Chlorine atom formation dynamics in the dissociation of CH3CF2Cl(HCFC-142b) after UV laser photoexcitation

Article abstract of DOI:10.1063/1.479124

The dynamics of chlorine atom formation after UV photoexcitation of CH₃CF₂Cl(HCFC-142b) in the gas phase was studied by a pulsed laser photolysis/laser-induced fluorescence (LIF) pump-and-probe technique at room temperature. The parent molecule was excited at the ArF excimer laser wavelength (193.3 nm) and nascent ground state Cl(²P_3/2) and spin-orbit excited Cl*(²P_1/2) photofragments were detected under collision-free conditions via laser induced fluorescence in the vacuum ultraviolet spectral region. Narrow-band probe laser radiation, tunable over the wavelength range 133.5-136.4 nm, was generated via resonant third-order sum-difference frequency conversion of dye laser radiation in Krypton. Using HCl photolysis at 193.3 nm as a source of well-defined Cl(²P_3/2) and Cl*(²P_1/2) concentrations, values for the total Cl atom quantum yield (Φ_Cl+Cl*=0.90±0.17) and the [Cl*]/[Cl] branching ratio 0.39±0.11 were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the average relative translational energy of the fragments could be determined to be 125±25 kJ/mol. The corresponding value f_T=0.48±0.10 of the fraction of total available energy channeled into product translational energy was found to be (within experimental uncertainty) in agreement with the result f_T=0.39 of a dynamical simulation assuming a repulsive model for single C-Cl bond cleavage. Both the measured total Cl atom quantum yield and the energy disposal indicates that direct C-Cl bond cleavage is a primary fragmentation mechanism for CH₃CF₂Cl after photoexcitation at 193.3 nm.

Full text of DOI:10.1063/1.479124

Chlorine atom formation dynamics in the dissociation of CH 3 CF 2 Cl(HCFC-142b) after
UV laser photoexcitation
Richard A. Brownsword, Patricia Schmiechen, Hans-Robert Volpp, Hari P. Upadhyaya, Young Jae Jung, and
Kyung-Hoon Jung
Citation: The Journal of Chemical Physics 110, 11823 (1999); doi: 10.1063/1.479124
View online: http://dx.doi.org/10.1063/1.479124
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 110, NUMBER 24
22 JUNE 1999
Chlorine atom formation dynamics in the dissociation
of CH3CF2Cl„HCFC-142b… after UV laser photoexcitation
Richard A. Brownsword, Patricia Schmiechen, and Hans-Robert Volppa)
¨
Physikalisch-Chemisches Institut der Universitat Heidelberg, Im Neuenheimer Feld 253,
D-69120 Heidelberg, Germany
Hari P. Upadhyaya
Chemistry Division, Bhabha Atomic Research Centre, Bombay 400-085, India
Young Jae Junga) and Kyung-Hoon Jung
Center for Molecular Science and Department of Chemistry, Korea Advanced Institute of Science
and Technology, Taeduck Science Town, Taejon 305-701, Korea
͑Received 8 December 1998; accepted 30 March 1999͒
The dynamics of chlorine atom formation after UV photoexcitation of CH3CF2Cl͑HCFC-142b͒ in
the gas phase was studied by a pulsed laser photolysis/laser-induced fluorescence ͑LIF͒
‘‘pump-and-probe’’ technique at room temperature. The parent molecule was excited at the ArF
excimer laser wavelength ͑193.3 nm͒ and nascent ground state Cl(2P3/2) and spin-orbit excited
2
*
Cl ( P1/2) photofragments were detected under collision-free conditions via laser induced
fluorescence in the vacuum ultraviolet spectral region. Narrow-band probe laser radiation, tunable
over the wavelength range 133.5–136.4 nm, was generated via resonant third-order sum-difference
frequency conversion of dye laser radiation in Krypton. Using HCl photolysis at 193.3 nm as a
2
source of well-defined Cl(2P3/2) and Cl ( P1/2) concentrations, values for the total Cl atom
*
*
quantum yield (⌽ClϩCl ϭ0.90Ϯ0.17) and the ͓Cl ͔/͓Cl͔ branching ratio 0.39Ϯ0.11 were
*
*
determined by means of a photolytic calibration method. From the measured Cl and Cl atom
Doppler profiles the average relative translational energy of the fragments could be determined to be
125Ϯ25 kJ/mol. The corresponding value fTϭ0.48Ϯ0.10 of the fraction of total available energy
channeled into product translational energy was found to be ͑within experimental uncertainty͒ in
agreement with the result fTϭ0.39 of a dynamical simulation assuming a repulsive model for single
C–Cl bond cleavage. Both the measured total Cl atom quantum yield and the energy disposal
indicates that direct C–Cl bond cleavage is a primary fragmentation mechanism for CH3CF2Cl after
photoexcitation at 193.3 nm. © 1999 American Institute of Physics. ͓S0021-9606͑99͒00624-8͔
I. INTRODUCTION
fore besides atmospheric concentration measurements and
advanced 3D modeling calculations,9 laboratory photodisso-
ciation dynamics studies of HCFCs and in particular quanti-
tative measurements of primary photochemical product
yields are needed to assess their environmental impact.10
For a number of HCFCs, optical absorption spectra in
the infrared ͑IR͒, ultraviolet ͑UV͒, and vacuum-ultraviolet
͑VUV͒ spectral region were measured.5,11 Pyrolysis12 and
decomposition of chemically activated13 CH3CF2Cl as well
As a consequence of the Copenhagen Amendment of the
Montreal Protocol, which allows the use of hydrochlorofluo-
rocarbons ͑HCFCs͒, on a transitional basis, as replacements
of the ozone-destroying chlorofluorocarbons ͑CFCs͒,1 it can
be expected that the atmospheric HCFC concentrations will
continue to increase rapidly over the next years. Although
HCFC compounds, which have at least one C–H bond, can
be oxidized by OH free radicals and O(1D) atoms in the
troposphere,2 they still have the potential to lead to signifi-
cant chlorine transportation to the stratosphere3 where they
are destroyed by photochemical processes initiated by light
absorption in the 190–230 nm wavelength region.4 In addi-
tion, HCFCs, like CFCs, absorb long-wavelength radiation
and should therefore also be considered as potential green-
house gases.5 In recent measurements of stratospheric pro-
files of CH3CF2Cl ͑HCFC-142b͒, which has been used since
the late 1980s to replace CF2Cl2 ͑CFC-12͒ as a foam blowing
agent in industry,6 it was found that CH3CF2Cl is not yet in
equilibrium, due to its rapidly increasing emissions.7,8 There-
14,15
as the infrared multiple photon decomposition ͑IRMPD͒
of CH3CF2Cl was studied in detail. The results of these stud-
ies indicate that decomposition on the ground state electronic
potential energy surface ͑PES͒ can be characterized by a
four-center unimolecular elimination pathway leading to
CH2ϭCF2ϩHCl as major products and to a minor extent to
CH2ϭCFClϩHF products.
