The photodissociation of toluene studied by forward photofragment translational spectroscopy

Article abstract of DOI:10.1063/1.469533

The translational energies of different fragments produced in laser induced unimolecular reactions are investigated in a molecular beam experiment.The time-of-flight of the photoproducts is measured in the forward direction with a mass specrometer.By operating the ion source at low electron impact energies it is possible to nearly eliminate fragmentation and observe the nascent products at their parent mass.With this technique the primary and secondary reaction channels can be identified and branching ratios and product translational energy distribution down to small kinetic energies can be measured.The method is illustrated in a study of the photodissociation of toluene (methylbenzene) at three excitation energies (E_ex=51 590, 52 240, and 63 240 cm^-1).From the measured product time-of-flight spectra the branching ratio as well as the translational energy distributions of the product molecules benzyl+H and phenyl+CH3 in the two reaction channels were determined.The measured product energy distributions are compared with RRK (Rice-Ramsperger-Kassel) and SACM (statistic adiabatic channel model) statistical theories.The experimental results agree very well with the SACM theory for the phenyl, but not for the benzyl products.This is attributed to a barrier in the exit channel for the latter product.

Full text of DOI:10.1063/1.469533

The photodissociation of toluene studied by forward photofragment translational
spectroscopy
Ralf Fröchtenicht
Citation: The Journal of Chemical Physics 102, 4850 (1995); doi: 10.1063/1.469533
View online: http://dx.doi.org/10.1063/1.469533
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The photodissociation of toluene studied by forward photofragment
translational spectroscopy
a),b)
¨
Ralf Frochtenicht
¨
¨
¨
Max-Planck-Institut fur Stromungsforschung, Bunsenstrasse 10, D-37073 Gottingen, Germany
͑Received 26 August 1994; accepted 15 December 1994͒
The translational energies of different fragments produced in laser induced unimolecular reactions
are investigated in a molecular beam experiment. The time-of-flight of the photoproducts is
measured in the forward direction with a mass spectrometer. By operating the ion source at low
electron impact energies it is possible to nearly eliminate fragmentation and observe the nascent
products at their parent mass. With this technique the primary and secondary reaction channels can
be identified and branching ratios and product translational energy distribution down to small kinetic
energies can be measured. The method is illustrated in a study of the photodissociation of toluene
͑methylbenzene͒ at three excitation energies ͑Eexϭ51 590, 52 240, and 63 240 cmϪ1͒. From the
measured product time-of-flight spectra the branching ratio as well as the translational energy
distributions of the product molecules benzylϩH and phenylϩCH3 in the two reaction channels
were determined. The measured product energy distributions are compared with RRK ͑Rice–
Ramsperger–Kassel͒ and SACM ͑statistic adiabatic channel model͒ statistical theories. The
experimental results agree very well with the SACM theory for the phenyl, but not for the benzyl
products. This is attributed to a barrier in the exit channel for the latter product. © 1995 American
Institute of Physics.
I. INTRODUCTION
in the center-of-mass frame, as is the case for the molecular
systems studied in this work. By varying the distance be-
tween laser excitation and detection zone, the time-of-flight
resolution and the desired product signal ͑given by the de-
tection solid angle͒ can be optimized as required. The detec-
tion of reaction products on the axis of the incident reactant
beam is possible by the reduced ion fragmentation of the
reactant molecules at low energy of the ionizing
electrons.15,16 By choosing the electron energies just above
the ionization potential of the product radical but below the
appearance potential of the reactant molecule, only the prod-
uct radicals are detected.16–19
The advantages and implications of this technique are
demonstrated in the present article for the laser-induced uni-
molecular bond fission of toluene ͑methylbenzene͒. After
photoexciting the molecules at 193 nm to the electronically
excited S3 state, high vibrational states in the electronic
ground state are populated via fast internal conversion
͑Ͻps͒.20,21 The excited complex lives a few hundred nano-
seconds ͑cf. Table I͒ before it decays to products with small
kinetic energies. The reaction has been investigated exten-
sively in several groups with different methods and makes
toluene an ideal system for demonstrating the new experi-
mental technique. In previous experiments two reaction
paths R1 and R2 were found
Unimolecular reactions are important intermediates in
complex reaction processes, for example, in atmospheric
chemistry,1,2 in combustion processes,3 in plasma physics or
in reactions in liquids.4 They also provide an ideal means for
the study of molecular dynamics since the number of reac-
tion parameters is reduced compared to bimolecular reac-
tions.
