H atom formation dynamics in the dissociation of CH3-CF2Cl (HCFC-142b) after UV and VUV laser photoexcitation

Article abstract of DOI:10.1063/1.474376

Using the laser photolysis/laser-induced fluorescence (LIF) "pump-and-probe" technique, the dynamics of H atom formation in the photodissociation of CH₃-CF₂Cl (HCFC-142b) after excitation at 193 nm and the Lyman-α wavelength were studied under collision-free conditions in the gas-phase at room temperature. The H atoms produced were detected by (2p²P←1s²S)-LIF using tunable narrow-band Lyman-α laser radiation (λ_Lα ≈ 121.6 nm) generated by resonant third-order sum-difference frequency conversion of pulsed dye laser radiation. In the VUV photodissociation experiments the Lyman-α laser radiation was used both to photodissociate the parent molecules and to detect the produced nascent H atoms via laser induced fluorescence. The following quantum yields Φ_H for H atom formation were determined by a photolytic calibration method: Φ_H(193 nm) = (0.06±0.02) and Φ_H(L_α) = (0.53±0.12). From the measured H atom Doppler profiles the average H atom kinetic energy was determined to be E_T(193 nm) = (51 ±10) kJ/mol and E_T(L_α) = (72±4) kJ/mol, respectively.

Full text of DOI:10.1063/1.474376

H atom formation dynamics in the dissociation of CH 3 –CF 2 Cl (HCFC-142b) after UV
and VUV laser photoexcitation
R. A. Brownsword, M. Hillenkamp, T. Laurent, H.-R. Volpp, J. Wolfrum, R. K. Vatsa, and H.-S. Yoo
Citation: The Journal of Chemical Physics 107, 779 (1997); doi: 10.1063/1.474376
View online: http://dx.doi.org/10.1063/1.474376
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H atom formation dynamics in the dissociation of CH3–CF2Cl (HCFC-142b)
after UV and VUV laser photoexcitation
R. A. Brownsword, M. Hillenkamp, T. Laurent, H.-R. Volpp,a) and J. Wolfrum
¨
Physikalisch-Chemisches Institut der Universitat Heidelberg, Im Neuenheimer Feld 253,
D-69120 Heidelberg, Germany
R. K. Vatsa
Chemistry Division, Bhabha Atomic Research Centre, Bombay 400-085, India
H.-S. Yoo
Department of Chemistry, Chungbuk National University, Cheongju 361-763, Korea
͑Received 14 February 1997; accepted 8 April 1997͒
Using the laser photolysis/laser-induced fluorescence ͑LIF͒ ‘‘pump-and-probe’’ technique, the
dynamics of H atom formation in the photodissociation of CH3–CF2Cl ͑HCFC-142b͒ after
excitation at 193 nm and the Lyman-␣ wavelength were studied under collision-free conditions in
the gas-phase at room temperature. The H atoms produced were detected by (2p2P←1s2S)-LIF
using tunable narrow-band Lyman-␣ laser radiation (␭ Ϸ121.6 nm) generated by resonant
L
␣
third-order sum-difference frequency conversion of pulsed dye laser radiation. In the VUV
photodissociation experiments the Lyman-␣ laser radiation was used both to photodissociate the
parent molecules and to detect the produced nascent H atoms via laser induced fluorescence. The
following quantum yields ⌽H for H atom formation were determined by a photolytic calibration
method: ⌽H(193 nm)ϭ(0.06Ϯ0.02) and ⌽H(L␣)ϭ(0.53Ϯ0.12). From the measured H atom
Doppler profiles the average H atom kinetic energy was determined to be ET(193 nm)ϭ(51
Ϯ10) kJ/mol and ET(L␣)ϭ(72Ϯ4) kJ/mol, respectively. © 1997 American Institute of Physics.