Early gas-phase photodecomposition studies of
CH3CF2Cl after irradiation with VUV light ͑147 nm;16 123.6
nm17͒ were carried out under static conditions as a function
of irradiation time ͑30–180 min͒, CH3CF2Cl pressure ͑3.6–
20.6 Torr͒ and/or in the absence and presence of radical
scavenging agents (NO,H2S,HI) with stable end-product
analysis ͑CH2CF2, CH2CHF, C2H2, CH4, and CH3Cl͒ being
a͒
Authors to whom correspondence should be addressed.
0021-9606/99/110(24)/11823/7/$15.00
11823
© 1999 American Institute of Physics
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J. Chem. Phys., Vol. 110, No. 24, 22 June 1999
Brownsword et al.
performed by flame ionization gas chromatography. Only re-
cently laser-based CH3CF2Cl low-pressure gas-phase pho-
tolysis experiments have been carried out in which primary
photochemical product yields could be determined under
collision-free conditions.18,19 In Ref. 18, the H atom forma-
tion dynamics after UV ͑193.3 nm͒ and VUV ͑121.6 nm͒
laser excitation was investigated and absolute quantum
yields ⌽H for H atom formation were reported. The mea-
sured values of ⌽H͑193.3 nm͒ϭ0.06Ϯ0.02 and ⌽H͑121.6
nm͒ϭ0.53Ϯ0.12 indicate the increasing importance of the H
atom formation pathway in the CH3CF2Cl fragmentation in
going from UV to VUV excitation wavelengths. In Ref. 19,
photolysis experiments the CH3CF2Cl pressure in the cell
was typically 30–45 mTorr. For the calibration measure-
ments, HCl ͑Messer Griesheim, 99.99%͒ was flowed through
the reaction cell at pressures of typically 7–13 mTorr.
CH3CF2Cl flow rates were regulated by a calibrated mass
flow controller. The HCl flow was regulated by a needle
valve. The pressure in the cell was monitored by a MKS-
Baratron. Gas flow through the cell was maintained at a high
enough rate to ensure renewal of the gas between successive
laser shots at a laser repetition rate of 6 Hz.
A UV excimer laser ͑Lambda Physik LPX 205͒ operat-
ing with an ArF mixture ͑193.3 nm emission wavelength͒
was used to photodissociate both the CH3CF2Cl parent mol-
ecules as well as HCl. A circular aperture was used to skim
off the homogeneous part of the rectangular excimer laser
profile in order to provide the photolysis beam. A cylindrical
lens ͑1 m focal length͒ was used to slightly focus the pho-
tolysis beam. Pump laser intensities were typically between
4–12 mJ/cm2. The analysis of nascent H atom Doppler pro-
files recorded in the HCl photolysis showed that the pump
laser beam was essentially unpolarized ͓see, e.g., Fig. 2͑c͒ of
Ref. 22͑a͔͒ and it can therefore be expected that any possible
anisotropy of the photodissociation process is averaged out.
H atom, ground state Cl(2P3/2) and spin-orbit excited
2
*
Cl ( P1/2) atoms, and HCl product formation were investi-
gated after laser excitation at 193.3 nm. Using time-of-flight
mass spectrometry ͑TOF-MS͒ in combination with (2ϩ1)
resonance-enhanced multiphoton ionization ͑REMPI͒ for
*
product detection H, Cl, and Cl atom formation was ob-
served. In these experiments, no HCl( ϭ0,1) product mol-
v
Љ
ecules could be detected and an upper limit of 1%
(⌽HCl(v ϭ0,1)(193.3 nm)Ͻ0.01) was estimated for the HCl
Љ
elimination channel. In addition, the relative product branch-
*
ing ratio ͓H͔/͓͑Cl͔ϩ͓Cl ͔͒ and the Cl atom spin-orbit state
*
distribution ͓Cl ͔/͓Cl͔ were determined to be 0.67Ϯ0.21 and
*
For VUV-LIF detection of Cl and Cl atoms, narrow-
0.18Ϯ0.04, respectively.
band probe laser radiation, tunable in the wavelength region
133.5–136.4 nm was generated using Wallenstein’s method
for resonant third-order sum-difference frequency conversion
(␻VUVϭ2␻RϪ␻T) of pulsed dye laser radiation in
Krypton.25 In the four-wave mixing process the frequency
In this article we report on the extension of our earlier
work on the H atom formation in the UV/VUV photolysis of
22
CHF2Cl͑HCFC-22͒,20,21 chloromethanes (CHnCl4Ϫn
)
and
CH3CF2Cl18 towards the direct measurements of absolute Cl
atom formation yields via the pulsed laser photolysis ͑LP͒/
VUV laser-induced fluorescence ͑LIF͒ ‘‘pump-and-probe’’
␻
(␭Rϭ212.55 nm) is two-photon resonant with the Kr
R
4p–5p ͑1/2,0͒ transition. The second frequency ␻ could be
T
*
method. In the present work, the Cl/Cl fine-structure
tuned from 480 to 521 nm to cover the four allowed
branching ratio and the absolute Cl atom formation yield
Cl(4s2Pj ←3p2Pj ; j ϭ1/2,3/2←j ϭ1/2,3/2)
transi-
Ј
Љ
Ј
Љ
⌽
in the 193.3 nm photolysis of CH3CF2Cl were de-
tions.26 Ground-state Cl(2P3/2) and excited-state Cl ( P1/2
)
2
ClϩCl
*
*
termined by means of a photolytic calibration method using
HCl photolysis as a reference source for well-defined Cl and
atoms were detected via the (j ϭj )-transitions, which have
Ј
Љ
the highest transition probabilities f3/2,3/2ϭ0.114 and f1/2,1/2
ϭ0.088, respectively.27 The fundamental laser radiation was
obtained from two tunable dye lasers ͑Lambda Physik FL
2002͒, simultaneously pumped by a XeCl excimer laser
͑Lambda Physik EMG 201 MSC͒. The first dye laser pro-
*
Cl atom concentrations. In addition, from the analysis of the
*
measured Cl and Cl atom Doppler profiles, information
*
about the energy partitioning in the Cl/Cl -forming photo-
fragmentation step could be derived. A possible primary
fragmentation mechanism for CH3CF2Cl after photoexcita-
tion at 193.3 nm will be discussed in light of the new results.
