Photodissociation dynamics of the chloromethanes at the Lyman-a
wavelength (121.6 nm)
R. A. Brownsword, M. Hillenkamp, T. Laurent, R. K. Vatsa,a) H.-R. Volpp,b)
and J. Wolfrum
Physikalisch-Chemisches Institut der Universität Heidelberg, Im Neuenheimer Feld 253,
D-69120 Heidelberg, Germany
~Received 21 August 1996; accepted 14 October 1996!
The gas-phase dissociation dynamics of CH3Cl, CH2Cl2 , and CHCl3 after photoexcitation at the
Lyman-a wavelength ~121.6 nm! were studied under collision-free conditions at room temperature.
Narrow-band tunable Lyman-a laser radiation ~l L a '121.6 nm! was generated by resonant
third-order sum-difference frequency conversion of pulsed-dye-laser radiation and used both to
photodissociate the parent molecules and to detect the nascent H atom products via (2 p 2 P←1s 2 S)
laser induced fluorescence. Absolute H atom quantum yields FH~CH3Cl!5~0.5360.05!,
FH~CH2Cl2!5~0.2860.03!, and FH~CHCl3!5~0.2360.03! were determined employing a photolytic
calibration method where the Lyman-a photolysis of H2O was used as a reference source of
well-defined H atom concentrations. H atom Doppler profiles were measured for all chlorinated
methanes. In the case of CH3Cl the line shapes of the profiles indicate a pronounced bimodal
translational energy distribution suggesting the presence of two H atom formation mechanisms
leading to a markedly different H atom translational energy release. The observed ‘‘slow’’
component of the H atom translational energy distribution corresponds to an average kinetic energy
of ~5565! kJ/mol, while the ‘‘fast’’ component leads to an average kinetic energy of ~320617!
kJ/mol. The relative branching ratio between the ‘‘fast’’ and the ‘‘slow’’ H atom channel was
determined to be ~0.7160.15!. For CH2Cl2 and CHCl3 no bimodal translational energy distributions
were observed. Here the translational energy distributions could each be well described by a single
Maxwell–Boltzmann distribution, corresponding to an average translational energy of ~8169! kJ/
mol and ~7564! kJ/mol, respectively. © 1997 American Institute of Physics.
@S0021-9606~97!01004-0#
I. INTRODUCTION
In industry, chlorinated methanes ~CHn Cl42n ! are
widely used in many areas: methyl chloride ~CH3Cl! for production of silicones and other materials, methylene chloride
~CH2Cl2! is widely used for its solvent properties in paint
strippers, aerosols, metal cleaning, electronics manufacture,
and general industrial processing, and chloroform ~CHCl3! is
used in the production of hydrochlorofluorocarbon 22
~HCFC-22!.1 CH3Cl is the only precursor of stratospheric
ClOx that is established to be natural.2 In the stratosphere
CH3Cl is destroyed either by reaction with hydroxyl radicals
or, above 30 km, photolytically.2 In both cases Cl atoms
become able to enter the ozone destroying ClOx cycle.3
For CH3Cl, CH2Cl2 , and CHCl3 optical ultraviolet ~UV!
and vacuum-ultraviolet ~VUV! absorption spectra,4–7 photoelectron spectra,8,9 and fluorescence yields from photodissociative excitation by VUV radiation10 have been measured.
Very low fluorescence quantum yields ~,0.02%! were obtained after excitation at the Lyman-a wavelength. The observed fluorescence was, in the case of CH3Cl, attributed to
CH2(b̃-ã) emission produced by a CH2(b̃)1HCl channel.
For CH2Cl2 the photolytic pathways CH2(b̃)1Cl2 ,
a!
On sabbatical leave from Chemistry Division, Bhabha Atomic Research
Centre, Bombay, India.
b!
Author to whom correspondence should be addressed.