͓S0021-9606͑97͒01027-1͔
I. INTRODUCTION
Huber, who found, using the photofragment translational
spectroscopy, that at a photolysis wavelength of 193 nm,
C–Cl bond fission is the primary photochemical process.6 In
CHF2Cl ͑HCFC-22͒ dissociation dynamics studies carried
out by Melchior et al., H atoms were observed besides Cl
atom formation after photoexcitation at the same UV
wavelength.7 Using time-of-flight mass spectrometry ͑TOF-
MS͒ in combination with (2ϩ1) resonance-enhanced multi-
Following the Montreal Protocol hydrochlorofluorocar-
bons ͑HCFCs͒ can be used, on a transitional basis, as re-
placements of the chlorofluorocarbons ͑CFCs͒, which are be-
lieved to be responsible for the dramatic stratospheric ozone
reductions observed in the polar regions.1 The HCFC com-
pounds have at least one C–H bond and can therefore be
oxidized by OH free radicals in the troposphere. Although
this leads to shorter lifetimes compared to those of the CFC
compounds, HCFCs still have the potential to lead to chlo-
rine transportation to the stratosphere. Recently measure-
ments of stratospheric profiles of CH3–CF2Cl ͑HCFC-142b͒,
a principal replacement choice for CF2Cl2 ͑CFC-12͒ in, e.g.,
polystyrene foams,2 were carried out and it was found that
CH3–CF2Cl is not yet in equilibrium, due to the rapidly in-
creasing emissions.3 Therefore, besides atmospheric in situ
observations detailed laboratory studies on the UV and VUV
photochemistry of these compounds and in particular mea-
surements of primary photochemical product branching ra-
tios are clearly needed in order to assess their environmental
impact.4
photon ionization ͑REMPI͒ product detection,
a
2
H/ Cl(2P3/2)ϩCl ( P1/2) ratio of (0.19Ϯ0.06) and an up-
*
͕
͖
per limit of 1% for the HCl elimination channel was deter-
mined. In recent experiments, Brownsword et al. investi-
gated the H atom formation dynamics and determined
quantum yields (⌽H) for H atom formation after photoexci-
tation of CHF2Cl, CH3Cl, and CH2Cl2 at 193 nm and the
Lyman-␣ wavelength͑121.6 nm͒ and found that while in the
193 nm photolysis H atom formation is of minor importance
only it becomes a major channel in the Lyman-␣ photolysis.8
In the present article we report on the extension of these
studies in order to provide experimental dynamical quantities
such as photolytic H atom yields and information about the
energy partitioning in the UV ͑193 nm͒ and Lyman-␣ pho-
tolysis also for the CH3–CF2Cl molecule. The measured H
atom formation yields ͑in particular the one measured at the
Lyman-␣ wavelength9͒ will help to assess the relative impor-
tance of this photochemical product pathway in the strato-
sphere and can serve as an absolute calibration point for
photodissociation experiments where only relative product
Optical absorption spectra in the UV and in the VUV
were measured for a number of HCFCs5 while, on the other
hand, the product formation dynamics after excitation in this
spectral region have only been studied for a limited number
of HCFCs. Experiments on the UV photodissociation of
CHFCl2 ͑HCFC-21͒ were carried out by Yang, Felder, and
*
branching ratios ͑e.g., H/͕ClϩCl ͖, H/HCl͒ can be
Author to whom correspondence should be addressed.
measured.10 A detailed comparison of the present results ob-
a͒
J. Chem. Phys. 107 (3), 15 July 1997
0021-9606/97/107(3)/779/7/$10.00
© 1997 American Institute of Physics
779
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Brownsword et al.: Dissociation of CH3–CF2Cl
tained under collision-free conditions with the results of ear-
lier 147 and 123.6 nm photodecomposition studies carried
The photolysis laser beam was aligned so as to overlap
the probe beam at right angles in the viewing region of the
LIF detector. The delay time between the UV pump and
VUV probe pulses was typically 80 ns ͑with a time jitter of
about Ϯ5 ns͒, which allowed the collision-free detection of
the H atoms produced under the low-pressure conditions of
the experiment. A delay time of 80 ns was chosen to avoid
that pump and probe laser do overlap in time, which could
lead to unwanted multiphoton photochemical processes. The
quality of the spatial overlap of the pump and probe laser
beams was checked in order to ensure that no photolytically
produced H atoms can escape the detection region during the
typical delay time of 80 ns. This was done by recording H
atom Doppler profiles in the 193 nm photolysis of HCl at
pump/probe delay times between 20 and 150 ns. HCl pho-
out in
a ‘‘static’’ apparatus in the pressure range
p
CH –CF Clϭ3.6–20.6 Torr11–13 will be made. Possible pri-
3
2
mary photochemical fragmentation mechanisms will be dis-
cussed in the light of the present results.