vided ␻ by utilizing Coumarin 307 dye. The second dye
T
laser was operated with Coumarin 120 to generate light with
a wavelength of 425.10 nm from which ␻ was obtained by
R
II. EXPERIMENT
frequency doubling with a BBO II crystal. The VUV-light
generated in the four-wave mixing process was carefully
separated from the unconverted laser radiation by a lens
monochromator followed by a light baffle system ͑for details
see Ref. 24͒. The bandwidth of the VUV probe laser radia-
tion ͑0.4–0.5 cmϪ1͒ was determined by measuring Doppler
profiles of Cl atoms generated in the 193.3 nm photolysis of
HCl both under collision-free and thermalized conditions. In
the latter case Cl atoms were generated by HCl photolysis in
3 Torr of N2 and probed after 12 ␮s. The Cl atom Doppler
profiles measured under thermalized conditions could be
nicely fit by a Gaussian function indicating that the probe
laser lineshape itself can be represented by a Gaussian func-
tion. The VUV probe beam was aligned to overlap the pho-
tolysis beam at right angles in the viewing region of a LIF
detector. The delay time between the photolysis and probe
Photodissociation studies were carried out in a flow cell
at mTorr level pressures using the LP/VUV-LIF pump-and-
probe setup previously used to study the H atom formation
dynamics19 in the UV and VUV photodissociation of
CH3CF2Cl. A detailed description can be found in Ref. 23.
The setup was recently modified to allow VUV-LIF detec-
*
tion of Cl and Cl atoms produced in bimolecular reactions
as described in Ref. 24. In the following only a brief sum-
mary of the experimental method will be given.
CH3CF2Cl ͑Messer Griesheim, purity Ͼ98%͒ was
pumped through the cell at room temperature. As stated by
the manufacturer, the amount of HCFC-141b (CH3CFCl2) in
the sample was less than 0.1%, with the remaining impurity
being CH3CF3 which exhibits a negligible absorption at
193.3 nm. The water content was less than 5 ppm. In the
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J. Chem. Phys., Vol. 110, No. 24, 22 June 1999
Brownsword et al.
11825
laser pulses was controlled by a pulse generator and moni-
tored on a fast oscilloscope. The delay time between pump-
ing and probing was kept short enough, typically 80–170 ns
͑with a time jitter of about Ϯ10 ns͒, to allow the collision-
*
free detection of the nascent Cl and Cl atoms produced in
the CH3CF2Cl and HCl photolysis. On the other hand, these
delay times were long enough to avoid any time-overlap be-
tween pump pulse ͑duration ϳ15 ns͒ and probe laser pulse
͑duration ϳ10 ns͒, which would lead to unwanted multipho-
ton photochemical processes. Under the described experi-
*
mental conditions, relaxation of Cl by quenching as well as
*
fly-out of Cl͑Cl ͒ atoms and secondary reactions with
CH3CF2Cl and HCl were negligible.
*
Cl and Cl LIF signals were measured through a band
pass filter ͑ARC model 130-B-1D; transmission of 29% at
134 nm͒ by a solar blind photomultiplier ͑THORN EMI
model 9413 B͒ positioned at right angles to both pump-and-
probe laser. The VUV probe beam intensity was monitored
after passing through the reaction cell by an additional pho-
tomultiplier of the same type. Because the VUV probe beam
*
FIG. 1. Dependence of the observed Cl atom LIF signal from CH3CF2Cl
photolysis on the photolysis laser intensity. The slope of the fitted linear
log–log plot is given in the figure.
*
0.67Ϯ0.09 for the nascent ͓Cl ͔/͓Cl͔ spin-orbit state branch-
ing ratio in the 193.3 nm photolysis of HCl.28 This value is
within the combined experimental error in good agreement
with a value recently reported by Wittig and co-workers.29
Because the 193.3 nm photolysis of HCl generates
*
itself produced an appreciable Cl, Cl atom LIF signal via
photolysis of CH3CF2Cl and HCl, respectively, it was neces-
*
sary to subtract these ‘‘background’’ Cl, Cl atoms from the
*
Cl, Cl atoms produced in the 193.3 nm photolysis of
*
HϩCl͑Cl ͒ products with a quantum yield of unity (␾
ϭ␾ ϩ␾Clϭ1), values ␾ ϭ0.40 and ␾Clϭ0.60 can be
derived for the Cl and Cl fine-structure quantum yields
H
CH3CF2Cl and HCl. Therefore an electronically controlled
mechanical shutter was inserted into the photolysis beam
Cl
Cl
*
*
*
*
path, such that at each point of the Cl and Cl spectra, the
*
from the ͓Cl ͔/͓Cl͔ ratio.
signal could first be 30 times averaged with the shutter
opened and again be 30 times averaged with the shutter
closed. Finally a point-by-point subtraction procedure was
adopted,22 to obtain directly and on-line a LIF signal repre-
*
Quantum yields ⌽Cl for Cl and ⌽Cl for Cl formation
in the 193.3 nm photolysis of CH3CF2Cl were determined
using Eqs. ͑1a͒ and ͑1b͒, respectively:
*
*
senting the contribution from Cl and Cl atoms generated
*
solely by the 193.3 nm photolysis laser pulse. The Cl, Cl
LIF signals, VUV probe beam and photolysis laser intensity
͑the latter was monitored by a photodiode͒ were recorded
with a boxcar system and transferred to a microcomputer
where the LIF signals were normalized to both pump-and-
probe laser intensities. During the experiments special care
*
was taken that the Cl and Cl atom signals generated by the
193.3 nm photolysis laser pulse showed a linear dependence
on the pump and probe laser intensity. Slopes of the linear
*
log–log plots of Cl and Cl LIF signal intensity against pho-
tolysis laser intensity were 0.9Ϯ0.1 and 1.1Ϯ0.1, respec-
tively ͑see, for example, Fig. 1͒.
