State-to-state reactive scattering of F+H2 in supersonic jets: Nascent rovibrational HF(v,J) distributions via direct IR laser absorption

Article abstract of DOI:10.1063/1.475120

Pulsed discharge sources of supersonically cooled F radicals have been crossed with supersonically cooled H₂ to study the F(²P_3/2,²P₁₂) + H₂→HF(v,J) + H reaction under single collision conditions with a collision energy of 1.8(2) kcal/mol. The HF(v,J) product states are probed via direct absorption of a single mode, tunable near IR laser perpendicular to the plane of intersection of the two jet axes. The high spectral resolution (Δv≈0.0001 cm^-1) permits the quantum state HF(v,J) distribution to be determined with complete resolution of final rovibrational levels. The J-dependent integral cross sections for HF(v = 3,J) are compared with exact quantum scattering calculations by Castillo and Manolopoulous on the ground adiabatic potential energy surface of Stark and Werner. Agreement between theory and experiment is quite good for low J (<3) states. However, theory substantially underpredicts the experimental distributions for high J (≥3) states near the energetic cutoff for ground spin orbit state F(²P_3/2) atoms, which may indicate the presence of non-adiabatic reaction channels involving spin orbit excited F*(²P_1/2) atoms.

Full text of DOI:10.1063/1.475120

State-to-state reactive scattering of F+H 2 in supersonic jets: Nascent rovibrational HF
v,J) distributions via direct IR laser absorption
(
William B. Chapman, Bradley W. Blackmon, and David J. Nesbitt
Citation: The Journal of Chemical Physics 107, 8193 (1997); doi: 10.1063/1.475120
View online: http://dx.doi.org/10.1063/1.475120
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/107/19?ver=pdfcov
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State-to-state reactive scattering of F؉H in supersonic jets: Nascent
2
rovibrational HF„v,J… distributions via direct IR laser absorption
a)
William B. Chapman, Bradley W. Blackmon, and David J. Nesbitt
JILA, University of Colorado and National Institute of Standards and Technology and Department of
Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440
͑
Received 29 July 1997; accepted 9 September 1997͒
Pulsed discharge sources of supersonically cooled F radicals have been crossed with supersonically
2
2
cooled H to study the F( P , P )ϩH →HF(v,J)ϩH reaction under single collision conditions
2
3/2
1/2
2
with a collision energy of 1.8͑2͒ kcal/mol. The HF(v,J) product states are probed via direct
absorption of a single mode, tunable near IR laser perpendicular to the plane of intersection of the
Ϫ1
two jet axes. The high spectral resolution (⌬␯Ϸ0.0001 cm )permits the quantum state
HF(v,J)distribution to be determined with complete resolution of final rovibrational levels. The
J-dependent integral cross sections for HF(vϭ3,J) are compared with exact quantum scattering
calculations by Castillo and Manolopoulous on the ground adiabatic potential energy surface of
Stark and Werner. Agreement between theory and experiment is quite good for low J ͑Ͻ3͒ states.
However, theory substantially underpredicts the experimental distributions for high J͑у3͒ states
2
near the energetic cutoff for ground spin orbit state F( P3 )atoms, which may indicate the presence
/2
2
of non-adiabatic reaction channels involving spin orbit excited F*( P1/2) atoms. © 1997 American
Institute of Physics. ͓S0021-9606͑97͒05043-5͔
2
2–24
I. INTRODUCTION
beam methods by both Lee and co-workers at Berkeley
and Toennies and co-workers at Goettingen2
5–27
have been
Ever since the pioneering work of Polanyi and co-
workers on atomϩdiatom reactive collision dynamics,1–3
there has been an intense interest in reactive scattering on
used to investigate the differential reactive scattering of
FϩH ͑and isotropic variants͒ for a series of center-of-mass
2
collision energies. Due to limited energy resolution in these
time-of-flight studies, however, only vibrational product lev-
els could be resolved, with limited information on HF rota-
tional distributions inferred from contour analysis. There has
recently been a report from the Keil group of a measurement
based on HF chemical laser excitation and bolometric detec-
tion that provides angularly resolved reactive scattering in-
formation on a single HF product quantum state.28
The thrust of this communication is to report a new IR
laser based method for obtaining nascent product state dis-
tributions from FϩH under single collision conditions. Our
approach is based on the following combination: ͑i͒ A pulsed
supersonic discharge source of F atoms is collided with a
FϩH →HF(v,J)ϩH
from
the
chemical
physics
2
4
community. The reasons for strong theoretical interest in
this fundamental chemical reaction are readily appreciated.
