Dynamics of the H+HCl gas-phase reaction: Absolute reactive cross-section for Cl(2P3/2) atom formation

Article abstract of DOI:10.1016/S0009-2614(98)00404-7

The reaction of atomic hydrogen with HCl was studied in the gas phase using the laser photolysis/vacuum-UV laser-induced fluorescence (LP/VUV-LIF) 'pump-and-probe' technique. Translationally energetic H atoms with an average energy of E_cm=1.0 eV in the (H-HCl)-center-of-mass system were generated by laser photolysis of H₂S at 248 nm. Doppler profiles of nascent Cl(²P_3/2) atoms produced in the reactive collision of the H atoms with room temperature HCl molecules were measured under single collision conditions by VUV-LIF. For the H+HCl→H₂+Cl(²P_3/2) reaction an absolute reaction cross-section σ_R(1.0 eV)=(0.34±0.05) A² was determined by means of a photolytic calibration method using the 248 nm laser photolysis of PCl₃ as reference for well-defined concentrations of Cl atoms. Small amounts of Cl in the (²P_1/2) excited state were observed in the H+HCl experiments. An upper limit of (0.02±0.01) A² was measured for the Cl^*(²P_1/2) formation cross-section in the H+HCl reaction at 1.0 eV.

Full text of DOI:10.1016/S0009-2614(98)00404-7

12 June 1998
Ž
.
Chemical Physics Letters 289 1998 241–246
Dynamics of the HqHCl gas-phase reaction: absolute reactive
ž 2
/
cross-section for Cl P3r2 atom formation
)
R.A. Brownsword, C. Kappel, P. Schmiechen, H.P. Upadhyaya 1, H.-R. Volpp
Physikalisch-Chemisches Institut der UniÕersitat Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany
¨
Received 3 March 1998
Abstract
The reaction of atomic hydrogen with HCl was studied in the gas phase using the laser photolysisrvacuum–UV
Ž
.
laser-induced fluorescence LPrVUV–LIF ‘pump-and-probe’ technique. Translationally energetic H atoms with an average
Ž
.
energy of Ecms1.0 eV in the H–HCl –center-of-mass system were generated by laser photolysis of H2S at 248 nm.
Doppler profiles of nascent Cl P3
Ž2
.
atoms produced in the reactive collision of the H atoms with room temperature HCl
r2
Ž2
.
molecules were measured under single collision conditions by VUV–LIF. For the HqHCl™H2qCl P3
reaction an
r2
2
˚
.
Ž
.
Ž
absolute reaction cross-section sR 1.0 eV s 0.34"0.05 A was determined by means of a photolytic calibration method
using the 248 nm laser photolysis of PCl3 as reference for well-defined concentrations of Cl atoms. Small amounts of Cl in
Ž2
the P1
.
Ž
2
˚
.
excited state were observed in the HqHCl experiments. An upper limit of 0.02"0.01 A was measured for
r2
the Cl 2P1
)Ž
.
formation cross-section in the HqHCl reaction at 1.0 eV. q 1998 Elsevier Science B.V. All rights reserved.
r2
1. Introduction
the Cl–H–H system played an important role in the
development of the London–Eyring–Polanyi–Sato
Ž
.
w
x
The gas-phase reactions
LEPS semi-empirical valence bond method 8,9 .
Earlier work on the Cl–H–H potential energy sur-
HqHCl™H2 qCl
D H8O sy4.31 kJrmol
Ž
.
w x
face PES is reviewed in Ref. 10 . The first glob-
ally realistic Cl–H–H PES was calculated by Baer
1
Ž .
w
x
and Last 11 and a more recent PES was published
ClqH2 ™HqHCl
as important elementary steps in the H2–Cl2 reac-
-1
Ž
.
w
x
Ž
by Truhlar et al. 12 . The latter one denoted as
.
GQQ–PES was used by Polanyi et al. in quasi-
w
x
tion system 1–5 have played a major role in the
development of chemical kinetics 6 . For reactions
Ž
.
classical trajectory QCT calculations to study the
branching between D atom abstraction and hydrogen
atom exchange in the reaction of translationally ener-
getic H atoms with DCl 13 . The same group carried
out D atom abstraction and hydrogen atom exchange
reaction cross-section measurements for the HqDCl
reaction in the energy range of 1.0–2.4 eV 13 .