III. RESULTS
A. Primary photolytic chlorine atom quantum yield
⌽
Cl؉Cl
*
In order to determine the total chlorine atom quantum
*
yield ⌽ClϩCl quantum yields ⌽Cl for Cl and ⌽Cl for Cl
*
*
formation were measured separately by calibrating the Cl,
*
Cl atom signals SCl͑CH3CF2Cl͒, SCl (CH3CF2Cl) mea-
*
*
*
sured in the CH3CF2Cl photodissociation against the Cl, Cl
atom signals SCl͑HCl͒, SCl (HCl) from well-defined Cl, Cl
atom number densities generated in the 193.3 nm photolysis
FIG. 2. Doppler profiles of Cl atoms produced in the 193.3 nm laser pho-
tolysis of ͑a͒ 37 mTorr of CH3CF2Cl and ͑b͒ 10 mTorr of HCl. The Doppler
profiles were recorded 170 ns after the photolysis laser pulse. Line centers
*
of HCl ͑see Figs. 2 and 3͒. Recent kinetics experiments in
correspond to the (4s 2Pj
←3p 2Pj
) transition of the Cl atom
Љϭ3/2
Јϭ3/2
͑74225.8 cmϪ1͒. The laser bandwidth ͑FWHM͒ was determined to be 0.42
*
*
which the relaxation of Cl ͑Cl ϩM→MϩCl; MϭAr, N2,
HCl, H2, D2, and HD͒ was investigated, provided a value of
Ϯ0.06 cmϪ1
.
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11826
J. Chem. Phys., Vol. 110, No. 24, 22 June 1999
Brownsword et al.
from the known distances inside the flow cell and the relative
difference in the VUV probe laser attenuation measured in
the CH3CF2Cl photolysis and HCl calibration runs. Under
typical experimental conditions (pCH CF Clϭ30 mTorrϭ3
3
2
ϫpHCl) the absorption corrections were ␥Clϭ0.87Ϯ0.04 and
ϭ0.86Ϯ0.04. The experimental data set evaluated via
␥
Cl
*
Eq. ͑1a͒ to determine ⌽Cl consisted of ten Cl atom profiles
obtained in combination with ten Cl profiles from HCl cali-
*
bration runs. The Cl atom quantum yield was evaluated via
Eq. ͑1b͒ in the same manner. The quoted errors were deter-
mined from the 1␴ statistical uncertainties of the experimen-
tal data, the uncertainties of the CH3CF2Cl and HCl optical
absorption cross sections and the uncertainty of the
*
͓Cl ͔/͓Cl͔ value in the 193.3 nm photolysis of HCl using
simple error propagation. The following values for the Cl
*
and Cl
atom quantum yields were obtained:
(193.3 nm)ϭ0.25
⌽Cl(193.3 nm)ϭ0.65Ϯ0.12 and
Ϯ0.05 leading to a value of ⌽ClϩCl (193.3 nm)ϭ0.90
Ϯ0.17 for the total quantum yield. From the measured Cl
⌽
Cl
*
*
*
and Cl atom quantum yields, the spin-orbit branching ratio
of the Cl fragment in the 193.3 nm photolysis of CH3CF2Cl
*
can be calculated: Cl / Cl ϭ0.39Ϯ0.11.
͔ ͓
͓
͔
*
FIG. 3. Doppler profiles of Cl atoms produced in the 193.3 nm laser
photolysis of ͑a͒ 37 mTorr of CH3CF2Cl and ͑b͒ 10 mTorr of HCl. The
Doppler profiles were recorded 170 ns after the photolysis laser pulse. Line
*
centers correspond to the (4s 2Pj
←3p 2Pj
) transition of the Cl
Љϭ1/2
Јϭ1/2
B. Average Cl atom translational energy
atom ͑73983.1 cmϪ1͒. The laser bandwidth ͑FWHM͒ was determined to be
0.45Ϯ0.07 cmϪ1
.
The measured Cl atom spectral profiles were analyzed in
order to determine the average kinetic energy of the frag-
ments in the CH3CF2Cl photolysis. The Doppler profiles di-
⌽Clϭ␥Cl
S
CH CF Cl͒␾ ␴HClpHCl /
͑
͖
Cl 3 2 Cl
͕
rectly reflect, via the linear Doppler shift ␯Ϫ␯ /␯
͓
͔
0
0
ϭ
/c, the distribution f( ) of the velocity component
vz
vz
vz
S
HCl͒␴CH CF ClpCH CF Cl
,
͑1a͒
͑1b͒
͑
͕
͖
Cl
3
2
3
2
of the absorbing atoms along the propagation direction of the
probe laser beam. Hence, for an isotropic velocity distribu-
tion the average Cl atom kinetic energy in the laboratory
⌽
ϭ␥Cl
S
CH CF Cl͒␾
␴
HClpHCl
/
͑
͕
͖
Cl
Cl
3
2
Cl
*
*
*
*
v2
v2
repre-
͘
z
S
HCl͒␴CH CF ClpCH CF Cl .
͖
3 2 3 2
͑
͕
system is given by ELab͑Cl͒ϭ3/2 mCl
, where
Cl
͗
͘
͗
*
z
sents the second moment of the laboratory velocity distribu-
␴
and ␴
are the optical absorption cross sections
CH CF Cl
HCl
3
2
tion f( z) of the photolytically produced Cl atoms. How-
v
of HCl and CH3CF2Cl at Tϭ295 K and 193.3 nm SCl and
SCl are the integrated areas under the measured Cl and Cl
atom Doppler profiles and pHCl and pCH CF Cl are the pres-
ever, since the measured spectral profiles represent a
convolution of the laser spectral profile and the Doppler pro-
file of the absorbing Cl atoms, a deconvolution was carried
out to obtain the correct Cl velocity distribution f(vz) using
a Gaussian function to describe the VUV laser spectral pro-
file. The width of the laser spectral profile was determined
*
*
3
2
sures of HCl and CH3CF2Cl, respectively. For the HCl ab-
sorption cross sections, we used the value ␴HClϭ(8.1Ϯ0.4)
ϫ10Ϫ20 cm2 which has been recently measured for the ArF
excimer laser emission wavelength ͑193.3 nm͒.30 This value
is in good agreement with the value of 8.26ϫ10Ϫ20 cm2
which can be obtained by nonlinear interpolation from the
recommended data31͑a͒ of Inn.32 For the CH3CF2Cl absorp-
tion cross section we used the value ␴CH CF Clϭ0.58
*
from Cl (Cl ) profiles measured in the HCl photolysis ͓see,
*
e.g., Figs. 2͑b͒ and 3͑b͔͒ recorded right after the Cl (Cl )
profiles from the CH3CF2Cl photolysis ͓see, e.g., Figs. 2͑a͒
and 3͑a͔͒. Evaluation of the complete set of 20 experimental
*
Cl (Cl ) profiles taking into account the actual laser band-
3
2
ϫ10Ϫ20 cm2, which represents an average value calculated
from the recommended data listed in Ref. 31͑a͒ and the re-
sults of the most recent measurements by Nayak et al.33 The
uncertainty of the CH3CF2Cl absorption cross section at
193.3 nm was estimated to be Ϯ5% based on the scatter of
the data reported in Refs. 33–36 for the 180–200 nm region.
width revealed that the kinetic energies of both fragments in
the CH3CF2Cl photolysis are the same within the experimen-
tal uncertainty (ELab͑Cl͒ϷELab͑Cl ). Calculation of the sec-
͒
*
ond moments of f( z) gave an average value of ELab͑Cl͒
v
ϭ82Ϯ16 kJ/mol for the laboratory kinetic energy of the
fragments. The quoted error includes the 1␴ statistical uncer-
tainties obtained in the individual fits used to evaluate the
experimental spectral profiles from CH3CF2Cl and the HCl
photolysis using error propagation. The corresponding rela-
tive kinetic energy in the Cl–CH3CF2 center-of-mass ͑cm͒
system was calculated to be Ecmϭ125Ϯ25 kJ/mol.