II. EXPERIMENT
The photodissociation studies were carried out in a flow
cell at mTorr level pressures using a setup which was used
previously to investigate the H atom formation in the VUV
photodissociation of CHF2Cl,8 chloromethanes,14 and the re-
action dynamics of bimolecular gas-phase reactions.15 In the
following a brief summary of the experimental method will
be given.
tolysis at 193 nm leads to very fast H atoms (
vH
Ϸ19 300 m/s) and the invariance of the line shape, in par-
ticular the invariance of the area around the center of the
profile ͑which originates from H atom with a velocity vector
perpendicular to the probe laser propagation direction͒ is a
very sensitive check for ‘‘fly-out.’’ H atom Doppler profiles
measured in the 193 nm HCl photolysis were invariant
against changes in the pump/probe delay time in the range
between 20 and 150 ns. It can therefore be ruled out that the
measured H atom yields we report for CH3–CF2Cl are af-
fected by ‘‘fly-out’’ of photolytically produced H atoms.
In the two color ͑193 nm pump and Lyman-␣ probe͒
experiments, although the probe laser intensities were low,
small contributions to the LIF signal from H atoms produced
directly by the Lyman-␣ photolysis of CH3–CF2Cl and HCl,
respectively, were observed. In order to be able to com-
pletely discriminate between this ‘‘Lyman-␣-background’’
signal and that produced in the 193 nm photolysis, an elec-
tronically controlled mechanical shutter was inserted into the
photolysis beam path. At each point of the H atom Doppler
profile, signal was recorded first with the shutter open, and
then with the shutter closed, the difference between the two
signals representing the contribution from H atoms generated
solely by the 193 nm photolysis laser pulse. This procedure
is similar to the ‘‘Lyman-␣-background’’ subtraction method
extensively used in our recent H/D atom formation studies in
the UV photolysis of HNCO/DNCO.17
In a second set of experiments, the Lyman-␣ VUV laser
light was used to photodissociate the CH3–CF2Cl molecule
as well as to detect the photolytically produced nascent H
atoms within the same laser pulse. The duration of the laser
pulse was about 15–20 ns. In all experiments care was taken
to separate the Lyman-␣ VUV laser light from the uncon-
verted fundamental laser beams. Therefore, a lens monochro-
mator followed by a light baffle system was used. A band-
width of 0.4 cmϪ1 was determined for the VUV radiation in
separate experiments by measuring under thermalized condi-
tions Doppler profiles of H atoms generated in a microwave
discharge. The H atom LIF signal was measured through a
band pass filter ͑ARC, model 122-VN-ID, ␭centerϭ122 nm,
CH3–CF2Cl ͑Messer Griesheim, purity Ͼ98%͒ was
pumped through the cell at room temperature. As stated by
the manufacturer the amount of R141b (CH3–CFCl2) in the
sample was less than 0.1%, with the remaining impurity be-
ing CH3–CF3 which did not interfere. The water content was
less than 5 ppm. In the 193 nm photodissociation studies the
CH3–CF2Cl pressure in the cell was typically 50–100
mTorr, in the Lyman-␣ photolysis studies the pressure was
50–80 mTorr. For the calibration measurements at 193 nm,
HCl ͑Messer Griesheim, 99.99%͒ and for the calibration
measurements at the Lyman-␣ wavelength, H2O ͑deionized
and double distilled͒ was pumped through the reaction cell at
pressures of typically 5–10 and 40–70 mTorr, respectively.
The flow rates of CH3–CF2Cl and HCl were regulated by
mass flow controllers, the H2O flow was regulated using a
glass valve.
In one part of the study a UV excimer laser operating
with an ArF mixture ͑193 nm emission wavelength, pulse
duration 15–20 ns͒ was used to photodissociate the parent
molecules. A circular aperture was used to skim off a homo-
geneous 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 photolysis beam. Pump
laser intensities were typically between 4–10 mJ/cm2. The
pump laser beam was determined to be essentially unpolar-
ized, and it is therefore expected that any possible anisotropy
of the photodissociation process would be largely averaged
out.
Narrow-band VUV laser light—tunable around the
H͑121.567 nm͒ atom Lyman-␣ transition—was generated by
resonant third-order sum-difference frequency conversion of
pulsed-dye-laser radiation ͑pulse duration 15–20 ns͒ in a
phase-matched Kr/Ar mixture16 in order to detect the pho-
tolytically produced nascent H atoms via (2p2P←1s2S) la-
ser induced fluorescence ͑LIF͒. The maximum VUV energy
which can be obtained with this method is about 20
␮J/pulse.16͑b͒ However, in the present experiments the energy
was considerably lower due to absorption processes in the
cell windows and VUV optics and in all experiments the
actual VUV laser intensity was reduced until the desired in-
tensity dependence of the H atom LIF signal was obtained.