Good agreement between the experimental and QCT
abstraction cross-sections was found. However, the
w x
Ž .
Ž
.
1 and y1 a large number of experimental investi-
gations were carried out including detailed kinetic
w
x
Ž
w x.
isotope effect studies see, e.g., Ref. 7 . In addition,
w
x
)
Corresponding author. E-mail: aw2@ix.urz.uni.heidelberg.de
1 On sabbatical leave from Chemistry Division, Bhabha Atomic
Research Centre, Bombay, India.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 98 00404-7

(
)
242
R.A. Brownsword et al.rChemical Physics Letters 289 1998 241–246
) Ž2
.
Ž .
values of the experimental HqDCl™DqHCl ex-
change cross-sections for energies higher than 1.0 eV
could not be well reproduced in the QCT calcula-
tions on the GQQ–PES.
Cl P1r2 formation cross-sections for reaction 1
under single-collision conditions. In addition, the
result of this work can directly be compared with the
Ž
.
result of recent quantum-scattering QMS calcula-
So far there has been only one measurement of an
tions carried out on the GQQ–PES for the transla-
Ž .
Ž
.
w x
absolute reaction cross-section for reaction 1 .
tional energy 1.0 eV of the present experiment 15 .
2
˚
.
Ž
Valentini et al. determined a value of 2"1 A at
an average translational energy of 1.6 eV 14 . This
value is considerably higher than the average value
of 0.16"0.06 A for the abstraction cross-section
w
x
2. Experimental
2
˚
.
Ž
Ž2
.
measured in the HqDCl™HDqCl P3r2 dynam-
Reaction dynamics studies were carried out in a
Ž .
ics studies at collision energies of 1.2 and 1.9 eV
flowing mixture of H2 S UCAR electronic grade
w
x
w
x
Ž
and room temperature HCl Messer Griesheim,
13 . However, in the experiments of Ref. 14 coher-
Ž
.
.
w
x w
x
ent anti-Stokes Raman scattering CARS spec-
troscopy for H2 product detection was used. As a
consequence, the measured cross-section represents
the sum of the cross-sections for abstraction leading
99.999% with the ratio of H2 S : HCl typically
being 1:2 at a total pressure between 60 and 100
mTorr using a flow reactor apparatus depicted
schematically in Fig. 1. A similar experimental appa-
ratus was used recently to measure absolute reaction
cross-sections and isotope branching ratios for the
Ž2
.
to the formation of both ground-state Cl P3r2 and
) Ž2
.
electronically excited Cl P1r2 atoms. The aim of
the work to be presented in this Letter was to
develop a photolytic calibration method which al-
w
x
w x
HqD2O 16 and OHqHD reactions 17 . For the
ClrCl) atom calibration measurements PCl3 Al-
Ž
Ž2
.
.
lows the measurement of absolute Cl P3r2 and
drich, 99.99% was pumped through the reaction cell
Ž
.
Fig. 1. Schematic diagram of the experimental apparatus LM, lens monochromator; PM, photomultiplier . Inset: the four-wave mixing
scheme for generation of tunable VUV laser radiation in Kr for Cl and Cl) atom LIF detection.

(
)
R.A. Brownsword et al.rChemical Physics Letters 289 1998 241–246
243
at a typical pressure of 1–2 m Torr. PCl3 was
degased prior to use by a freeze–pump–thaw cycle.
VUV-probe beam intensity was monitored after pass-
ing through the reaction cell with an additional solar
Ž
.
Ž
A KrF excimer laser lpump s248 nm was used
to dissociate H2 S in order to generate translationally
energetic H atoms with an average translational en-
blind photomultiplier of the same type PM2 in Fig.
.
1 . In order to obtain a satisfactory SrN ratio, each
point of the Cl and Cl) atom Doppler profiles shown
in Fig. 2, and Fig. 3 was averaged over 30 laser
shots. Experiments were carried out at a laser repeti-
tion rate of 6 Hz. The ClrCl) LIF signal, VUV
probe beam and photolysis laser intensity were
recorded with a boxcar system and transferred to a
microcomputer where the LIF signal was normalized
to both pump and probe laser intensities.