The factors ␥Cl ,␥Cl in Eqs. ͑1a͒ and ͑1b͒ are correc-
*
tions accounting for the different degrees of absorption of the
*
Cl (␥probeϷ134.72 nm) and Cl (␭probeϷ135.16 nm) VUV
probe laser radiation by CH3CF2Cl and HCl, respectively.
The absorption corrections could be directly determined
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J. Chem. Phys., Vol. 110, No. 24, 22 June 1999
Brownsword et al.
11827
IV. DISCUSSION
*
͓Cl ͔/͓Cl͔ branching ratio from photodissociation out of the
ground vibrational state was for a considerable time subject
to controversy.42,43 A recent study of the 193.3 nm photodis-
sociation of HCl by Wittig and co-workers, in which the
high-n Rydberg time-of-flight ͑HRTOF͒ technique was uti-
Absorption of a 193.3 nm photon by a CH3CF2Cl mol-
ecule leads to an excitation in the first absorption band ͓de-
noted as A band in Ref. 11͑a͔͒. ‘‘A band absorption’’ origi-
*
nates from a 3p␲→␴ (C–Cl) transition within the valence
lized to record H atom product TOF spectra, yielded a value
*
shell, where 3p␲ is a Cl lone pair orbital and ␴ is an
29
*
of ͓Cl ͔/͓Cl͔ϭ0.69Ϯ0.02. This value was found to be in
antibonding C–Cl ␴–MO. Therefore it is expected that the
initial excitation is localized in the C–Cl bond11͑c͒ leading
preferentially to the breaking of this bond.31͑a͒ Furthermore
the A band of CH3CF2Cl is structureless,11͑b͒ suggesting a
direct dissociation mechanism.37
excellent agreement with the theoretical value of 0.71 calcu-
lated for the 193.3 nm excitation wavelength.44 In a recent
experimental and theoretical study, spin-orbit branching in
the HCl photolysis was investigated at long excitation wave-
lengths ͑193.3–235.3 nm͒.45 The experimental spin-orbit
branching ratios, obtained by ͑2ϩ1͒ REMPI using the
4p 2D3/2←3p 2P3/2 two-photon transition for Cl and the
4p 2D3/2←3p 2P1/2 and 4p 2P1/2←3p 2P1/2 two-photon
A. Primary chlorine release channels and energy
disposal
The present result for the primary photolytic chlorine
atom quantum yield suggests that the chlorine contents of
room-temperature CH3CF2Cl, after photoexcitation at 193.3
nm under collision-free conditions, is released to a large ex-
tent as atomic chlorine. The value for the chlorine atom
quantum yield measured in the present work is consistent
with the earlier measured H atom quantum yield18 of
⌽H(193.3 nm)ϭ0.06Ϯ0.02 and with the upper limit for the
HCl quantum yield reported in Ref. 19. In the latter experi-
*
transitions for Cl detection, were found to be in good agree-
ment with the theoretical results only after scaling to agree
with the Wittig value at 193.3 nm.46 As a consequence, we
tend to believe that the latter one can serve as a general
calibration point for the measurement of Cl atom spin-orbit
branching ratios, in particular because it has been determined
by a technique which is independent of the detection effi-
*
ciency of Cl and Cl atoms.
*
The value ͓Cl ͔/͓Cl͔ϭ0.39Ϯ0.11 determined in the
ments, no HCl( ϭ0,1) product molecules could be ob-
v
Љ
present investigation for the 193.3 nm photolysis of
served and the quantum yield was estimated to be
*
CH3CF2Cl was obtained by calibrating the Cl and Cl VUV-
⌽
HCl(v ϭ0.1)(193.3 nm)Ͻ0.01. However, IRMPD experi-
Љ
LIF signals against those obtained in the HCl photolysis. In
ments revealed that in the four-center elimination, a large
fraction of the total excess energy ͑Ͼ85%͒ is left as internal
excitation of the CH2ϭCF2 and HCl fragments and it was
suggested that the HCl fragments must be highly vibra-
tionally excited.14 As a consequence, the actual HCl yield in
the 193.3 nm photolysis of CH3CF2Cl could be considerably
higher than the upper limit estimated in Ref. 19 from the
*
Ref. 19, a significant lower value of ͓Cl ͔/͓Cl͔ϭ0.18Ϯ0.04
was reported for the 193.3 nm photolysis of CH3CF2Cl
in a molecular beam using the ͑2ϩ1͒ REMPI method for Cl
*
and Cl detection. The latter value was obtained using the
*
‘‘old’’ HCl chlorine branching ratio ͓Cl ͔/͓Cl͔ϭ0.50 as a
reference.43 Rescaling using the new HCl chlorine branching
ratio of Wittig et al.29 changes the value to ͓Cl ͔/͓Cl͔
*
absence of HCl( ϭ0,1) products. The result of the present
v
Љ
ϭ0.24Ϯ0.05 and brings it closer to the result of the present
work.
work puts an upper limit of ⌽HCl(193 nm)Ͻ0.27 for the total
HCl quantum yield. Hence, to fully elucidate the dynamics
of a possible HCl formation channel after UV excitation,
further studies are clearly needed which should include de-
Spin-orbit branching of atomic products is usually gov-
erned by the nature of the PES͑s͒ excited in the Franck–
Condon region as well as by subsequent nonadiabatic cou-
plings between the surfaces as the products depart. As a
result, for the correct interpretation of measured spin-orbit
branching ratios, information is required about the PESs in-
volved in the initial optical excitation and subsequent frag-
mentation process. Although such information is so far not
available for CH3CF2Cl, comparison can be made with
CH3Cl for which more detailed spectroscopic and dynamical
data is available. For CH3Cl it was found that photoabsorp-
tection of HCl( Ͼ1) product vibrational states. HCl elimi-
v
Љ
nation has been observed in the 193.3 nm photolysis of
CH3CH2Cl and attributed to a mechanism which involves
internal conversion to the electronic ground state.38
The average Cl translational energy determined in the
present work corresponds to a value of fTϭEcm /Eavlϭ0.48
Ϯ0.10. This value is—within experimental uncertainty—in
agreement with the value of fTϭ0.39 calculated using a re-
pulsive model.39 This confirms that formation of chlorine
atoms proceeds predominantly by a direct dissociation
mechanism on a PES that is repulsive along the C–Cl bond
coordinate.