FWHMϭ20 nm͒ by
a
solar blind photomultiplier
͑Hamamatsu model R1259͒. During all measurements the
relative VUV laser beam intensity was monitored with an ad-
J. Chem. Phys., Vol. 107, No. 3, 15 July 1997
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Brownsword et al.: Dissociation of CH3–CF2Cl
FIG. 1. H atom Doppler profiles obtained in the Lyman-␣ photolysis of ͑a͒
57 mTorr of CH3–CF2Cl and ͑b͒ 45 mTorr of H2O. The centres of the LIF
spectra correspond to the H atom Lyman-␣ transition (82 259 cmϪ1). Com-
parison of the total H atom signal ͑defined as the integrated area of the line
profile͒ produced in the Lyman-␣ photolysis of CH3–CF2Cl with the signal
observed in the case of H2O is also shown. Details of the calibration method
for the H atom quantum yield measurement in the CH3–CF2Cl photolysis
are described in the text.
FIG. 2. ͑a͒ Dependence of the H atom LIF signal on the 193 nm photolysis
͑‘‘pump’’͒ laser and ͑b͒ dependence of the H atom LIF signal on the
Lyman-␣ laser intensity in the CH3–CF2Cl photolysis experiments. In case
of ͑b͒ the Lyman-␣ laser acts as ‘‘pump’’ and ‘‘probe’’ laser. Solid lines are
the results of a fit to the experimental data to determine the power-
dependence n of the H atom LIF signal. The obtained values for n are given
in the figure.
ditional solar blind photomultiplier of the same kind. In or-
der to obtain a satisfactory S/N ratio, each point of a re-
corded H atom Doppler profiles ͑as shown in Fig. 1͒ was
averaged over 30 laser shots. Measurements were carried out
at a laser repetition rate of 6 Hz. The LIF signal and UV
photolysis and VUV probe beam intensities were recorded
with a boxcar integrator system and then transferred to a
microcomputer.
In the two color ͑UV pump–VUV probe͒ photodissocia-
tion studies care was taken that the observed H atom signals
measured in the HCl and CH3–CF2Cl photolysis showed a
linear dependence on the 193 nm pump laser ͓see Fig. 2͑a͔͒
and the probe laser intensities. In these experiments the H
atom LIF signal was normalized point-by-point to the UV
pump and Lyman-␣ VUV ͑probe͒ laser intensity.
VUV laser radiation was varied. In Fig. 2͑b͒ a plot of the
measured H atom LIF signal versus the Lyman-␣ VUV laser
intensity is shown for CH3–CF2Cl. Similar measurements
were carried out in case of the H2O Lyman-␣ photolysis,
where for comparable experimental conditions a value of n
ϭ(2.0Ϯ0.2) was observed. We therefore conclude that
higher order processes like the secondary photodissociation
of photolysis products can be neglected and need not be
considered in the analysis and interpretation of the results.
III. RESULTS
A. Photolytic H atom quantum yields
In the one color ͑VUV pump–probe͒ experiments the H
atom LIF signal was normalized point-by-point to the square
of the Lyman-␣ VUV ͑pump/probe͒ laser intensity. There-
fore, the (nϭ1ϩ1)-photon nature ͑one-photon dissociation
of the CH3–CF2Cl parent molecule followed by a one-
photon H atom LIF-detection step͒ of the process was veri-
fied in separate experiments in which the intensity of the
Absolute quantum yields ⌽H for photolytic H atom for-
mation were obtained by calibrating the H atom signal
SH͑CH3–CF2Cl͒ as measured in the CH3–CF2Cl photodisso-
ciation against the H atom signal SH͑HCl/H2O͒ from well-
defined H atom number densities generated in the 193 nm
photolysis of HCl and Lyman-␣ photolysis of H2O, respec-
tively. 193 nm photolysis of HCl generates H atoms with a
J. Chem. Phys., Vol. 107, No. 3, 15 July 1997
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Brownsword et al.: Dissociation of CH3–CF2Cl
quantum yield of unity and for the Lyman-␣ photolysis of
H2O an H atom quantum yield of ⌽͑H2O͒ϭ1.02 was
determined.18 In the Lyman-␣ photolysis experiments, for
example, the H atom quantum yield was determined via the
following equation:
⌽
L ͒ϭ␥ S ͑CH –CF Cl͒⌽͑H O͒␴H OpH O /
͕ ͖
␣ H 3 2 2
2 2
͑
H
S ͑H O͒␴CH –CF ClpCH –CF Cl
.