J. Chem. Phys. 106 (4), 22 January 1997
CCl2(Ã)1H2 , CCl2(Ã)1H1H, and for CHCl3 the channels
CCl2(Ã)1HCl and CCl2(Ã)1H1Cl were suggested as
sources of the observed fluorescence.10 Matrix-isolation
studies of the VUV photolysis of CHCl3 were carried out by
Jacox and Milligan.11a UV-multiphoton ionization experiments in argon matrices were reported for CH3Cl and
CH2Cl2 .11b In the gas phase, UV-multiphoton dissociation
studies were carried out for all chlorinated methanes.12 The
single photon UV photodissociation dynamics of the chlorinated methanes have been studied by several groups using
different experimental techniques. Using photofragment
translational spectroscopy, the dissociation dynamics of
CH3Cl ~Ref. 13! and CHCl3 ~Ref. 14! were investigated at a
photolysis wavelength of 193.3 nm. In Ref. 14, Huber and
co-workers found that at this wavelength, HCl molecular
elimination is unimportant and C–Cl bond fission is the primary photolytic process for CHCl3 . For the chlorinated
methanes @Cl*~2P 1/2!/Cl~2P 3/2!# fine structure branching ratios were measured at photolysis wavelengths of 193.3 nm
and 157.6 nm using the ~211! resonance enhanced multiphoton ionization ~REMPI! technique15–17 as well as the laser
induced fluorescence ~LIF! method18 for Cl* and Cl atom
detection. In Ref. 19a an overview of the measured values
can be found. H atom formation was observed in the 157.6
nm photolysis of the chlorinated methanes.19b The measured
relative yields @H#/@Cl*1Cl# were found to decrease with the
0021-9606/97/106(4)/1359/8/$10.00
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Brownsword et al.: Photodissociation of the chloromethanes
FIG. 1. Experimental apparatus used for the CHn Cln24 VUV photolysis studies. The Kr four-wave mixing scheme for generation of tunable Lyman-a laser
radiation is shown as an inset.
number of H atoms present in the molecule.20 The observed
bimodal H atom Doppler profiles could be described by a
combination of a Gaussian and a non-Gaussian profile.20 The
presence of the two distinct H atom velocity distributions
was explained by the possibility for the CHn Cl42n molecules
to undergo both direct and indirect photolytic C–H bond
scission at 157.6 nm.
The aim of the work presented in this article is to extend
the photodissociation dynamics studies to the Lyman-a
wavelength in order to provide further dynamical quantities
such as absolute H atom quantum yields, as well as information about the energy partitioning in the VUV photolysis of
the chlorinated methanes. The Lyman-a wavelength was
chosen due to its very strong presence in the solar emission
spectrum2 and because of the possibility for simultaneous H
atom product detection via (2 p 2 P←1s 2 S) laser induced
fluorescence ~LIF!.
II. EXPERIMENT
The VUV photodissociation studies were carried out in a
flow apparatus, as depicted schematically in Fig. 1. The experimental technique is similar to the one used in recent
UV21a and VUV photodissociation21b and bimolecular reaction dynamics experiments.22 Therefore, only a brief summary of the experimental method will be given in the following.
Room temperature CH3Cl ~>99.8%!, CH2Cl2 ~>99.7%!,
and CHCl3 ~>99.7%! was pumped through the reactor.
CH3Cl was used without further purification. CH2Cl2 and
CHCl3 were degassed by freeze–thaw cycling at liquid N2
temperature. The CH3Cl flow was controlled by a Tylan
flowmeter. CH2Cl2 and CHCl3 flows were controlled using a
needle valve. For the calibration measurement H2O ~deionized and double-distilled! was passed through the reactor.
The H2O flow was regulated by a glass valve. Typical pressures during the photodissociation experiments were 15–90
mTorr, measured by an MKS Baratron.
Narrow-band VUV laser light—tunable around the H
atom Lyman-a transition at 121.567 nm—was generated by
resonant third-order sum-difference frequency conversion of
pulsed-dye-laser radiation in a phase-matched Kr-Ar
mixture23 and used to photodissociate the CHn Cl42n molecule, as well as to detect the photolytically produced nascent H atoms via (2 p 2 P←1s 2 S) laser induced fluorescence
~LIF! within the same laser pulse. The duration of the laser
pulse was about 15–20 ns, which ensures the collision-free
detection of the produced H atoms under the low-pressure
conditions of the experiment. In the Kr mixing scheme
~shown as an inset in Fig. 1! via which the VUV radiation
~vVUV52 v R 2 v T ! was generated, the laser radiation of vR
~lR 5212.55 nm! is resonant with the Kr 4p25p ~1/2, 0!