tion at 193.3 nm is associated with a parallel transition from
1
the X A1 ground state to the 3Q0 state, which correlates
˜
*
diabatically with Cl products. The observed Cl formation
3
was attributed to a level crossing of the Q0 potential curve
by the electronically excited 1Q1 state which diabatically
B. Chlorine atom spin-orbit branching ratio
*
correlates with Cl products. Therefore the ͓Cl ͔/͓Cl͔ branch-
Accurate measurements of spin-orbit branching ratios
ing ratios should depend on the relative velocity of the prod-
uct fragments while the system passes through the crossing
2
X ( P ) / X(2P ) of halogen atoms (XϭCl, Br, I) pro-
*
͓
͔ ͓
͔
3/2
1/2
*
duced in the photodissociation of halogenated compounds
play an important role in the elucidation of the influence of
nonadiabatic couplings on molecular dynamics.40,41 How-
ever, even for HCl as one of the simplest systems, the
region and the ͓Cl ͔/͓Cl͔ branching ratio is expected to in-
crease in going to shorter photolysis wavelengths. Such a
behavior was indeed observed in Ref. 47 where ͓Cl ͔/͓Cl͔
branching ratios of 0.27Ϯ0.01 and 0.62Ϯ0.06 were reported
*
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11828
J. Chem. Phys., Vol. 110, No. 24, 22 June 1999
Brownsword et al.
for photolysis wavelengths of 235 and 193.3 nm, respec-
tively. Assuming that for both CH3CF2Cl and CH3Cl, the
spin-orbit branching is determined as the fragments separate
considerably higher ͑ca. 70%͒.50,51 As a consequence of the
markedly different parent molecule vibrational state distribu-
tions in the molecular beam and the room temperature gas
phase experiments, UV photon absorption could lead ͑due to
a different Franck–Condon overlap between the ground vi-
bronic and the electronically excited states wave functions͒
to the excitation of different PESs. The assessment of differ-
ent electronically excited PESs or—in the case that only one
electronically excited PES is involved—the assessment of
different regions of the excited PES could be one possible
explanation of the observed difference in the dissociation
dynamics. However, further theoretical work on the elec-
tronically excited molecular state͑s͒ structure in combination
with additional experimental work in the gas-phase and un-
der well-characterized molecular beam conditions is needed
before a more detailed picture of the H atom formation dy-
namics can be drawn. On the experimental side, measure-
ments of the product state distributions of the CH2CF2Cl,
and CH2CF2 co-fragments and the investigation of vector
correlations for different photolysis wavelengths would be
most useful in helping to identify the excited electronic
state͑s͒ involved in the UV photolysis of CH3CF2Cl.
*
͑and not in the initial excitation͒, the higher ͓Cl ͔/͓Cl͔
branching ratio observed in the 193.3 nm photolysis of
CH3Cl can be attributed to the higher relative velocity of the
separating fragments. For the CH3ϩCl fragments, a relative
velocity of 6300 m/s can be calculated from the fT value of
0.78 reported in Ref. 48, while the present work yields a
value of 3200 m/s for the CH3CF2ϩCl fragments. Hence the
CH3ϩCl fragmentation process should be more diabatic
*
leading to a higher amount of Cl fragments formed on the
initially excited Q0PES.
3
*
C. H atom formation dynamics and †H‡/†Cl؉Cl ‡
branching ratio
In a study of the H atom formation dynamics in the
193.3 nm laser photolysis of room-temperature CH3CF2Cl, a
value of ⌽H(193.3 nm)ϭ0.06Ϯ0.02 was reported for the H
atom product quantum yield and the relative translational
energy of the H atom fragments was determined to be
E
T(H)(193.3 nm)ϭ51Ϯ10 kJ/mol.18 From the results of our
*
earlier H atom study and the present Cl and Cl atom quan-
V. SUMMARY
*
tum yield measurements a ͓H͔/͓ClϩCl ͔ branching ratio of
0.07Ϯ0.03 can be calculated. Comparison of Eq. ͑1͒ of the
In a gas-phase laser photolysis/laser-induced fluores-
cence ‘‘pump-and-probe’’ study absolute quantum yields
present paper which was used to determine ⌽ClϩCl with the
*
corresponding Eq. ͑1͒ for ⌽H in Ref. 18 shows that the ac-
tual values of the HCl and CH3CF2Cl optical absorption
cross sections are not needed if one wants to determine only
*
for photolytic formation of Cl and Cl atoms were deter-
mined after UV laser excitation of room-temperature
CH3CF2Cl(HCFC-142b) at 193.3 nm. In agreement with our
earlier H atom quantum yield measurements,18 which gave a
*
the relative branching ratio H / ClϩCl ϭ⌽ /⌽
͓ ͔ ͓
.
͔
H
ClϩCl
*
In that case the HCl photolysis measurements serve as a
value of ⌽H(193.3 nm)ϭ0.06Ϯ0.02, the value of ⌽ClϩCl
*
calibration of the relative H versus Cl atom LIF detection
ϭ0.90Ϯ0.17 obtained in the present work clearly indicates
that C–Cl bond cleavage is the dominant dissociation path-
way after A band excitation. The fraction of total available
energy channeled into CH3CF2ϩCl product translational en-
ergy was found to be in agreement with the result of a dy-
namical simulation assuming a repulsive model for single
C–Cl bond cleavage. Both the measured total Cl atom quan-
tum yield and the energy disposal indicates that direct C–Cl
bond cleavage is a primary fragmentation mechanism for
CH3CF2Cl after photoexcitation at 193.3 nm.