First of all, the FϩH system is an excellent prototype of a
2
low barrier exothermic chemical reaction, yet small enough
in total electron number and nuclear degrees of freedom to
be tractable via high level ab initio calculations. This has led
5
–10
to the development of several potential energy surfaces,
classical,1
1,12
which has facilitated detailed
and
1
3–15
quasi-classical
studies of the FϩH reaction dynamics,
2
2
as well as prediction of electron energy distributions from
Ϫ
16,17
͓
FH2͔ photodetachment studies.
Most importantly,
there have been breakthroughs in three atom quantum reac-
tive scattering that make feasible a numerically exact treat-
ment of the reaction dynamics for a given adiabatic potential
second pulsed jet source of H molecules under sufficiently
2
low densities to ensure single collision, molecular beam con-
ditions; ͑ii͒ the product HF(v,J) is probed in the intersection
region by high sensitivity direct absorption of a single mode
tunable IR laser; and ͑iii͒ as a function of laser tuning, these
spectral data yield Doppler limited absorbance profiles on
reactively scattered product HF(v,J)with complete resolu-
tion of final vibration/rotation quantum state. A complete
description of the experimental method and results will be
1
7–21
surface.
As a result, FϩH has evolved to become the
2
‘‘benchmark’’ chemical reaction system with which to com-
pare experiment against theory at a fully rigorous level.
This theoretical interest has been stimulated by corre-
sponding experimental efforts. Arrested relaxation methods
of Polanyi and co-workers were first used to probe the
HF(v,J) rotational distributions via low pressure FTIR
1
29
chemiluminescence methods. Due to long residence times in
presented elsewhere; this communication focuses on the
the FTIR detection region, however, substantial collisional
redistribution of the nascent HF product could occur; thus,
highest vibrational manifold ͓i.e., HF(vϭ3,J)] that is ener-
getically accessible at 1.8͑2͒ kcal/mol center of mass colli-
‘‘nascent’’ rovibrational distributions were estimated by ex-
sion energy, Ecom .
trapolation to the zero pressure limit. Crossed molecular
Such results provide the first opportunity for a fully rig-
orous comparison with exact quantum theoretical predictions
of reactive scattering by Castillo and Manolopoulos21 on the
a͒Staff member, Quantum Physics Division, National Institute of Standards
and Technology.
lowest adiabatic FϩH potential energy surface of Stark and
2
J. Chem. Phys. 107 (19), 15 November 1997
0021-9606/97/107(19)/8193/4/$10.00
© 1997 American Institute of Physics
8193
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8194
Letters to the Editor
tion on the ␯ CH stretch absorption band. Boltzmann analy-
3
sis of the CH rotational distribution estimates the rotational
4
temperature of the H reagent to be Ͻ30 K. Even at 100 K,
2
H is cooled essentially completely down into its lowest
2
nuclear spin allowed states, namely jϭ1 ͑ortho͒ and jϭ0
͑para͒ in a 3:1 ratio, with the first excited para (jϭ2) and
ortho (jϭ3)states down to 100- to 1000-fold.
The jets intersect 4.5 cm downstream of the nozzles,
where for a typical 200 Torr H backing pressure, the F
2
atoms have a reaction probability of only Ϸ2%. Thus the
probability of secondary inelastic collisions of the reaction
products can be neglected, as explicitly verified by H stag-
2
nation pressure studies. The temporal evolution of both gas
pulses is monitored with miniature hearing aid microphones
mounted inside the vacuum chamber on translational stages;
time delay studies of the gas pulses are used to measure the
velocity distributions for each beam. The H and F atom
2
5
5
beam velocities are 2.47(13)ϫ10 cm/s and 1.45(7)ϫ10
cm/s, respectively, which for the right angle collision geom-
etry translates into Ecomϭ1.8(2)kcal/mol. The 0.2 kcal/mol
uncertainty arises predominantly from the finite spread in
collision angles and is experimentally determined from Dop-
pler profiles and Monte Carlo modeling. This energy width is
more than threefold smaller than the rotational energy spac-
ing between Jϭ4 and 5, and thus has a negligible effect on
the product state distributions. The center-of-mass collision
energy is essentially equal to the 1.84 kcal/mol value used in
previous quantum calculations of Castillo and Manolopou-
los, which forms the basis of all comparison with theory in
this paper.
