~c! molecule is thus determined
primarily by strong spin-orbit interactions, effective in the
FC region, which ensure that a coherent superposition of
continuum wave packets on three ~adiabatic! PESs is created
by absorption. Subsequent wave packet evolution takes place
via an adiabatic mechanism, i.e., dissociation occurs predominantly on the initially populated PES.
In contrast to HI, spin-orbit interactions appear to be
negligible for HF and HCl in the FC region. Angular distributions of photofragments and the product branching fraction G following excitation of HF ( v 53) molecules at 193
nm have been resolved and show that both product channels
are formed by purely perpendicular transitions ~b F521;
b F* 521!, with G50.42.24 Similar behavior is observed for
HCl: the product recoil anisotropies reported for excitation at
193 nm illustrate that Cl and Cl* photofragments are formed
via predominantly perpendicular excitations ~b Cl521;
b Cl* ;21!, and G was measured to be 0.41.3 These results,
together with a detailed wavelength-dependent study2 of the
spin-orbit branching fractions of HCl photolysis that demonstrate the preferential formation of Cl over Cl*, have been
successfully rationalized in terms of an initial ~spin-allowed!
perpendicular excitation to the 1 P(1) state, with subsequent
nonadiabatic coupling to other V51 states at extended bond
Regan et al.
lengths: in particular, coupling to the 3 S 1 (1) state, which
forms Cl* photofragments. Excitation to the 3 P(0 1 ) state is
calculated to make a small contribution to the total absorption cross section. This state is ~weakly! spin-orbit coupled
with the ground state. This contribution is thus expected to
be more significant at long wavelengths, when excitation occurs at large internuclear distance and the mixing of states is
stronger. Theoretical predictions25 indicated that this contribution is ,0.5% of the total absorption, although experimental evidence3 ( b Cl* 520.9460.07) suggests a value of
0.8%. We have made a preliminary measurement of photofragment distributions following photolysis of HCl at 205.5
nm, as a prelude to a more extensive study.4 Branching fractions of 0.4860.05 and 0.4160.04 were measured with u set
at 54.7°, and 90°, respectively, suggesting that there is a
parallel component to the initial excitation at this wavelength. The signal measured with u50° is—as expected—
very low, but it is evident from the TOF spectrum that the
H1Cl* contribution to this spectrum has greater intensity
than the H1Cl channel. This, too, supports the view that a
parallel excitation populates the 3 P(0 1 ) state which correlates adiabatically with the H1Cl* channel.
The difference in behavior of HI and HCl can be summarized qualitatively using an adiabatic picture: the nuclear
kinetic energy operator couples adiabatic PESs, and the
smaller the difference in energy of the adiabats, the greater
the influence of nonadiabatic coupling. For HI, PESs are
always well separated in energy, either by electrostatic interactions, or by strong spin-orbit splitting ~both at short and
long bond lengths! whereas for HCl, the large energy separation of the adiabats ~caused by electrostatic interactions!
decreases at extended bond lengths and asymptotically becomes the small spin-orbit splitting of the isolated Cl atom.
Thus, the indication is that the adiabats of Hl are energetically too dissimilar for significant nonadiabatic coupling to
occur, although the opposite is true for HCl. Given our
knowledge of the relative spin-orbit constants of the halogen
atoms, HBr can reasonably be expected to have spin-orbit
interactions of intermediate character between HCl and HI.
This study illuminates how the intermediate spin-orbit coupling is revealed in the UV photodissociation process, by
comparison with the known behavior of the other hydrogen
halides.
Production of the ground state H1Br channel by a perpendicular transition at all wavelengths suggests a mechanism similar to formation of H1I: initial excitation, and
subsequent dissociation, involves the 1 P(1) and/or 3 P(1)
states. Calculations8 show that at short wavelengths ~l5193
nm! the partial absorption cross section to the 1 P(1) state is
greater than to the 3 P(1) state, but at long wavelengths
~l5243 nm!, the transition probabilities to these two states
become comparable, as in HI. In stark contrast to the recoil
anisotropies of all other HX photofragments, the b parameter
for the H1Br* channel has a very marked dependence on
initial excitation energy. At 193 nm10,11 this channel is produced primarily by a perpendicular transition ( b Br* ;21)
but, as Fig. 5 shows, it becomes progressively more parallel
in character with increasing photolysis wavelength to the
limit of pure parallel photodissociation ( b Br* 512) at 243
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Regan et al.