*
efficiency. Hence the above ͓H͔/͓ClϩCl ͔ branching ratio
can be directly compared with the results of Ref. 19 in which
H atoms were observed during 193.3 nm laser photolysis of
a ‘‘cold’’ molecular beam of neat CH3CF2Cl. Here the H
atoms were detected by ͑2ϩ1͒ REMPI via the 2s 2S
←1s 2S two-photon transition around 243.135 nm and the
4p 2D3/2←3p 2P3/2 and 4p 2D3/2←3p 2P1/2 two-photon
*
transitions were utilized for Cl and Cl atom ͑2ϩ1͒ REMPI
*
detection, respectively. In this study a relative ͓H͔/͓ClϩCl ͔
branching ratio of 0.67Ϯ0.21 was measured using HCl pho-
tolysis to calibrate the H versus Cl atom REMPI detection
efficiency.19 The same experiments yielded an average value
of ET(H)(193.3 nm)ϭ91Ϯ19 kJ/mol for the H atom transla-
tional energy. Both values are significantly higher than those
obtained in the room temperature gas-phase photolysis ex-
periments. On the other hand, the Cl atom translational en-
ergy release seems to be reduced compared to the gas-phase
results.49 The difference between the molecular beam and the
gas-phase results could indicate that cooling of the parent
molecules leads to a significant modification of the product
branching and fragmentation dynamics.
For the molecular beam studies of Ref. 19, a CH3CF2Cl
rotational temperature of 100 K was reported. Based on this,
one can estimate that under the conditions of the molecular
beam experiment ͑assuming TvibϷTrot͒ only a small fraction
͑ca. 4%͒ of the CH3CF2Cl molecules are in vibrational levels
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support of
the European Union ͑Contract No. ISC -CT940096 of the
*
International Scientific Cooperation program between the
University of Heidelberg and the Ben-Gurion-University of
the Negev, Beer-Sheva, Israel͒, the Korea Research Founda-
tion ͑nondirected research fund 1995–1998͒, and the Korea
Science and Engineering Foundation ͑Korea-Germany Joint
Project, 1997–1998͒. HPU wishes to acknowledge a fellow-
ship provided by the DLR Bonn ͑Indo-German bilateral
agreement, Project No. CHEM-19͒. HRV would like to
thank I. Bar, A. Melchior, S. Rosenwaks, M. Cameron, and
R. K. Vatsa for helpful communications and J. Wolfrum ͑Di-
rector of the Institute of Physical Chemistry, Ruprecht-Karls-
University Heidelberg͒ for his continuous support and his
interest in the ongoing work.
with Ͼ0, while for a room temperature sample this value is
v
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J. Chem. Phys., Vol. 110, No. 24, 22 June 1999
Brownsword et al.
11829
1 P. J. Crutzen, in Physics and Chemistry of Upper Atmospheres, edited by
B. M. McCormac ͑Reidel, Dordrecht, 1973͒; M. J. Molina and F. S. Row-
land, Nature ͑London͒ 249, 810 ͑1974͒.
23 R. A. Brownsword, M. Hillenkamp, T. Laurent, R. K. Vatsa, and H. R.
Volpp, J. Chem. Phys. 106, 4436 ͑1997͒.
24 R. A. Brownsword, C. Kappel, P. Schmiechen, H. P. Upadhyaya, and
H.-R. Volpp, Chem. Phys. Lett. 289, 241 ͑1998͒.
2 R. P. Wayne, The Chemistry of Atmospheres, 2nd ed. ͑Oxford University
Press, Oxford, 1991͒ and references therein.
25 G. Hilber, A. Lago, and R. Wallenstein, J. Opt. Soc. Am. B 4, 1753
͑1987͒.
3 ATLAS3 Public Affairs Status Report #20, NASA Science Operation
Space Flight Center, Huntsville, Nov. 13, 1994.
26 K. Tonokura, Y. Matsumi, M. Kawasaki, S. Tasaki, and R. Bersohn, J.
Chem. Phys. 97, 8210 ͑1992͒ and references therein.
27 W. L. Wiese, M. W. Smith, and B. M. Miles, Atomic Transition Prob-
abilities, Vol. 2, Nat. Stand. Ref. Data Ser. Nat. Bur. Stand. 22 ͑1969͒.
28 R. A. Brownsword, C. Kappel, H. P. Upadhyaya, and H.-R. Volpp ͑to be
published͒.
4 A. K. Nayak, T. J. Buckley, M. J. Kurylo, and A. Fahr, J. Geophys. Res.
101, 9055 ͑1996͒.
5 S. Pinnock, M. D. Hurley, K. P. Shine, T. J. Wallington, and T. J. Smyth,
J. Geophys. Res. 100, 23227 ͑1995͒.
6 E. Linak and P. Yau, Chemical Economics Handbook ͑SRI International,
Menlo Park, CA, 1995͒.
29 J. Zhang, M. Dulligan, and C. Wittig, J. Chem. Phys. 107, 1403 ͑1997͒.
30 Y. Mo, K. Tonkua, Y. Matsumi, M. Kawasaki, T. Sato, T. Arikawa, P. T.
A. Reilly, Y. Xie, Y. Yag, Y. Huag, and R. J. Gordon, J. Chem. Phys. 97,
4815 ͑1992͒.
7 J. M. Lee, W. T. Sturges, S. A. Penkett, D. E. Oram, U. Schmidt, A.
Engel, and R. Bauer, Geophys. Res. Lett. 22, 1369 ͑1995͒.
8 T. Shirai and Y. Makide, Chem. Lett. 4, 357 ͑1998͒.
9 M. Kanakidou, F. J. Dentener, and P. J. Crutzen, J. Geophys. Res. 100,
18781 ͑1995͒.
31
͑a͒ R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson, Jr., J. A. Kerr,
10 Scientific Assessment of Ozone Depletion: 1991, World Meteorological
Organization Global Research and Monitoring Project Report No. 25
͑World Meteorological Organization, Geneva, Switzerland, 1992͒.
and J. Troe, J. Phys. Chem. Ref. Data 21„6…, 1125 ͑1992͒; ͑b͒ C. F.
Melius, Sandia National Laboratories, BAC-MP4 Heats of Formation
Data File ͑Vers. July 16, 1996͒.
32 E. C. Y. Inn, J. Atmos. Sci. 32, 2375 ͑1975͒.
11
͑a͒ M. B. Robin, Higher Excited States of Polyatomic Molecules, Vol. I
33 A. K. Nayak, T. J. Buckley, M. J. Kurylo, and A. Fahr, J. Geophys. Res.
101, 9055 ͑1996͒.
͑Academic, New York, 1974͒; ͑b͒ J. Doucet, P. Sauvageau, and C. San-
dorfy, J. Chem. Phys. 62, 355 ͑1975͒; ͑c͒ H. Okabe, Photochemistry of
Small Molecules ͑Wiley, New York, 1978͒; ͑d͒ W. B. DeMore, S. P.
Sander, D. M. Golden, R. F. Hampson, M. J. Kurylo, C. J. Howard, A. R.
Ravishankara, C. E. Kolb, and M. J. Molina, Chemical Kinetics and Pho-
tochemical Data for Use in Stratospheric Modeling, No. 12 ͑NASA, JPL
Publication 97-4, 1997͒.