Ž
.
ergy of Ecm s1.0 eV in the H–HCl –center-of-mass
system. An aperture was used to skim off a homoge-
neous part of the rectangular excimer laser profile in
order to provide a homogeneous photolysis beam of
about 3–5 mJrpulse.
Ž
Narrow-band VUV-probe laser radiation DvVUV
s0.4 cmy1 , tunable in the wavelength region of
.
133.5–136.4 nm was generated using Wallenstein’s
It was found that the VUV probe beam itself
produced an appreciable ClrCl) atom LIF signal via
photolysis of HCl and PCl3, respectively. In order to
subtract these ‘background’ ClrCl) atoms from the
ClrCl) atoms produced in the HqHCl reaction and
in the 248 nm photolysis of PCl3, an electronically
controlled mechanical shutter was inserted into the
photolysis beam path see Fig. 1 . At each point of
the Cl and Cl) atom line scan, the signal was first
averaged with the shutter opened and again averaged
method for resonant third-order sum-difference fre-
quency conversion vVUV s2vR yvT of pulsed
dye laser radiation in krypton 18 . The four-wave
Ž
.
w
x
mixing scheme is shown as an inset in Fig. 1. The
Ž
.
frequency vR lR s212.55 nm is resonant with the
Ž
.
Kr 4p–5p 1r2, 0 two-photon transitions. The fre-
Ž
.
quency vT could be tuned from 480 to 521 nm to
2
2
cover the four allowed Cl 4s Pj §3p Pj ; jX s
Ž
X
Y
Y
.
w x
1r2,3r2§j s1r2,3r2 transitions 19 . Ground-
Ž2
.
Ž2
were detected using the j sj -transitions, which
have the highest transition probabilities f3r2, 3r2
.
state Cl P3r2 and excited-state Cl P1r2 atoms
X
Y
Ž
.
s
w
x
0.114 and f1r2, 1r2 s0.088, respectively 20 .
The fundamental laser radiation was obtained from
two tunable dye lasers, simultaneously pumped by an
XeCl excimer laser, one of which, vR was fre-
quency doubled with a BBO II crystal. VUV-light
was carefully separated from the fundamental lasers
Ž
.
by a lens monochromator LM . The probe beam
was aligned to overlap the photolysis beam at right
angles in the viewing region of an LIF detector. The
delay time between the photolysis and probe pulse
was controlled by a pulse generator and could be
monitored on a fast oscilloscope. Delay times be-
tween ‘pump’ and ‘probe’ laser pulses were typi-
Ž
.
Ž
.
cally between 120"5 and 250"5 ns allowing
the collision-free detection of the nascent Cl and Cl)
atoms produced in the reaction. Under these experi-
mental conditions relaxation of Cl) by quenching
was negligible. The Cl and Cl) LIF signal was
Ž .
Fig. 2. Doppler profiles of
a
Cl atoms produced in the reaction
HqHCl experimental conditions: 1:2 mixture of H2 S and HCl;
Ž .
Ž
measured through a band pass filter ARC model
Ž
.
130-B-1D with a transmission 29% at 134 nm by a
solar blind photomultiplier THORNEMI model
B214FV, denoted as PM1 in Fig. 1 positioned at
right angles to both the pump and probe laser. The
.
ptot s86 mTorr; Dts180 ns ,
photolysis of 1.6 mTorr PCl3 at 248 nm. Line centers correspond
b Cl atoms produced in the
Ž
2
2
.
Ž
X
Y
.
to the 4s Pj s3r2 §3p Pj s3r2 transition of the Cl atom
y1
Ž
.
.
74225.8 cm

(
)
244
R.A. Brownsword et al.rChemical Physics Letters 289 1998 241–246
with the shutter closed. A point-by-point subtraction
curves were determined under identical experimental
w
x
Ž .
procedure was adopted 21 , to obtain directly and
on-line a signal free from ‘probe-laser-generated’
background Cl and Cl) atoms.
conditions and evaluated using formula 2 . 22 cali-
bration measurements, which consisted of 44 indi-
vidual Cl atom Doppler profiles yielded an average
value for the HqHCl™H2 qCl reaction cross-sec-
2
˚
.