J. Chem. Phys., Vol. 110, No. 1, 1 January 1999
nm, consistent with the result of Kinugawa and Arikawa.13
These observations are readily understood if we assume that,
at shorter wavelengths, the dominant route to H1Br* products involves initial excitation to the 1 P(1) state and subsequent nonadiabatic coupling with the 3 S 1 (1) state as the
H–Br bond extends ~c.f. HCl! but that at longer wavelengths
direct 3 P(0 1 )←X 1 S 1 (0 1 ) excitation ~c.f. HI! takes over
as the dominant route for formation of this channel as the
mixing of the 3 P(0 1 ) and the X 1 S 1 (0 1 ) states lends oscillator strength. This picture of the photodissociation is supported by detailed theoretical calculations8 and illustrates
that HBr shows, in part, the excitation to the triplet and singlet states evident in HI, and also the long-range nonadiabatic transitions more characteristic of HCl.
With two different dissociation mechanisms at work, it
is perhaps surprising that the branching flux between the two
exit channels is relatively insensitive to the fragmentation
dynamics. However, this proposal of two mechanisms, sustaining a near constant flux into the H1Br* channel, is supported by the theoretical predictions of Péoux et al.8 based
on ab initio calculated electronic potential energy curves,
rotational and spin-orbit coupling elements, and electronic
transition moments: as the significance of one mechanism
declines, the other increases in importance and coincidentally compensates almost exactly in terms of Br* yield. The
small cusp in the plot of G against wavelength suggests that,
at around 235 nm, there is a cooperative effect of the two
mechanisms to produce Br* products, and replicating the occurrence of this cusp will be a good test for theoretical models of HBr photodissociation.
287
orbit mixings. This study has illustrated that the photodissociation of HBr can be understood only if substantial spinorbit coupling affects the dynamics in both of these regimes,
in qualitative agreement with the theoretical predictions8
based on ab initio electronic structure calculations. The small
discrepancy between experimentally observed and theoretically calculated branching fractions suggests the need for
some refinement of the ab initio data. This improvement
could be greatly expedited by a detailed study of the photodissociation of vibrationally excited HBr molecules which
could resolve better the relative placements of the excited
state potential energy surfaces.
ACKNOWLEDGMENTS
Financial support from the Engineering and Physical
Sciences Research Council ~EPSRC! is gratefully acknowledged, as is the help and advice of our colleagues: Professor
R.N. Dixon, Professor G.G. Balint-Kurti, K. N. Rosser and
P. A. Cook. We thank also Professor M. H. Alexander ~University of Maryland! for helpful discussions. M.N.R.A.,
S.R.L. and P.M.R. are grateful to EPSRC for, respectively,
the award of a Senior Research Fellowship, postdoctoral financial support, and a studentship. A.J.O.E. is grateful to the
Royal Society for the award of the Eliz. Challenor Research
Fellowship.
1
S. R. Langford, P. M. Regan, A. J. Orr-Ewing, and M. N. R. Ashfold,
Chem. Phys. 231, 245 ~1998!.
2
H. M. Lambert, P. J. Dagdigian, and M. H. Alexander, J. Chem. Phys.
108, 4460 ~1998!.
3
J. Zhang, M. Dulligan, and C. Wittig, J. Chem. Phys. 107, 1403 ~1997!.
V. CONCLUSION
4
P. M. Regan, S. R. Langford, A. J. Orr-Ewing, and M. N. R. Ashfold
~unpublished!.
The technique of H Rydberg atom PTS has been applied
5
B. J. Huebert and R. M. Martin, J. Chem. Phys. 72, 3046 ~1968!.
to the photodissociation of jet-cooled HBr molecules at 15
6
R. S. Mulliken, Phys. Rev. 51, 310 ~1937!; J. Chem. Phys. 8, 382 ~1940!.
7
different wavelengths between 201 and 253 nm. The meaG. N. A. van Veen, K. A. Mohamed, T. Baller, and A. E. de Vries, Chem.
sured branching fractions G and photofragment angular disPhys. 80, 113 ~1983!.