two-photon transition and held fixed during the experiments,
while vT is tuned from 844 nm to 846 nm to generate VUV
laser radiation covering the H atom Lyman-a transition. The
laser radiation was obtained from two dye lasers ~Lambda
Physik FL 2002!, simultaneously pumped by a XeCl excimer
laser ~Lambda Physik EMG 201 MSC!. In the first dye laser
~denoted as A in Fig. 1!, Coumarin 120 was used to generate
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Brownsword et al.: Photodissociation of the chloromethanes
the 425.10 nm radiation which was subsequently frequency
doubled in a BBO II crystal in order to obtain lR 5212.55
nm. lT 5844–846 nm was obtained by operating the second
dye laser ~denoted as B in Fig. 1! with Styryl 9 dye.
The generated Lyman-a light was carefully separated
from the fundamental laser light by a lens monochromator
~denoted as LM in Fig. 1! followed by a light baffle system.
A bandwidth of DnL a 50.4 cm21 was determined for the
Lyman-a laser radiation in separate experiments by measuring H atom profiles under thermalized conditions ~T trans'300
K!. The H atom LIF signal was measured through a band
pass filter ~ARC, model 122-VN-ID, lcenter5122 nm,
FWHM520 nm! by a solar blind photomultiplier
~Hamamatsu model R1259! positioned at right angles to the
VUV laser beam ~PM 1 in Fig. 1!. During the experiments
the change of the VUV laser beam intensity was monitored
with an additional solar blind photomultiplier of the same
kind ~PM 2 in Fig. 1!. In order to obtain a satisfactory S/N
ratio, each point of the recorded H atom Doppler profiles was
averaged over 30 laser shots. The measurements were carried
out at a laser repetition rate of 6 Hz. H atom LIF signal and
VUV beam intensity were recorded with a two-channel boxcar integrator system ~SRS 250! and transferred to a microcomputer where the H atom LIF signal was normalized
point-by-point to the square of the VUV laser intensity. Because sequential multiphoton absorption ~n.2! may distort
the results, the ~n5111!-photon nature ~one-photon dissociation of the parent molecule followed by one-photon H
atom LIF detection! of the process was checked in separate
experiments where the VUV laser intensity was varied. In
Fig. 2, log–log plots of the measured H atom LIF signal
versus the Lyman-a VUV laser intensity are shown for
CH3Cl and H2O photolysis, respectively. Also for CH2Cl2
and CHCl3 no significant deviations from n52 were observed within the experimental errors. It is therefore concluded that the secondary photodissociation of photolysis
products is negligible and need not be considered in the
analysis of the results.
III. RESULTS
A. H atom quantum yields
Absolute quantum yields FH~CHn Cl42n ! for photolytic
H atom formation were obtained by calibrating the H atom
signal SH~CHn Cl42n ! measured in the CHn Cl42n photodissociation against the H atom signal SH~H2O! from well-defined
H atom number densities generated by photolyzing H2O. The
UV and VUV photodissociation of H2O has been studied in
great detail24–27 and a H atom quantum yield of
F~H2O!51.02 was measured after H2O excitation at the
Lyman-a wavelength.28a In the present study, the absolute H
atom quantum yields for the different chlorinated methanes
were determined using the following equation:
F H~ CHn Cl42n ! 5 $ S H~ CHn Cl42n ! F ~ H2O! s H2Op H2O% /
$ S H~ H2O! s CHn Cl42n p CHn Cl42n %
~1!
1361
FIG. 2. Dependence of the observed H atom LIF signal on the Lyman-a
laser intensity: ~a! for CH3Cl ~15 mTorr! and ~b! for H2O ~90 mTorr!. The
values for the slopes of the linear log–log plots are given in the figure.
where s H2O and s CHn Cl42n are the optical absorption cross
sections of H2O and CHn Cl42n at the Lyman-a wavelength.
Optical absorption cross sections for H2O and the chlorinated
methanes at the Lyman-a wavelength were measured in the
course of the present experiments. The following values
were obtained: s H2O5(1.660.1)310217 cm2, s CH3Cl5(8.8
60.2)310217 cm2, s CH2Cl25(4.060.1)310217 cm2, and
s CHCl3 5(3.560.2)310217 cm2 and found to be in good
agreement with earlier measurements.10,28b S H are the integrated areas under the measured H atom Doppler profiles,
and p H2O and p CHn Cl42n are the pressure of H2O and
CHn Cl42n , respectively. Care was taken to choose the H2O
and CHn Cl42n pressures in the calibration measurements in
such a way that ‘‘pre-absorption’’ of the VUV-laser beam
before reaching the observation region was comparable in
each case.