͑1͒
͕
͖
H
2
3
2
3
2
␴
H O and ␴CH –CF Cl are the optical absorption cross sections
2
3
2
of H2O and CH3–CF2Cl at the Lyman-␣ wavelength. The
optical absorption cross section for H2O at the Lyman-␣
wavelength was measured in a previous study ͑where the H
atom yield in the Lyman-␣ photolysis of methane was deter-
19
mined with a similar calibration method͒ to be ␴
H O
2
ϭ(1.6Ϯ0.1)ϫ10Ϫ17 cm2. The Lyman-␣ absorption cross
sections ␴CH –CF Clϭ͑3.1Ϯ0.2͒ϫ10Ϫ17 cm2 was also mea-
3
2
sured in the course of the present experiments and was found
to be in very good agreement with the results of earlier
measurements.5͑d͒ SH are the integrated areas under the mea-
sured H atom Doppler profiles ͑see, e.g., Fig. 1͒ and pH O
2
and pCH –CF Cl are the pressures of H2O and CH3–CF2Cl,
3
2
respectively. For the evaluation of the 193 nm photolysis
experiments ⌽͑H2O͒, pH O , ␴H O , and SH͑H2O͒ have to be
2
2
replaced by the corresponding HCl values. Literature
values of ␴HClϭ͑8.1Ϯ0.4͒ϫ10Ϫ20 cm2 and ␴
CH –CF Cl
3
2
ϭ0.53ϫ10Ϫ20 cm2 for the optical absorption cross sections at
193 nm were used.20,5͑c͒ In Eq. ͑1͒, the factor ␥ is a correc-
tion which accounts for the different degrees of absorption of
the Lyman-␣ laser radiation by CH3–CF2Cl, HCl, and
H2O, respectively.
FIG. 3.
H atom Doppler profiles obtained in the dissociation of
CH3–CF2Cl after photoexcitation at ͑a͒ 193 nm and ͑b͒ at the Lyman-
␣ wavelength ͑121.6 nm͒. The dashed lines represent results of a fit using
a
symmetric double sigmoidal function ͑for details see the text͒.
Arrows indicate the maximum possible H atom Doppler shifts for the
•CH2–CF2ClϩH͑i͒ and CH2ϭCF2ϩHϩCl͑ii͒ product channels.
In the 193 nm photolysis studies the experimental data
set which was evaluated consisted of 13 H atom profiles
obtained in 6 independent calibration measurements in com-
bination with 8 ‘‘HCl calibration lines.’’ The H atom quan-
tum yield in the Lyman-␣ photolysis was evaluated using 10
H atom profiles measured in 5 independent calibration runs
where 8 ‘‘H2O calibration lines’’ were recorded. All the pro-
files had a S/N ratio similar to the profiles shown in Figs. 1
and 3. The quoted errors were determined from the 1␴ sta-
tistical uncertainties of the experimental data, the uncertain-
ties of the measured optical absorption cross sections, and in
case of ⌽H(L␣), from the uncertainty of the H atom quan-
tum yield of water ͑estimated to be 20%͒ using simple error
propagation. The following values for the H atom quantum
yields were obtained: ⌽H͑193 nm͒ϭ(0.06Ϯ0.02) and
⌽H(L␣)ϭ(0.53Ϯ0.12).
v2
v2
represents the second moment
͘
ETϭ3/2 mH
, where
͗
͘
͗
z
z
of the laboratory velocity distribution f( z) of the photolyti-
v
cally produced H atoms. The velocity distribution f( z) was
v
determined by fitting the measured H atom line shapes using
a symmetric double sigmoidal function.21 The results of the
least-squares fit procedure are shown in Fig. 3. The analysis
of the results revealed that all the H atom line shapes ob-
tained in the 193 nm photolysis experiments can be well
described by a symmetric double sigmoidal function which
very closely resembles a Gaussian. For the H atom line
shapes obtained in the Lyman-␣ photolysis this was not the
case. Here, the use of a Gaussian fit function would lead to a
systematic trend in the residuals of the fit21͑c͒ in particular in
the wings of the measured profiles and, as a consequence,
later in the evaluation to an underestimation of the average H
atom kinetic energy of about 25%.