8
G. Péoux, M. Monnerville, T. Duhoo, and B. Pouilly, J. Chem. Phys. 107,
tributions ~b parameters! are found to be in good agreement
70 ~1997!.
with other, more limited, experimental data and with theoret9
F. Magnotta, D. J. Nesbitt, and S. R. Leone, Chem. Phys. Lett. 80, 21
ical predictions. The branching fractions show that the H1Br
~1981!.
10
Z. Xu, B. Koplitz, and C. Wittig, J. Chem. Phys. 87, 1062 ~1987!.
products are formed in preference to H1Br* at all wave11
Z. Xu, B. Koplitz, and C. Wittig, J. Phys. Chem. 92, 5518 ~1988!.
lengths. There is a small variation of G with excitation wave12
Y. Matsumi, K. Tonokura, M. Kawasaki, and T. Ibuki, J. Chem. Phys. 93,
length, although not as pronounced as in HCl and HI, with a
7981 ~1990!.
13
distinct cusp at l;235 nm. The photofragment angular disT. Kinugawa and T. Arikawa, J. Chem. Phys. 96, 4801 ~1992!.
14
L. Schnieder, W. Meier, K. H. Welge, M. N. R. Ashfold, and C. M.
tribution for the H1Br channel, at all reported wavelengths,
Western, J. Chem. Phys. 92, 7027 ~1990!.
is well characterized by b521, whereas, for the H1Br*
15
G. P. Morley, I. R. Lambert, M. N. R. Ashfold, and C. M. Western, J.
channel, b512 at long wavelengths ~l5243 nm! and beChem. Phys. 97, 3157 ~1992!.
16
comes increasingly more perpendicular in character as the
D. H. Mordaunt, I. R. Lambert, G. P. Morley, M. N. R. Ashfold, R. N.
Dixon, C. M. Western, L. Schnieder, and K. H. Welge, J. Chem. Phys. 98,
excitation wavelength decreases. These results have been
2054 ~1993!.
discussed with reference to the recent extensive studies of
17
S. H. S. Wilson, M. N. R. Ashfold, and R. N. Dixon, J. Chem. Phys. 101,
HCl and HI photolysis in order to develop an understanding
7538 ~1994!.
18
of the photodissociation dynamics of the hydrogen halide
M. N. R. Ashfold, D. H. Mordaunt, and S. H. S. Wilson, in Advances in
Photochemistry, edited by D. C. Neckers, D. H. Volman and G. von
series. In particular, the influence of spin-orbit interactions in
Bünau ~Wiley, New York, 1996!, Vol. 21, p. 217.
HX molecules is shown to be significant for different regions
19
C. E. Moore, Atomic Energy Levels ~U.S. Government Printing Office,
of internuclear separation as X changes, as revealed by the
Washington, D.C., 1971!, NSRDS-NBS 35.
20
symmetry of the initial absorption step and by the extent of
H. Okabe, Photochemistry of Small Molecules ~Wiley, New York, 1978!.
21
K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structhe nonadiabatic couplings of PESs in the subsequent dissoture. IV. Constants of Diatomic Molecules ~Van Nostrand-Reinhold, New
ciation process. For HCl, spin-orbit couplings are effective
York, 1979!.
22
only when the bond has lengthened significantly, whereas the
R. N. Zare, Angular Momentum ~Wiley, New York, 1988!.
23
U. Buck ~private communication!.
Franck–Condon region of HI is dominated by strong spinThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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288
24
Regan et al.
J. Chem. Phys., Vol. 110, No. 1, 1 January 1999
J. Zhang, C. W. Riehn, M. Dulligan, and C. Wittig, J. Chem. Phys. 104,
7027 ~1996!.
25
M. H. Alexander, B. Pouilly, and T. Duhoo, J. Chem. Phys. 99, 1752
~1993!.
26
D. A. Chapman, K. Balasubramanian, and S. H. Lin, Chem. Phys. 118,
333 ~1987!.
27
R. Schinke, Photodissociation Dynamics ~Cambridge University Press,
1993!. Cambridge, Chap. 15.
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