In Fig. 3, typical H atom Doppler profiles from the H2O
and CHn Cl42n Lyman-a photodissociation are shown. In independent calibration runs, integrated areas under the fluorescence curves were determined for the H2O and CHn Cl42n
photodissociation under identical experimental conditions
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Brownsword et al.: Photodissociation of the chloromethanes
FIG. 3. Comparison of the total H atom signal ~defined as the integrated
area of the line profile! produced in the Lyman-a photolysis of 15 mTorr
CH3Cl with the signal observed for 77 mTorr of H2O. Details of the calibration method are explained in the text.
which gave, using Eq. ~1!, the following average values for
the H atom quantum yields: FH~CH3Cl!5~0.5360.05!,
FH~CH2Cl2!5~0.2860.03!, and FH~CHCl3!5~0.2360.03!.
B. Average H atom translational energy
From the H atom Doppler profiles the average kinetic
energies, i.e., E T ~CHn Cl42n !, of the H atoms in the laboratory frame, were determined. Because the measured H atom
Doppler profiles reflect, via the linear Doppler shift
n2n05v z n 0 /c, directly the distribution of the velocity component v z of the absorbing H atoms along the propagation
direction of the probe laser beam, for an isotropic velocity
distribution the average translational energy is given by
E T 5(3/2)m H^ v 2z & , where ^ v 2z & represents the second moment
of the laboratory velocity distribution of the H atoms. Evaluation of the measured profiles by a direct calculation of the
second moment gave the following values for the average H
atom kinetic energies: E T ~CH3Cl!5~164615! kJ/mol,
E T ~CH2Cl2!5~8169! kJ/mol, E T ~CHCl3!5~7564! kJ/mol.
The measured H atom Doppler profiles showed a pronounced bimodal structure in the case of CH3Cl. For CH2Cl2
and CHCl3 such a bimodal feature was not present. Following the suggestions of Matsumi and co-workers,20 the bimodal H atom Doppler profiles from the CH3Cl photolysis were
fitted by a superposition of a wide non-Gaussian component
which describes a speed distribution centred at a high velocity, and a narrow Gaussian component which corresponds to
a statistical Maxwell–Boltzmann-like velocity distribution.
The results of such a numerical least-squares fit to a measured Doppler profile are shown in Fig. 4~a!. For CH3Cl the
average width ~FWHM! of the non-Gaussian contribution
@dotted line in Fig. 3~a!, represented by a symmetric double
sigmoidal function# was ~13.560.6! cm21, the FWHM of the
Gaussian @dashed line in Fig. 3~a!# one was ~3.960.2! cm21.
The ratio of the areas of the two different Doppler profile
contributions ~non-Gaussian divided by Gaussian! was determined to be Gnon-G/GG5~0.7160.15!. The quoted uncertainties reflect one standard deviation of the values ~Gaussian
and non-Gaussian widths and areas, respectively! obtained in
the evaluation of the complete set of measured profiles. The
quality of each individual Doppler profile fit was much
higher, as can be seen in Fig. 4~a!, and in no case was a
systematic trend in the residuals of the fit observed. The
experimental results are summarized in Table I and compared to results obtained at a photolysis wavelength of 157.6
nm. In Fig. 5~a! the translational energy distribution P(E T )
is depicted ~solid line! which corresponds to the observed
bimodal H atom Doppler profile. The parts of the energy
distribution which belong to the non-Gaussian and the
Gaussian component of the Doppler profile, respectively, are
depicted as dotted and dashed lines. Evaluation of these two
distinct translational energy distributions yielded average
values of E T(non-G) 5 (320 6 17) kJ/mol and E T(G) 5 (55 6 5)
kJ/mol for the non-Gaussian and Gaussian components of
the Doppler profile, respectively; evaluation of the data assuming an anisotropy parameter of b51/3 for the nonstatistical component ~this value has been suggested in Ref. 19b
for the nonstatistical component observed in the 157.6 nm
photolysis of CH3Cl! would lead to a slight reduction of only
the E T(G) value which is, however, still within our experimental uncertainty. The same holds for the value of the
Gnon-G/GG ratio, which in that case would increase slightly.