FIG. 1. Schematic diagram of the crossed jet direct absorption reactive
scattering experiment. Fluorine atoms produced in a discharge pulsed jet
expansion are intersected at a 90° angle 4.5 cm downstream with a pulse of
supersonically cooled H . Tunable single mode IR laser light is multipassed
2
perpendicular to the collision plane and probes HF(v,J) products by direct
absorption.
Werner.10 The agreement is found to be reasonably good, but
theory substantially underpredicts the high Jrotational distri-
butions near the energetic upper limit. These data provide
indications that nonadiabatic channels involving both ground
The HF(v,J) reaction products are probed by direct ab-
sorption of a single mode color center laser that is multi-
passed 16 times through the intersection region in a cylindri-
2
2
31
F( P )and spin orbit excited F*( P1/2) atoms may be par-
cal Herriot cell. Absorption measurements are performed
3
/2
ticipating in the reaction dynamics.
on the ⌬vϭϩ1 fundamental HF band, and measure popula-
tion differences between the upper and lower levels. Shot
noise limited absorption sensitivity is achieved by a combi-
nation of ͑i͒ dual beam differential detection on matched
InSb detectors and ͑ii͒ electrooptic servoloop control of the
color center laser intensity. All HF product signals are moni-
tored on ⌬vϭϩ1P or R branch transitions, the frequencies
for which are well determined and measured with a traveling
II. EXPERIMENT
The experimental apparatus for state-to-state reactive
scattering of FϩH is based on a modification of our earlier
2
apparatus for state-to-state inelastic scattering, and is de-
picted schematically in Fig. 1. A pulsed discharge is used to
generate high densities of F atoms upstream of the limiting
expansion orifice ͑800 ␮m diameter͒ of an axisymmetric su-
32
wave meter of the Hall and Lee design. This yields readily
Ϫ5
detectable HF signals at 10
absorbance levels, which
8
3
translate into sensitivities of Ͻ1ϫ10 /cm /quantum state.
personic jet. The stagnation gas is 5% F in He obtained
2
from a commercial excimer laser gas premix. The discharge
is struck by negatively biasing the orifice with respect to the
valve body by Ϫ600 V, which results in 35 mA currents
stabilized by a 5 k⍀ ballast resistor in series with the dis-
charge. Pulsed timing circuits are used to confine the dis-
charge to a 200 ␮s window near the peak of the full gas
pulse ͑800 ␮s͒.
III. RESULTS AND ANALYSIS
For each rotationally resolved transition, the time re-
solved HF signals are captured by a transient digitizer, inte-
grated over the pulse duration, and stored on computer as a
function of laser detuning. Sample results for J-dependent
absorption signals in the HF(vϭ3,J) manifold are shown in
Fig. 2, demonstrating velocity resolved Doppler profiles for
the nascent HF product. Since this is a coherent absorption
measurement, the HF(vϭ3,J) signals rigorously reflect
population differences between the upper and lower rovibra-
tional states. However, HF(vϭ4,Jϭ0) is energetically inac-
The H supersonic jet is formed through a 200 ␮m di-
2
ameter pinhole with a piezoelectric actuator based on the
design of Proch and Trickl.30 The column integrated number
density of H in the intersection region is directly calibrated
2
in a separate experiment by doping CH into the stagnation
4
gases at Ϸ1% level, and then monitoring direct IR absorp-
J. Chem. Phys., Vol. 107, No. 19, 15 November 1997
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Letters to the Editor
8195
FIG. 2. Sample absorption signals from HF(vϭ3) produced by reactive
scattering of F atoms with H . Nascent population is evident in all J levels
2
up the energetic limit for the 1.8͑2͒ kcal/mol center of mass collision en-
ergy.