For the study of laser-induced unimolecular reactions of
polyatomic molecules in the gas phase under nearly collision
free conditions, essentially two experimental techniques have
been exploited.5 Several variations on time-resolved pump–
probe techniques6 involving for detection either absorption
spectroscopy,7,8
laser-induced
fluorescence
͑LIF͒,9
resonance-enhanced multiphoton ionization ͑REMPI͒,10 and
direct light emission11 have been used. The other method is
photofragment translational spectroscopy12,13 under molecu-
lar beam condition. The goal is to determine the primary
products and their branching ratio, to measure the lifetime of
the excited reactant molecule, and to determine the distribu-
tion of the excess energy on the internal quantum states and
translational degrees of freedom.6,14
In the present work photofragment translational spectra
in a molecular beam experiment have been measured after
nanosecond laser pulse excitation of the reactant molecules.
In contrast to conventional photofragment translational spec-
troscopy the product molecules are detected in the laboratory
frame only in forward direction. Products with low kinetic
energy can now be observed and no rotatable source or de-
tector is needed. The interpretation of the measured data is
straightforward if the product angular distribution is isotropic
a͒
Thermal rate constants for the reaction channel R1 at 300
K and in the temperature range from 1200 to 1500 K to-
E-mail: froechtenich@msfd1.dnet.gwdg.de
b͒
¨
Present address: BMWAG, 80788 Munchen, Germany.
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¨
Ralf Frochtenicht: Photodissociation of toluene
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TABLE I. Survey of experimental studies of the specific rate constants of the photodissociation of toluene.
Specific rate constants
͑sϪ1
Detected
product
Eex
͑cmϪ1
͒
͒
Method
Beam/cell
Reference
46 020
52 080
52 080
52 730
52 730
52 730
52 730
52 730
52 730
64 450
1.5Ϯ1ϫ105
LIF
H atom
Benzyl
H atom
Benzyl
Benzyl
H atom
Methyl
Methyl
H atom
Benzyl
Cell
Cell
Cell
Cell
Cell
Cell
Cell
Beam
Beam
Cell
32
25
31
28
8
32
33
35
35
30
1.9Ϯ0.2ϫ106
2.6Ϯ0.5ϫ106
2.01Ϯ0.04ϫ106
1.9Ϯ0.2ϫ106
3.0Ϯ0.3ϫ106
2.2Ϯ1.0ϫ106
2.5Ϯ0.2ϫ106
2.3Ϯ0.2ϫ106
9.3Ϯ0.7ϫ107
UV Absorption
LIF
UV Absorption
UV Absorption
LIF
͑2ϩ1͒REMPI
͑3ϩ1͒REMPI
͑1ϩ1͒REMPI
UV Absorption
gether with the rate constants for the reverse reaction have
been previously determined from extensive shock wave stud-
ies using time-resolved UV absorption.22–24 The same time-
resolved UV absorption technique was used to measure
specific rate constants of laser-induced unimolecular reac-
tions of toluene and other alkyl benzenes.7,8,25,27–30 Instead
of probing the benzyl radical via UV absorption, the reaction
channel R1 was also studied via Lyman-␣ laser-induced fluo-
rescence ͑LIF͒ of the H atom reaction product.31,32 With this
detection method specific rate constants from the temporal
behavior ͑cf. Table I͒ as well as mean translational energies
from the LIF Doppler profile ͑cf. Table III͒ were
obtained.31,32
The second reaction path R2 to a phenyl radical and a
methyl radical was investigated by Troe and co-workers us-
ing ͑2ϩ1͒ resonance-enhanced multiphoton ionization
͑REMPI͒ of the methyl radical at 333.5 nm33 and nonreso-
nant vuv ͑121 nm͒ one photon ionization34 in a gas phase
cell experiment. From the temporal increase of the methyl
ion signal the specific rate constant was determined ͑cf.
Table I͒. The product yield directly provides the branching
ratio of the reaction paths ͑cf. Table V͒. In a very recent
molecular beam experiment ͑3ϩ1͒ REMPI detection of the
methyl radicals at 450.8 nm and ͑1ϩ1͒ REMPI detection of
the hydrogen molecule at 121.6 nm was used in order to
measure the specific rate constants after exciting toluene at
193 nm.35,36
enthalpies of formation of Ϸ11 000 cmϪ1 between the two
isomers39 provides for an additional energy to that of the
photon energy. Accidentally, direct excitation of toluene at
␭ϭ193 nm and via cycloheptatriene at ␭ϭ248 nm leads to
nearly the same internal energies of 51 740 cmϪ1 and 51 090
cmϪ1, respectively. Following photoexcitation, bond fission
to either benzylϩH or to phenylϩmethyl takes place. In Fig.