The H atom Doppler profiles for CH2Cl2 and CHCl3
could be fitted very well by a single Gaussian function @see
Figs. 4~b! and ~c!# with a FWHM of ~4.960.2! cm21 and
~4.660.2! cm21, respectively. A Gaussian line shape for a
Doppler profile is consistent with an isotropic Maxwell–
Boltzmann distribution of velocities.19b The corresponding
distributions of the translational energy are depicted in Figs.
5~b! and ~c!.
IV. DISCUSSION
A. H atom yields
The total H atom quantum yields FH~CH3Cl!5~0.53
60.05!, FH~CH2Cl2!5~0.2860.03!, and FH~CHCl3!5~0.23
60.03! measured after excitation at the Lyman-a wave-
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Brownsword et al.: Photodissociation of the chloromethanes
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yields of FH~CH3Cl!50.29, FH~CH2Cl2!50.23 and
FH~CHCl3!50.13, assuming that FH1FCl1Cl*51. At the
longer wavelength of 193.3 nm, however, no C–H bond fission could be observed in case of CH3Cl,13a and CHCl3 .14
These results suggest that in contrast to the VUV photodissociation, 193 nm photolysis, in which a ~n Cl→s*!-transition
is excited,6 does not lead to an efficient C–H bond cleavage.
This parallels the results of the early flash photolysis experiments by Levy and Simons29 who reported that selective
C–H rather than C–I bond scission is the primary process in
alkyl iodide photodissociation in the wavelength region 140–
170 nm, while at longer wavelengths ~.170 nm! the quantum yield of C–H bond scission is considerable smaller
~,1022!. A value of ;0.03 was reported by Lee and
co-workers13a for the quantum yield of the C–H bond scission channel in the 193.3 nm laser photolysis of CH3I.
B. H atom Doppler profiles and velocity distributions
In the 157.6 nm photolysis of all chloromethanes, ‘‘bimodal’’ H atom Doppler profiles were observed which could
be simulated by a superposition of a ‘‘wide’’ non-Gaussian
and a ‘‘narrow’’ Gaussian component.20 The non-Gaussian
contributions were attributed to a ‘‘direct’’ photodissociation
channel while the Gaussian components were explained by
the existence of an ‘‘indirect’’ hot molecule decomposition
mechanism as suggested by Bersohn and co-workers.30 In
the present Lyman-a photodissociation study, bimodal H
atom Doppler profiles were observed only in case of CH3Cl
while for CH2Cl2 and CHCl3 the measured H atom Doppler
profiles showed a single Gaussian component.
1. CH3Cl
FIG. 4. H atom Doppler profiles obtained in the Lyman-a photolysis. ~a!
CH3Cl: The solid line represents the result of a fit of composite function
which consists of superposition of a symmetric double sigmoidal function
~dotted line! and a Gaussian function ~dashed line! to the measured bimodal
H atom Doppler profile. ~b! CH2Cl2 and ~c! CHCl3 . In ~b! and ~c! the solid
lines represent results of a fit using a single Gaussian function. The centers
of the LIF spectra correspond to the Lyman-a transition of the H atom
~82 259 cm21!.
length decrease with decreasing number of C–H bonds in the
CHn Cl42n molecules. At a photolysis wavelength of 157.6
nm, a similar trend was observed for the relative
@H#/@Cl1Cl*# yields,20 which can be converted into H atom
In the case of CH3Cl, photon absorption below 165 nm
can induce B band transitions, originating from a
3 p p (e)→4s(a 1 ) Cl atom Rydberg excitation, which have
been assigned to be 1E and 1A 1 molecular states.19b In Ref.
20, the fast H atoms from the non-Gaussian component observed at a photolysis wavelength of 157 nm were attributed
to a direct dissociation from the 1E state of CH3Cl, while for
the slow Gaussian component H atoms it was suggested that
they are formed by a indirect predissociation process via a
‘‘hot’’ CH3Cl‡ molecule mechanism. The bimodal structure
in the H atom Doppler profile observed in the present study
@see Figs. 3~a! and 4~a!# suggests a similar explanation for
the Lyman-a photodissociation of CH3Cl. Although measurements of H atom Doppler profiles for different pumpprobe polarization geometries to determine the anisotropy
parameter b ~a value of b51/3 has been estimated for the
1
E2X̃ 1 A 1 optical transition19b! for the direct H atom channel were not possible in the present case ~as the Lyman-a
laser acts as both pump and probe source, the polarization of
the dissociating light is always perpendicular to the probe
beam propagation direction!, the observed similarity in the
energy release suggests that the non-Gaussian component
observed in the Lyman-a photodissociation of CH3Cl also
originates from a direct C–H bond dissociation via the 1E
state.