B. Average H atom translational energies
The measured H atom Doppler profiles were analysed in
order to determine the average kinetic energy. Because the
measured Doppler profiles directly reflect, via the linear
Calculation of the second moments of f( z) gave the
v
following values for the average H atom kinetic energies:
ET(193 nm)ϭ(51Ϯ10) kJ/mol and ET(L␣)ϭ͑72Ϯ4͒ kJ/
mol. The quoted error represents the 1␴ statistical uncer-
tainty obtained in the evaluation of the complete experimen-
tal data set. 13 H atom profiles measured in the 193 nm
photolysis and 10 measured in the Lyman-␣ photolysis were
Doppler shift ␯Ϫ␯ ϭ ␯0 /c, the distribution of the velocity
vz
0
component
of the absorbing atoms along the propagation
vz
direction of the probe laser beam, for an isotropic velocity
distribution the average H atom kinetic energy is given by
J. Chem. Phys., Vol. 107, No. 3, 15 July 1997
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Brownsword et al.: Dissociation of CH3–CF2Cl
TABLE I. Reaction enthalpies ⌬HR(298 K) of product channels in the dissociation of CH3–CF2Cl which are
discussed in the text. Available energies to the products were calculated via Eavl͑␭
Ϫ⌬HR(298 K). Values are given in kJ/mol. Thermochemical data are from Ref. 24.
͒ϭប␻͑␭
͒
pump
pump
Product channel
⌬HR(298 K)
147 nma
123.6 nma
121.6 nm
Eavl : 193 nm
•CH2–CF2ClϩH
438.7
535.3
544.9
103.7
180.2
83.6
73.9
375.1
278.5
268.9
710.1
529.1
432.6
423.0
864.2
545.3
448.8
439.2
880.4
CH2vCF2ϩHϩCl
b
*
CH2vCF2ϩHϩCl
CH2vCF2ϩHCl
515.2
aWavelengths used in the static decomposition studies of Ref. 11 and 12, respectively.
b
*
The heat of formation of Cl was calculated from the Cl value using ⌬Eϭ0.1 eV ͑9.64 kJ/mol͒ for the
Cl(2P1/2Ϫ3/2) spin–orbit splitting.
evaluated. In the evaluation of the H atom profiles it has to
be taken into consideration that the measured line shape rep-
resents a convolution of the VUV laser spectral profile and
the Doppler profile of the absorbing atoms. However, the
measured probe laser bandwidth of our VUV laser system is
so small that a back-correction due to the finite probe laser
bandwidth was well below the experimental uncertainty.
photodissociation studies of CH3Cl and CH2Cl2, where also
rather small primary H atom yields were observed (⌽H
Ͻ0.02) after A band excitation at 193 nm.8
B. Photolysis of CH3–CF2Cl in the VUV spectral
region
In the 147 and in the 123.6 nm photolysis of
static samples of CH3–CF2Cl, using gas chromatography
for final product analysis, CH2vCF2 was found to be
the major photodecomposition product11,12 and the following
yields were reported: ⌽CH CF (147 nm)ϭ͑0.74Ϯ0.06͒ and
IV. DISCUSSION
The measured quantum yields demonstrate the increas-
ing importance of photolytic H atom formation in the
CH3–CF2Cl fragmentation in going from UV to VUV exci-
tation wavelengths. A similar behaviour was observed in UV
and VUV photodissociation studies of CHF2Cl and
chloromethanes8,14,22 as well as in earlier CH3I UV and VUV
photolysis studies.23
2
2
⌽
CH CF (123.6 nm)ϭ͑0.82Ϯ0.02͒.
2 2
From the known CH3–CF2Cl UV/VUV absorption
spectrum5͑d͒ it can be derived that at 147 nm, valence ͑A
band͒ as well as to a certain extent Rydberg excitation ͑B
Band͒ can occur.25 At 123.6 nm and at the Lyman-␣ wave-
length ͑121.6 nm͒ D band Rydberg excitation can occur as
well.5͑a͒,5͑d͒ B band absorption can be attributed to a Rydberg
process, related to a (nCl→4s) transition while D band
absorption26 originates from a (nCl→4p) Rydberg excita-
tion.