Ž
.
Ž
tion of sR 1.0 eV s 0.34"0.05 A . The total
3. Results and discussion
experimental error was calculated from the errors of
Ž .
the entries of formula 2 on the basis of simple error
The total absolute reaction cross-section for the
HqHCl™H2 qClrCl) reaction was obtained by
means of a photolytic calibration method using laser
flash photolysis of PCl3 as a reference source of
well-defined ClrCl) atom concentrations. The
ClrCl) branching ratio in the 248 nm photodissoci-
ation of PCl3 has been measured by several groups
using different techniques. By comparing the Cl
propagation. The uncertainty of the literature value
of the optical absorption cross-section of H2 S at 248
nm 26 was estimated to be 10%. The overall exper-
imental uncertainty of the calibration measurement
— namely the measurement of SR Cl rSCal Cl —
was determined to be 3% one standard deviation .
In Fig. 4, the reaction cross-section measured in the
present study is depicted and compared with results
w
x
Ž
.
Ž .
Ž
.
Ž
.
2
atom signal, SR Cl — defined as the area below the
Cl atom Doppler profile in Fig. 2a — produced in
the reaction HqHCl, with the Cl atom signal
˚
w
x
Ž
.
of QMS calculations 15 . A value of 2"1
A
w
x
reported in Ref. 14 for Ecm s1.6 eV was not
included in Fig. 4. Absolute reaction cross-sections
measured for the HqDCl™HDqCl reaction are
also shown together with QCT cross-sections for the
Ž
.
Ž
.
SCal Cl produced in the PCl3 photolysis Fig. 2b
the absolute reaction cross-section sR for the Hq
HCl™H2 qCl reaction can be obtained using the
w
x
following formula 22 :
w
x
fCl sPCl PCl3
SR Cl
Ž
.
.
3
sR s
=
.
2
Ž .
w
xw
x
SCal Cl
sH H2 S HCl ÕrelDt
Ž
2 S
Here Õrel is the relative velocity which corresponds
to the average translational energy of Ecm s1.0 eV
of the HqHCl reactant pair. The average transla-
tional energy was calculated using the HqSH en-
ergy partitioning data obtained in the 248 nm pho-
todissociation studies of H2 S as described in Ref.
w
x w x
23 . X denotes the gas-phase concentration of
species X calculated from the corresponding partial
pressures. fCl is the Cl quantum yield in the PCl3
photolysis. A value of fCl s0.56 was used, which
was calculated from the diode laser ClrCl) infrared
w
x
absorption data given in Refs. 24,25 . sPCl is the
optical absorption cross-section of PCl3 at t3he pho-
tolysis wavelength of 248 nm. sPCl was measured
3
Ž
in the course of the present study to be sPCl s 1.5
3
y19
cm2 . sH is the optical absorption
.
)
"0.2 =10
Ž .
2 S
Fig. 3. Doppler profiles of a Cl atoms observed in the 248 nm
w
x
cross-section of H2 S at 248 nm given in Ref. 26 .
Dt is the time delay between pump and probe laser
pulses.
Ž
photolysis of a 1:2 mixture of H2 S and HCl ptot s97 mTorr;
.
Ž .
b
Dts180 ns .
Cl) atoms produced in the photolysis of 1.4
2
Ž
X
mTorr PCl3 at 248 nm. Line centers correspond to the 4s Pj s1r2
2
)
y1
Y
.
Ž
.
§3p Pj s1r2 transition of the Cl atom 73983.1 cm . Com-
Ž . Ž .
a and b was used to
To determine the absolute reactive cross-section
for the HqHCl reaction in a number of experimen-
tal runs, integrated areas under the Cl fluorescence
parison of the Cl) signals shown in
determine an upper limit for the Cl) formation cross-section in
Ž
.
the HqHCl reaction for details see text .