1 the reaction mechanisms are sketched.
II. EXPERIMENTAL SET-UP
Figure 2 shows a schematic diagram of the apparatus
with important diameters and pressures. The molecular beam
vacuum apparatus consists of four differentially pumped
vacuum stages. The molecules are expanded through a
nozzle ͑diameter 100 ␮m͒ into the first chamber ͑pumped by
a baffled 6000 l /s diffusion pump͒. To prevent condensation
and clustering of the molecules, the stainless steel nozzle
chamber is resistively heated with a tantal wire between 300
and 500 K. Different expansion conditions were selected us-
ing helium and argon at different pressures as seed gases.
Photofragment translational spectroscopy with a rotat-
able mass spectrometer detector has been used by Lee and
co-workers to study both reaction channels. In these experi-
ments cycloheptatriene was first excited at 248 and 266 nm.
In a subsequent step the cycloheptatriene isomerizes to
highly excited toluene.37,38 The high electron energies used
for product ionization lead to an extensive ion fragmentation,
so that ions from different neutral precursors can fragment
on the same mass. To avoid a direct entrance of the reactant
beam in the detection chamber measurements were carried
out at laboratory angles larger than 5°, so that products with
a small excess velocity which only arise at small laboratory
angles could not be studied.
FIG. 1. The photodissociation of toluene is schematically described in an
energy diagram displaying the energy levels of the different molecules in-
volved in the reaction. To generate highly vibrationally excited toluene mol-
ecules either cycloheptatriene can be excited electronically at 193 or 248 nm
followed by a rapid internal conversion ͑IC͒ to the vibrationally excited
In the present work highly vibrationally excited toluene
molecules were generated either directly by 193 nm laser
excitation or via isomerization of cycloheptatriene following
excitation at ␭ϭ248 nm or ␭ϭ193 nm. The difference in the
*
levels of the electronic ground state S0 and a subsequent isomerization to
toluene or by direct excitation of toluene at 193 nm. Starting from the highly
*
vibrationally excited S0 state toluene fragments to either benzylϩH or to
phenylϩmethyl.
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Ralf Frochtenicht: Photodissociation of toluene
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FIG. 2. Schematic diagram of the molecular beam apparatus showing typi-
cal distances, aperture diameters, and total pressures with molecular beam
on and off.
This leads to different final velocity distributions of the re-
actant molecules. After the nozzle expansion the molecules
pass a 0.79 mm diameter skimmer, two differentially
pumped stages ͑pumped by a 2000 l /s diffusion pump and a
400 l /s turbo molecular pump͒ before the beam enters the
detection chamber ͑pumped by a 220 l /s turbo molecular
pump͒ where it is ionized by electron impact in a commer-
cial quadrupole mass spectrometer ͑Extranuclear #279-9͒
with crossed beam electron impact ionizer ͑Extranuclear
#041-2͒. A scheme of the vacuum apparatus together with
important lengths, typical diameter of apertures and pres-
sures is shown in Fig. 2.
The beam velocities depend sensitively on the source
temperature, the total stagnation pressure as well as on the
used seed gas and its relative portion. Therefore, in each
measurement cycle the beam velocities were determined
with a time-of-flight technique using the laser induced deple-
tion of the reactant molecules ͑see also below͒. Typical ve-
locities of about 550 and 900 m/s for Ar and He as seed
gases were found. The internal energy of the molecules after
the expansion is difficult to measure and depends on the
expansion conditions as well. However, in the range of ex-
pansion conditions used here no dependence in the extracted
product energy distributions could be detected. This reflects
the fact, that the translational energy distributions are quite
insensitive to small changes of the initial internal energy. A
mean value for the thermal energy was estimated to be about
500 cmϪ1. This is in accordance with measurements of the
specific rate constant,35 which are very sensitive to the inter-
nal energy.
FIG. 3. Time-of-flight spectra recorded at the masses ͑a͒ 91 amu, ͑b͒ 90
amu, ͑c͒ 89 amu, ͑d͒ 77 amu, and ͑e͒ 65 amu after laser excitation of
cycloheptatriene at 193 nm.