34 C. Hubrich and F. Stuhl, J. Photochem. 12, 93 ͑1980͒.
35 D. Gillotay and P. C. Simon, J. Atmos. Chem. 12, 269 ͑1991͒.
36 R. Talukdar, A. Mellouki, T. Gierczak, J. B. Burkholder, S. A. McKeen,
and A. R. Ravishankara, J. Phys. Chem. 95, 5815 ͑1991͒.
37
͑a͒ R. Bersohn, in Molecular Photodissociation Dynamics, edited by M.
12 For example, Z. F. Dong, M. Schneider, and J. Wolfrum, Int. J. Chem.
Kinet. 21, 387 ͑1989͒ and references therein.
N. R. Ashfold and J. E. Baggot ͑Royal Society of Chemistry, London,
1987͒; ͑b͒ R. Schinke, Photodissociation Dynamics ͑Cambridge Univer-
sity Press, Cambridge, 1992͒.
13 For example, Y. Jones, B. E. Homes, D. W. Duke, and D. L. Tipton, J.
Phys. Chem. 94, 4957 ͑1990͒ and references therein.
38 M. Kawasaki, K. Kasatani, H. Sato, H. Shinohara, and N. Nishi, Chem.
Phys. 88, 135 ͑1984͒.
14 R. N. Zitter and D. F. Koster, J. Am. Chem. Soc. 100, 2265 ͑1978͒; R. N.
Zitter, D. F. Koster, A. Cantoni, and J. Pleil, Chem. Phys. 46, 107 ͑1980͒;
R. N. Zitter, D. F. Koster, A. Cantoni, and A. Ringwelski, Chem. Phys.
51, 11 ͑1981͒.
39 A. F. Tuck, J. Chem. Soc., Faraday Trans. 2 73, 689 ͑1977͒.
40 R. D. Levine and R. B. Bernstein, Molecular Reaction Dynamics and
Chemical Reactivity ͑Oxford University Press, Oxford, 1987͒.
41 R. N. Dixon, Chem. Soc. Rev. 23, 375 ͑1994͒.
15 A. S. Sudbo, P. A. Schulz, E. R. Grant, Y. R. Shen, and Y. T. Lee, J.
Chem. Phys. 68, 1306 ͑1978͒; A. S. Sudbo, P. A. Schulz, Y. R. Shen, and
Y. T. Lee, J. Chem. Phys. 69, 2312 ͑1978͒; G. A. West, R. E. Weston, Jr.,
and G. W. Flynn, Chem. Phys. 35, 275 ͑1978͒; A. Sondag and G. H.
Wegdam, Chem. Phys. Lett. 92, 191 ͑1982͒.
42 Y. Matsumi, K. Tonokura, M. Kawasaki, and T. Ibuki, J. Chem. Phys. 93,
7981 ͑1990͒.
43 Y. Matsumi, K. Tonokura, M. Kawasaki, and T. Ibuki, J. Chem. Phys. 97,
5261 ͑1992͒.
16 T. Ichimura, A. W. Kirk, and E. Tschuikow-Roux, J. Phys. Chem. 81,
2040 ͑1977͒.
44 M. H. Alexander, B. Pouilly, and T. Duhoo, J. Chem. Phys. 99, 1752
͑1993͒.
17 T. Ichimura, A. W. Kirk, and E. Tschuikow-Roux, Int. J. Chem. Kinet. 9,
697 ͑1977͒.
45 H. M. Lambert, P. J. Dagdigian, and M. H. Alexander, J. Chem. Phys.
108, 4460 ͑1998͒.
18 R. A. Brownsword, M. Hillenkamp, T. Laurent, H.-R. Volpp, J. Wolfrum,
R. K. Vatsa, and H.-S. Yoo, J. Chem. Phys. 107, 779 ͑1997͒.
46 The authors of Ref. 45 noted that the use of the ratio of the REMPI line
intensities reported in Ref. 43 would lead to 35% lower values.
47 H. M. Lambert and P. J. Dagdigian, Chem. Phys. Lett. 275, 499 ͑1997͒.
48 Y. Matsumi, K. Tonokura, M. Kawasaki, G. Inoue, S. Satyapal, and R.
Bersohn, J. Chem. Phys. 94, 2669 ͑1991͒.
19
͑a͒ A. Melchior, I. Bar, and S. Rosenwaks, J. Chem. Phys. 107, 8476
͑1997͒; ͑b͒ A. Melchior, H. M. Lambert, P. J. Dagdigian, I. Bar, and S.
Rosenwaks, Isr. J. Chem. 37, 455 ͑1997͒; A comparison of Fig. 1 of Ref.
19 ͑a͒ with the data shown in Fig. 2 of Ref. 19 ͑b͒ suggests that the H and
Cl atom REMPI signals obtained for CH3CF2Cl ͑HCFC-142b͒ have been
erroneously assigned to CH3CFCl2 ͑HCFC-141b͒ in the latter article.
20 A. Melchior, P. Knupfer, I. Bar, S. Rosenwaks, T. Laurent, H.-R. Volpp,
and J. Wolfrum, J. Phys. Chem. 100, 13375 ͑1996͒.
49 From the Cl atom Doppler width (FWHMϭ0.55 cmϪ1) reported in Ref.
19͑a͒ the average relative kinetic energy can be estimated ͑assuming a
VUV effective laser bandwidth of 0.4 cmϪ1͒ to be ca. 28 kJ/mol which is
about 4.5 times smaller than the average relative Cl fragment kinetic en-
ergy determined in the present room temperature gas phase experiments.
50 D. McNaughton and C. Evans, J. Mol. Spectrosc. 182, 342 ͑1997͒.
51 The relative population of the vibrationally excited states was estimated
treating the CH3CF2Cl molecule as a quantum mechanical harmonic os-
cillator with 18 normal modes of vibration. The values for the 18 vibra-
tional frequencies were taken from Ref. 50.
21 R. A. Brownsword, M. Hillenkamp, T. Laurent, R. K. Vatsa, H.-R. Volpp,
and J. Wolfrum, J. Phys. Chem. A 101, 995 ͑1997͒.
22
͑a͒ R. A. Brownsword, M. Hillenkamp, T. Laurent, R. K. Vatsa, H.-R.
Volpp, and J. Wolfrum, J. Phys. Chem. A 101, 5222 ͑1997͒; ͑b͒ R. A.
Brownsword, M. Hillenkamp, T. Laurent, R. K. Vatsa, H.-R. Volpp, and
J. Wolfrum, J. Chem. Phys. 106, 1359 ͑1997͒.
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