A. Photolysis of CH3–CF2Cl at 193 nm
For CH3–CF2Cl, absorption of a 193 nm photon leads to
an excitation in the first absorption band ͓denoted as A band
*
in Ref. 5͑a͔͒, which originates from a 3p␲→␴ (C–Cl) va-
Although, concerning the final photochemical products
the results of the 147 and 123.6 nm photolysis studies were
found to differ very little—the only photochemical process
unambiguously assignable to the 123.6 nm wavelength was
the opening of an additional minor reaction pathway leading
lence shell transition, where 3p␲ is a lone pair orbital of the
*
Cl and ␴ is an antibonding C–Cl/␴–MO. Therefore, it can
be expected as suggested by Okabe5͑b͒ that the initial excita-
tion is localized in the C–Cl bond leading preferentially to
the breaking of this bond. H atom formation can occur, how-
ever, after intramolecular redistribution of the excitation en-
ergy, either via a single C–H bond fission or via a three-body
to CH2vCFCl formation (⌽CH CFClϭ0.05)12—the ‘‘stabil-
2
ity’’ of the major product molecule CH2vCF2 was found to
depend strongly on the excitation wavelength.11,12 In the 147
nm studies at low sample pressures, CHwCF was observed
as a product and its formation was attributed to a sequential
process of the form11
*
dissociation process in combination with Cl ͑or Cl ͒ atoms
and 1,1-difluoroethylene (CH2vCF2) formation ͑see Table
I͒. That both reaction channels would be consistent with the
observed energy release into H atom translational energy can
be seen in Fig. 3͑a͒, where the H atom Doppler profile ob-
served at the 193 nm photolysis wavelength is depicted; the
arrows ͑i͒ and ͑ii͒ indicate the maximum possible Doppler
shift for the above suggested two-body and three-body prod-
uct channels. Reaction enthalpies and the values of the ener-
gies available to the products as listed in Table I were cal-
culated using the heat of formations given in Ref. 24. The
observed small H atom quantum yield, however, clearly in-
dicates that H atom formation is only a minor product chan-
nel in the 193 nm photolysis of CH3–CF2Cl. The measured
value for the H atom yield is in line with results obtained in
h␯
‡
*
CH3–CF2Cl →CH3–CF2Cl →CH2ϭCF2ϩHCl
CH2ϭCF‡2→CHwCFϩHF
͑2a͒
͑2b͒
CH2ϭCF‡2ϩM→CHzϭCF2ϩM.
͑2c͒
‡
*
Here, ‘‘ ’’ denotes the electronically excited state, and ‘‘ ’’
an internally excited electronic ground state molecule. Based
on an extrapolation of the observed pressure dependence of
the ⌽CH CF /⌽CHCF ratio to collison-free conditions it was
2
2
suggested that most of the nascent CH2vCF‡2 molecules are
formed with an average internal energy higher than the acti-
J. Chem. Phys., Vol. 107, No. 3, 15 July 1997
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784
Brownsword et al.: Dissociation of CH3–CF2Cl
vation energy for HF elimination via reaction ͑2b͒.11 As a
consequence it was suggested that in the 147 nm photolysis
the primary product HCl must be formed in reaction ͑2a͒ for
the most part with insufficient energy to undergo further de-
composition to yield HϩCl.
Despite the fact that after dissociative excitation at 123.6
nm a considerably higher amount of energy is available to
the products, no pressure dependence of the final
CH2vCF2 yield was found in that case.12 In addition, con-
siderably lower concentrations of CHwCF were observed in
the 147 nm studies, and it was suggested that after excitation
at 123.6 nm CH2vCF2 molecules are initially produced in
an excited singlet state but are, under the pressure conditions
of the experiment, very efficiently quenched to the electronic
ground state with less internal energy than the activation
energy required for subsequent HF elimination.
nonical’’ statistical model of Baer et al.31—assuming equi-
partitioning of the available energy among the product de-
grees of freedom and treating the final fragmentation step of
Eq. ͑4͒ as a simultaneous three-body process—a ‘‘product
temperature’’ of TϷ3500 K can be obtained. The fact that
the experimentally observed average H atom translational
energy ET(L␣)ϭ(72Ϯ4) kJ/mol is considerably higher than
the statistical ‘‘prior’’ value of 3/2 kTϭ44 kJ/mol might
suggest that the actual dissociation process lies somewhere
29͑a͒
in between the statistical ͑strong coupling͒
and nonstatis-
tical ͑direct rapid predissociation͒ limit.32 However, for a
more detailed understanding of the CH3–CF2Cl UV and
VUV photochemistry a more sophisticated theoretical dy-
namical treatment of the fragmentation process and more in-
formation about the electronically excited states involved in
the fragmentation processes is clearly needed.