The laser radiation illuminates a photodiode which trig-
gers a multichannel scaler. The signal pulses from the elec-
tron multiplier of the detector are counted and add time-
resolved in the different channels of the scaler ͑dwell time
100 ns͒. The module is controlled by a personal computer via
a Camac bus. The spectra are displayed on the computer
monitor to control on-line the measurement.
A pulsed excimer laser ͑Lambda Physik LPX 110͒, op-
erating at 248 or 193 nm, excites the molecules. The light of
the laser is directed into the apparatus through a MgF2 win-
dow and crosses the molecular beam perpendicularly at ei-
ther one of two positions at Ϸ280 mm from the nozzle close
to the ionization volume for measurement of ionization po-
tentials and at Ϸ80 mm for measurement of translational
energies. The energy of the laser pulse directly after crossing
the molecular beam is measured with a pyroelectric detector
͑Gentec ED500͒. Together with the spot area ͑typical 2
mm2͒, the effective fluence at the excitation area was calcu-
lated. Typical operation conditions were repetition rates of
100 Hz at ␭ϭ248 nm and of 30 Hz at ␭ϭ193 nm with laser
fluences of Ϸ2 mJ/cm2.
III. RESULTS
A. Identification of reaction channels
Figure 3 shows a typical series of time-of-flight spectra
recorded for different product masses after exciting cyclo-
heptatriene at ␭ϭ193 nm. Each of the time-of-flight spectra
shows a peak superimposed on a constant background of
about the same intensity. This background signal results from
reactant molecules that are not excited by the laser and frag-
ment in the ionizer. The shape of the time-correlated contri-
bution depends both on the velocity distribution of reactant
and product molecules. The analysis of the product transla-
tional energy distributions, described in Sec. III C, identifies
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Ralf Frochtenicht: Photodissociation of toluene
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TABLE II. Measured branching ratios BR and relative reaction efficiencies
for benzylϩH and phenylϩCH3 .
b,c
Eex
Q2
͑cmϪ1
͒
Signal ratio I1/I2
BRb
͑10Ϫ2
͒
a
51 590
52 240
63 240
9.7Ϯ1.1
10.3Ϯ0.9
11.0Ϯ1.0
3.3Ϯ0.4
3.5Ϯ0.3
3.7Ϯ0.4
23Ϯ3
22Ϯ2.5
21Ϯ2.5
aI1 : benzyl signal; I2 : phenyl signal.
bOnly the statistical error.
cQ2ϭ1/͑1ϩBR͒.
Ni
Qiϵ
.
͑2͒
͚kNk
The branching ratio can be expressed in terms of the relative
efficiency by BR (i/j)ϭQi/Qj . The major difficulty in the
determination of Qi and BR (i/j) is the conversion of the
measured product signal to absolute particle numbers Ni ,
since the percentage of detected products has to be known
and since in the most cases the detection probability is dif-
ferent for different product molecules. To overcome the
former difficulty, in this experiment the laser was irradiated
in front of the ionizer, so that product molecules were de-
tected from the whole newton sphere. The area under the
measured time-of-flight spectrum after correcting for the ve-
locity dependent ionization cross section is then directly pro-
portional to the product yield. The latter problem in the case
of electron impact ionization detection is due to the fact that
the ionization cross section and fragmentation probability de-
pend sensitively on the molecule and the electron energy. In
order to take these values from the literature, the measure-
ments were carried out at higher electron impact energies.
FIG. 4. Time-of-flight spectra at the masses 91 and 65 amu after exciting
toluene at 193 nm. The intensities are normalized to 1 at the maximum and
0 at the background level. The shape of the time-of-flight spectra is identi-
cal.
the neutral product on mass 91 amu ͓Fig. 3͑a͔͒ being the
benzyl radical ͑C7H7͒ from the reaction channel R1 and the
product on mass 77 amu ͓Fig. 3͑d͔͒ being the phenyl radical
͑C6H5͒ of reaction channel R2 in agreement with the results
in the literature. The product on mass 90 amu ͑C7H6͒ appears
only at high laser fluences of more than 10 mJ/cm2. This fact
together with its time-of-flight spectrum identifies it as a de-
cay product of the benzyl radical after two photon absorption
of cycloheptatriene at 193 nm.40,41
In order to identify the spectrum measured at 65 amu we
show in Fig. 4 a comparison of the time-of-flight spectra at
masses 91 amu ͑C7H7ϩ͒ and 65 amu ͑C5Hϩ5 ͒, recorded under
the same experimental conditions. To enable a direct com-
parison both spectra were normalized to a peak intensity of
one. It is evident, that the spectra have the same shape, indi-
cating that the ion signals on mass 65 and 91 amu originate
from the same neutral precursor C7H7. The same is found for
the time-of-flight spectra at masses 90 amu ͑C7Hϩ6 ͒ and 89
amu ͑C7Hϩ5 ͒ which both stem from the same neutral radical
C7H6. These assignments are supported by measurements of
the ionization potential, which are described in detail in Refs.