(
)
R.A. Brownsword et al.rChemical Physics Letters 289 1998 241–246
245
Ž
.
reaction cross-section an upper limit of 0.02"0.01
2
˚
A was determined, which indicates that the forma-
tion of Cl) is only of minor importance in the
HqHCl reaction. The latter result is in good agree-
ment with earlier experiments on the HqDCl reac-
tion in which no Cl) atoms could be observed,
2
˚
leading to an estimated upper limit of 0.02 A for the
w
x
corresponding reaction cross-section 13 . Good
agreement between the experimental HqHCl™H2
qCl reaction cross-section and the results of QMS
w
x
w x
calculations 15 on the GQQ–PES 12 was found at
Ecm s1.0 eV. The result of the present study is also
in good agreement with the HqDCl™HDqCl
cross-sections measured at collision energies of 1.2
Fig. 4. Comparison of the experimental absolute reaction cross-
w
x
Ž
.
and 1.9 eV 13 as well as with QCT cross-sections
for the HqDCl™HDqCl reaction calculated in
the energy range of 1.0–2.4 eV using the GQQ–PES
section for the HqHCl™H2 qCl reaction
v
with results of
Ž
.
Ž
. w x
quantum-scattering QMS calculations ` 15 carried out using
w
x Ž .
the GQQ–PES 12 . B denotes absolute reaction cross-sections
w
x Ž
.
are re-
w x
measured for the HqDCl™HDqCl reaction 13 .
I
w
x
12 . This supports the conclusion drawn by Polanyi
Ž
.
sults of quasi-classical trajectory QCT calculations 13 for the
w
x
et al. 13 , namely that the GQQ–PES is quite ade-
quate for the hydrogen abstraction channel. How-
ever, further H q HCl ™ H2 q ClrCl) reaction
cross-section measurements in the energy range of
1.0–2.4 eV are needed to prove the dramatic in-
crease in the HqHCl abstraction cross-section as
w
x
HqDCl™HDqCl reaction using the GQQ–PES 12 .
w
x
same reaction 13 . In the QMS and QCT calcula-
w
x
tions the GQQ–PES was used 12 .
Besides the strong Cl signals shown in Fig. 2a,
small Cl) signals as depicted in Fig. 3a could also
be observed in the reaction dynamics studies. How-
ever, due to the poor SrN ratio it was not possible to
Ž
.
Ž
suggested by the Valentini value of sR 1.6 eV s 2
2
˚
.
w x
"1 A 14 .
confirm the linear increase of the Cl) concentration
)
w
) x
with pump–probe delay time, Cl AsR ÕrelDt,
Acknowledgements
which is expected if Cl) is formed by the reaction of
energetic H atoms with HCl under single-collision
Financial support of the Alexander von Humboldt
Stiftung, the Max Planck Gesellschaft and the
Deutsche Forschungsgemeinschaft via the Sonder-
)
conditions. Calibration of the Cl) signals SR Cl
Ž
.
observed in the HqHCl studies at Dts180 ns
)
)
Ž
.
Ž
.
Fig. 3a against the Cl signals SCal Cl obtained
forschungsbereich 359 ‘Reaktive Stromung, Diffu-
¨
Ž
.
in the 248 nm photolysis of PCl3 Fig. 3b yielded a
sion und Transport’ at the University of Heidelberg
2
value of sR) 1.0 eV s 0.02"0.01 A . In the
data evaluation, 12 measured Cl) atom Doppler
profiles were used, each was calibrated against Cl)
˚
.
Ž
.
Ž
is gratefully acknowledged. Collaboration with the
Ž
group of Professor J.P. Mittal Director of the Chem-
.
istry Group, BARC Bombay, India was supported
atom Doppler profiles from the PCl3 photolysis. We
by KFA Julich and DLR Bonn under the Indo-Ger-
¨
2
˚
.
Ž
Ž
.
suggest the value 0.02"0.01 A as an upper limit
for the HqHCl™H2 qCl) reaction cross-section
at Ecm s1.0 eV.
man bilateral agreement Project No. INI-207 . HRV
Ž
would like to thank Professor J. Wolfrum Director
of the Institute of Physical Chemistry, University of
.
Heidelberg for his continuous support and his stimu-
lating interest in the ongoing work.
4. Conclusion
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Ž
.
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Ž
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˚
.
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