The quantum yield of (0.53Ϯ0.12) for primary H atom
formation as measured in the present study after excitation at
the Lyman-␣ wavelength ͑121.6 nm͒ suggests, however, that
a considerable amount of CH2vCF2 could be produced di-
rectly in its electronic ground state via a three-body dissocia-
tion process of the form
V. SUMMARY
Using the laser photolysis/laser-induced fluorescence
‘‘pump-and-probe’’ technique absolute quantum yields for
photolytic formation of H atoms were determined after opti-
cal excitation of CH3–CF2Cl͑HCFC-142b͒ at 193 nm and at
the Lyman-␣ wavelength ͑121.6 nm͒. The measured quan-
tum yields ⌽H͑193 nm͒ϭ(0.06Ϯ0.02) and ⌽H(L␣)ϭ(0.53
Ϯ0.12) clearly indicate that H atom production is only a
minor photochemical product channel in the UV photolysis
but becomes a major product channel in the VUV photodis-
sociation. The relatively high value of ⌽H(L␣) suggests
CH3–CF2Cl to be an efficient precursor of translationally
excited H atoms in the earth’s stratosphere. In addition, the
experimentally observed energy release into the H atom
translational degree of freedom when combined with results
from previous static photodecomposition experiments sug-
gests that besides H atoms comparable amounts of atomic
chlorine and internally excited CH2vCF2 molecules are pro-
duced in the Lyman-␣ photolysis of CH3–CF2Cl.
h␯
‡
*
CH3–CF2Cl →CH3–CF2Cl →CH2ϭCF2ϩHϩCl.
͑3͒
In that case, the average H atom translational energy of 72
kJ/mol as determined in the present study sets a ‘‘natural’’
upper limit for the internal energy of the nascent
CH2vCF‡2 molecules of Eintр376.8 kJ/mol which is only
slightly above the activation energy of ⑀HFϭ372 kJ/mol27 for
HF elimination via Eq. ͑2b͒. In the calculation of the upper
limit for Eint it was assumed that the Cl atoms are produced
in the electronic ground state (2P3/2) with zero kinetic en-
ergy. Therefore, it can be expected that in the actual disso-
ciation process only very few of the CH2vCF‡2 molecules
produced by reaction ͑3͒ will have enough energy to undergo
further decomposition by HF elimination. Thus the very
small amount of CHwCF produced in the static photolysis
experiments at 123.6 nm as well as the observed pressure
independence of the CH2vCF2 yield would be consistent
with the above suggested primary dissociation mechanism
͑3͒.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support of
the European Union under Contract No. ISC -CT940096 of
the International Scientific Cooperation programme between
the University of Heidelberg and the Ben-Gurion-University
of the Negev ͑Beer-Sheva, Israel͒. R.K.V. wishes to ac-
*
The H atom Doppler profiles as observed in the present
study after excitation of CH3–CF2Cl at the Lyman-␣ wave-
length are similar to the ones observed in the Lyman-␣ pho-
¨
knowledge a fellowship provided by the KFA Julich and the
8͑a͒
tolysis of CH2Cl2 and CHCl3 where H atom formation has
DLR Bonn under the Indo-German bilateral agreement
͑Project No. CHEM-19͒. H.S.Y. acknowledges support from
the Center for Molecular Science at KAIST ͑Korea͒. Y.
been suggested to proceed via a Herzberg I predissociation
mechanism.28 This dissociation mechanism is of an indirect
type in which an internally highly excited reaction interme-
diate ͑indicated by †͒ can be formed by a nonadiabatic tran-
¨
Schneider-Kuhnle is thanked for his help in the experiments,
I. Bar, A. Melchior, and S. Rosenwaks for helpful commu-
nications, and C. F. Melius for providing us with the BAC-
MP4 Heats of Formation Data file.
*
sition from the initial excited Rydberg state ͑ ͒ to a dissocia-
tive state
‡
†
*
CH3–CF2Cl →CH3–CF2Cl →CH2ϭCF2ϩHϩCl. ͑4͒
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