18 and 40.
Relative ionization cross sections ␴ of 15.2ϫ10Ϫ16 cm2 and
r
12.3ϫ10Ϫ16 cm2 ͑calculated after Ref. 42͒ and fragmentation
probabilities of 0.23 and 0.138 for benzyl and phenyl, respec-
tively, were used.
The measurements to determine the branching ratio BR
ϭBR͑1/2͒ were carried out at mass 65 amu ͑signal I1͒ for
reaction channel R1 and at mass 77 amu ͑signal I2͒ for reac-
tion channel R2 at three different excitation energies. The
results are summarized in Table II. The signal ratios I1/I2 are
converted into the branching ratio BR as mentioned above.
The given results are mean values of several measurements
and are discussed in Sec. IV B.
C. Translational energy distributions
Benzyl time-of-flight spectra were measured at different
expansion conditions achieved by different seed gases, stag-
nation pressures, and nozzle temperatures to avoid system-
atic errors after exciting toluene at ␭ϭ193 nm and cyclohep-
tatriene at ␭ϭ248 nm and ␭ϭ193 nm. Two typical examples
for benzyl time-of-flight spectra are shown on the left-hand
side ͓Figs. 5͑a͒ and 5͑b͔͒. For comparison the calculated
spectra are plotted, which base on product translational en-
ergy distributions ͓cf. Figs. 5͑c͒ and 5͑d͔͒ calculated with the
phase space limit SACM/PST of the statistical adiabatic
channel model ͑SACM͒ theory43 and using expressions
based on the RRK ͑Rice–Ramsperger–Kassel͒ theory.44,45 In
B. Branching ratio
In an unimolecular reaction with more than one reaction
path the branching ratio BR (i/j) of two reaction channels i
and j is defined as the ratio of the product yields Ni and Nj
Ni
BR i/j͒ϵ
,
͑1͒
͑
Nj
whereas the relative reaction efficiency Qi of a reaction path
is defined as the relative portion of this channel i
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¨
Ralf Frochtenicht: Photodissociation of toluene
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FIG. 5. On the left hand time-of-flight spectra of the benzyl radical following excitation of ͑a͒ toluene and ͑b͒ cycloheptatriene at 193 nm are shown. Together
with the experimental data calculated spectra are plotted. It is evident, that the distributions based on the SACM/PST theory results in a too narrow
distribution, whereas the RRK calculations with qϭ10 and qϭ12, respectively, fits the data well. On the right hand calculated SACM/PST ͑--͒ and fitted
RRK ͑–͒ translational energy distributions are shown, which were used to fit the experimental spectra for maximum translational energies of ͑c͒ 21160 cmϪ1
and ͑d͒ 32160 cmϪ1
.
the calculations the velocity distribution of the parent mol-
ecules were taken into consideration in the convolution as
described in Refs. 40, 46 and 47. A dissociation energy for
toluene to benzylϩH of E0ϭ31 080 cmϪ148 was used. The
further molecular parameters for the SACM/PST calculation
are taken from Ref. 26, which base on the values from Refs.
19–56. It is evident, that the SACM/PST calculation results
in too narrow TOF distributions, i.e., the mean translational
energy is too small. In contrast, the RRK-type expression is a
phenomenological distribution which can be fitted to the data
by using an adjustable parameter q
Time-of-flight spectra of the phenyl radical ͑reaction
channel R2͒ were also measured at different expansion con-
ditions following excitation of toluene at ␭ϭ193 nm and
cycloheptatriene at ␭ϭ248 nm and ␭ϭ193 nm. Two ex-
amples are shown in Figs. 6͑a͒ and 6͑b͒. The measured spec-
tra are compared with calculations based on the SACM/PST
theory ͓Figs. 6͑c͒ and 6͑d͔͒. In these calculations the energy
for dissociating toluene to phenylϩmethyl was taken to be
E0ϭ36 000 cmϪ1 33
the measured spectra well. Mean translational energies of
.