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Brownsword et al.: Photodissociation of the chloromethanes
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TABLE I. FWHM values and quantum yields of H atoms from the photolysis of chloromethanes. The results obtained at a photolysis wavelength of 157.6
nm are from Ref. 20.
Molecule
lphoto/nm
Gaussian
CH3Cl
157.6
121.6
157.6
121.6
157.6
121.6
6.4
~3.960.2!
4.2
~4.960.2!
4.4
~4.660.2!
CH2Cl2
CHCl3
a
FWHM/cm21
non-Gaussian
13.2
~13.560.6!
13.1
10.7
-
Gnon-G/GG
FH
4
~0.7160.15!
0.2
0
1.5
0
0.29a
~0.5360.05!
0.23a
~0.2860.03!
0.13a
~0.2360.03!
The FH values at 157.6 nm are calculated from the relative hydrogen versus chlorine atom yield values given in Ref. 20 assuming that FH1FCl1Cl*51.
The overall shape of the observed H atom Doppler profile, in particular the details of the Gaussian component as
well as the ratio of the non-Gaussian to Gaussian contribution, is very similar to what was observed in the 157.6 nm
photolysis of CH3I ~see Fig. 4 of Ref. 19b!. This similarity
could be explained by the fact that due to the lower lying
2
E 1/2 and 2E 3/2 cation states of CH3I the same Rydberg bands
can be excited in CH3I at 157.6 nm as for CH3Cl at the
Lyman-a wavelength.6 In addition, in both cases, the available energy is high enough to allow direct scission of the
C–H bond from the parent molecule as well as H atom formation via unimolecular decomposition of the hot methyl
radical photolytically produced by the initial C–X ~X5Cl, I!
bond cleavage. In Ref. 19b, the consecutive pathway
CH3I1\v→CH‡31I~ 2 P 3/2! ,
~2a!
CH‡3→CH21H,
~2b!
was suggested to explain the observed narrow Gaussian
component in the 157.6 nm photolysis of CH3I.
After excitation of CH3Cl at the Lyman-a wavelength,
the following H atom formation pathway
CH3Cl1\v→CH‡31Cl~ 2 P 3/2 , 2 P 1/2!
~3a!
CH‡3→CH21H,
~3b!
as well as
CH3Cl1\v→CH2Cl‡1H,
~4a!
CH2Cl‡→CHCl1H,
~4b!
is energetically allowed ~see Table II!. Neither of reaction
sequences ~3! and ~4! is possible for CH3Cl at a photolysis
wavelength of 157.6 nm ~759.2 kJ/mol!, so the narrowing of
the Gaussian contribution ~see Table I! in going from 157.6
nm to the Lyman-a wavelength could be attributed to the
opening of the above sequential H atom formation pathways
at the shorter photolysis wavelength.
31
2. CH2Cl2 and CHCl3
For both molecules, photodissociation at the Lyman-a
wavelength leads to H atom Doppler profiles which can be
well described by a single Gaussian function, which corresponds to a Maxwell–Boltzmann distribution of the translational energy. Non-Gaussian components as observed in the
157.6 nm photodissociation experiments20 are not present in
FIG. 5. H atom translational energy distributions P(E T ). ~a! CH3Cl: The
dotted and dashed lines describe the two components of the distribution
which belong to the observed ‘‘statistical’’ and ‘‘nonstatistical’’ features
observed in the Doppler profile @see Fig. 4~a!#. In ~b! and ~c! the Maxwell–
Boltzmann-like distributions as derived from the Gaussian Doppler profiles
@Figs. 4~b! and ~c!# are shown.