FIG. 3. Nascent rotational distribution for FϩH →HF(vϭ3)ϩH at E
2
com
ϭ1.8(2) kcal/mol ͑circles͒ compared to full quantum reactive scattering
calculations ͑squares͒ by Castillo and Manolopoulos at 1.84 kcal/mol on the
lowest adiabatic surface of Stark and Werner. Note the substantial under-
prediction of populations in Jу3.
cessible to both F and F* at Ecomϭ1.8(2) kcal/mol; thus the
vϭ4←3 signals in Fig. 2 reflect pure absorbance due solely
to optical excitation out of the HF(vϭ3,J)manifold. These
absolute absorbances are rigorously converted33 to absolute
population densities per unit velocity subgroup by the IR line
strengths experimentally measured by Setser and co-workers
from chemiluminescence studies.34 We restrict our focus in
this paper on state resolved integral scattering cross sections
obtained by integrating over all Doppler velocity compo-
seen in Jϭ1 and 2. Given that these results are based on
fully ab initio calculations and exact quantum scattering
codes, this level of agreement serves to confirm the reliabil-
ity of the Stark and Werner potential surface for this ‘‘bench-
mark’’ atomϩdiatom reaction system.
nents, this yields the absolute column integrated populations
However, there are also substantial discrepancies be-
tween these theoretical predictions and experiment at higher
Jvalues. Specifically, theory systematically underpredicts the
populations in Jу3, by factors that greatly exceed the ex-
perimental uncertainty of the measurements. Indeed, the J
ϭ3 experimental values are nearly two-fold larger than theo-
retically predicted, while this factor grows to nearly six-fold
for Jϭ4. This effect is most dramatic in Jϭ5, which is an
energetically closed channel on the Stark and Werner surface
͑i.e., the integral cross section vanishes͒, whereas experimen-
tal signals clearly exist out to Jϭ5. Note that this is not
plausibly due to finite resolution, since the energy difference
between Jϭ4 and 5 is more than three-fold greater than
2
͑
i.e., molecules/cm )for a given final J state over the region
33
sampled by the probe laser beam.
IV. COMPARISON WITH THEORY
The highest level theoretical studies done to date on the
FϩH (j) system have been the quantum reactive scattering
2
21
calculations performed by Castillo and Manolopoulos,
which predict differential and integral cross sections into
given HF(v,J) states on the lowest adiabatic potential sur-
face of Stark and Werner.10 These differential cross sections
can be related to the experimentally observed column inte-
grated populations by center-of-mass to lab frame transfor-
mation. However, the resulting flux-to-concentration trans-
formation between integral cross sections and populations
for the current scattering geometry, kinematic mass combi-
nations and energetics proves to be essentially independent
of J. Thus to a very good approximation we can directly
compare the experimental column integrated populations
with the theoretical integral cross sections, averaged over the
experimental width in Ecom
.
As a final comment, it is worth speculating on what the
high J discrepancies between experiment and theory might
be due to. The quantum reactive scattering calculations of
Castillo and Manolopoulos are ‘‘exact’’ for reactions on a
given adiabatic potential surface; thus the simplest interpre-
tation would be that these high Jdiscrepancies may reflect
deficiencies in Stark and Werner’s lowest adiabatic surface
in the barrier region. Alternatively, it is possible that the
1:3 nuclear spin distribution of jϭ0 ͑para͒ and jϭ1 ͑ortho͒
H in the jet.
2
FϩH reactions do not take place exclusively on the single
2
This comparison is shown in Fig. 3, where the column
lowest adiabatic potential surface, though this has been ex-
integrated populations have been scaled to the integral cross
section into Jϭ1. Overall the agreement between experiment
and theory is quite good, with general trends in the experi-
mental data well reproduced by theory. Agreement for the
lower J values is especially quantitative, capturing the rise
from Jϭ0 to nearly equivalent populations experimentally
plicitly assumed in all FϩH quantum reactive scattering cal-
culations thus far. If nonadiabatic effects are important, then
2
one would also anticipate contributions to reactive scattering
2
from low lying spin orbit excited F*( P1 ) atoms also
/2
Ϫ1
present in the jet. Given the 1.16 kcal/mol ͑404 cm ) spin
orbit splitting between F and F*, this would help explain the
J. Chem. Phys., Vol. 107, No. 19, 15 November 1997
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8196
Letters to the Editor
1
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ACKNOWLEDGMENTS
This work was supported by grants from the Air Force
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J. Chem. Phys., Vol. 107, No. 19, 15 November 1997
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