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Brownsword et al.: Photodissociation of the chloromethanes
TABLE II. Standard enthalpies of formation DH of ~298! and reaction enthalpies DH R ~298! of different energetically possible H atom product channels
in kJ/mol ~Ref. 31!. The available energy to the products is given by E avl
5\ v L a (983.9 kJ/mol)2DH R (298 K).
Species
DH of ~298 K!
Reaction
DH R ~298 K!
E avl
CH3Cl
CH2Cl2
CHCl3
CH2Cl
CHCl2
CCl3
1
CH2
3
CH2
CCl2
CHCl
Cl
H
282
295.4
2103.3
121.8
98.3
71.1
430.1
392.5
239
297.49
121.30
217.997
CH3Cl→CH2Cl1H
CH3Cl→CHCl1H1H
CH3Cl→3CH21Cl1H
CH3Cl→1CH21Cl1H
CH2Cl2→CHCl21H
CH2Cl2→CCl21H1H
CH2Cl2→CHCl1Cl1H
CHCl3→CCl31H
CHCl3→CCl21Cl1H
421.8
815.5
813.8
851.4
411.7
770.4
732.2
392.4
681.6
562.1
168.4
170.1
132.5
572.2
213.5
251.7
591.5
302.3
1365
suggests that H atom formation proceeds via two mechanisms. A direct one—characterized by ‘‘primary’’ C–H
bond fission—which leads to a ‘‘fast’’ nonstatistical contribution in the translational energy distribution and an indirect
channel responsible for the ‘‘slow’’ statistical distribution
with the H atoms being produced via a ‘‘secondary’’ unimolecular decay of energized CH3 and/or CH2Cl fragments.
For CH2Cl2 and CHCl3 the observed H atom translational energy distributions could each be well described by a
single Maxwell–Boltzmann distribution, suggesting for both
molecules a similar H atom producing ‘‘hot molecule’’ decay mechanism.
ACKNOWLEDGMENTS
the corresponding H atom Doppler profiles obtained after
excitation at the Lyman-a wavelength. This indicates a clear
dominance of the suggested ‘‘indirect’’ hot molecule mechanism over the ‘‘direct’’ one at shorter VUV wavelengths.
The observed statistical distribution of the H atom translational energy could be explained by a presence of a strong
coupling between the internal states of the hot intermediate.
In this case the dissociation process is dominated by the final
state interaction32 and can be—if energy is completely
randomized—described by a half collision scattering matrix,
which gives rise to final state distributions which can be
obtained using statistical models.33 Since both the H atoms
from CH2Cl2 and CHCl3 which contains only one H atom
exhibit a similar energy distribution, the scission of two C–H
bonds cannot be essential for the observed Maxwell–
Boltzmann distributions.
In the VUV absorption spectra of CH2Cl2 and CHCl3 , in
addition to broad underlying absorption bands, a weak vibrational structure was observed in the Lyman-a region.10 This
might suggest that the observed ‘‘statistical’’ H atom formation proceeds via a Herzberg type I predissociation
mechanism,26,34 e.g.,
‡
CH2Cl21\ v L a →CH2Cl!!
2 →CH2Cl2→H1...,
in which a hot intermediate ~indicated by ‡! is formed by a
nonadiabatic transition from the initially excited bound Rydberg state ~!!! to a dissociative state.
V. CONCLUSION
Using the laser photolysis/laser-induced fluorescence
‘‘pump/probe’’ technique, total H atom quantum yields were
measured for the dissociation of the chloromethanes following photoexcitation at the Lyman-a wavelength. The measured values demonstrate that VUV photolysis of CHn Cl42n
in contrast to UV photolysis ~193.3 nm! can produce considerable H atom concentrations.
Under collision-free conditions, bimodal H atom Doppler profiles were observed in case of CH3Cl. An energetic
analysis of the bimodal H atom kinetic energy distribution
R.K.V. wishes to acknowledge a fellowship provided by
the KFA Jülich and DLR Bonn under the Indo-German bilateral agreement ~Project No. CHEM-19!. 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, Israel!, as well as support of the Deutsche Forschungsgemeinschaft, the Alexander von Humboldt
Stiftung and the Max-Planck-Gesellschaft. We would like to
thank P. Farmanara for help in the experiments and A. Suvernev ~Institute for Theoretical Astrophysics, University
Heidelberg! and P. Frantsuzev ~Department of Physics, The
Weizmann Institute of Science, Rehovot, Israel! for helpful
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