Here, the SACM/PST calculation fits
E
E
ϭ(900ϩ175) cmϪ1 after exciting toluene at 193 nm and
͗
͗
͘
͘
T
Ϫ90
ϭ(1240ϩ120
Ϫ150) cmϪ1 after exciting cycloheptatriene at
T
qϩ1
• EmaxϪET͒q.
͑3͒
193 nm were obtained.
PE E ͒ϭ
͑
͑
T
T
EmT ax qϩ1
EmT ax is the maximum translational energy and q can be re-
lated to the number of degrees of freedom participating at the
reaction, which determines the mean translational energy as
well as the width of the distribution. Best agreement was
achieved with qϭ10 ͑for excitation of toluene at ␭ϭ193 nm
and cycloheptatriene at ␭ϭ248 nm͒ and qϭ12 ͑for excita-
tion of cycloheptatriene at ␭ϭ193 nm͒. For this q values the
IV. DISCUSSION
A. Product translational energy distribution
For the reaction channel to benzylϩH mean translational
energies of
ϭ(2300ϩ380
E
ϭ(1760ϩ250
)
cmϪ1 and
E
͗ ͘
T
͗
͘
T
Ϫ245
Ϫ340) cmϪ1 were determined for excitation energies
of Eexϭ52 240 cmϪ1 and Eexϭ63 240 cmϪ1, respectively.
The SACM/PST calculation yielded too small energies of
970 and 1300 cmϪ1. Note the small percentage of E with
mean translational energy can be calculated via
E
͘
T
͗
ϭ EmT ax/(q ϩ 2) leading to
E
ϭ(1760ϩ250
Ϫ245) cmϪ1 and
Ϫ340) cmϪ1, respectively. In comparison the
͗
͘
͗
͘
T
T
E
ϭ(2300ϩ380
͘
respect to the maximum translational energy EmT ax of 8% and
7%, respectively, i.e., only a small amount of the excess
energy is distributed in the translational degrees of freedom,
and the shape of the distribution, which decreases monotoni-
cally. Both observations reflect the fact that the number den-
͗
T
mean translational energies of 970 and 1300 cmϪ1 calculated
with the SACM/PST theory for the two excitation energies
are definitely lower ͑see also Table III͒. These results are
discussed in Sec. IV.
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¨
Ralf Frochtenicht: Photodissociation of toluene
4855
TABLE III. Comparison of measured mean product translational energies for toluene→benzylϩH with calcu-
lations.
EmT ax
E
͘
T
a
͗
͑cmϪ1
͒
͑cmϪ1
͒
Used distribution
Method
Reference
ϩ250
Ϫ240
ϩ380
Ϫ350
ϩ130
Ϫ115
ϩ2.0
21 160
32 160
20 355
21 650
21 000
32 000
20 660
31 660
14 900
21 650
͑1760
͑2300
͑1564
͒
͒
͒
RRK with qϭ10
RRK with qϭ12
PTS of benzyl
PTS of benzyl
PTS of benzyl
LIF of H atoms
LIF of H atoms
LIF of H atoms
Calculation
Calculation
Calculation
Calculation
This work
This work
Ϫ1.5
ϩ2.5
Ϫ2.0
RRK with qϭ11Ϯ1
Maxwellian: Tϭ2630Ϯ87 K
Maxwellian: Tϭ2280Ϯ140 K
Maxwellian: Tϭ3330Ϯ235 K
SACM/PST
38
32
31
31
2728Ϯ175
2343Ϯ140
3462Ϯ245
970
1300
1295
This work
This work
32
SACM/PST
RRKM
RRKM
1610
32
aMaximum translational energy EmaxϭEexϪE0 .
sity of open channels and therefore the occupation of product
internal states increases strongly with increasing energy
above the dissociation threshold. Since internal and transla-
tional energy are directly related, PE(ET) decreases with in-
creasing ET . The results of this work are compared with
measured and calculated translational energies in Table III
and Fig. 7.
Besides our measurements, experimental results from
Bersohn’s and Lee’s group are available. In Lee’s group, the
translational energy distributions were also measured via
photofragment translational spectroscopy in a molecular
beam experiment.38 In contrast to our measurements, they
detected the reaction products with a rotatable mass spec-
trometer at laboratory scattering angles of more than 5°. Af-
ter excitation of cycloheptatriene at 266 nm a mean transla-
tional energy of 1560 cmϪ1 was determined for reaction
pathway R1 ,38 which is in an excellent agreement with the
results of this work.
The group of Bersohn measured the Doppler profile of
the Lyman-␣ line of the H atom product. The translational
temperatures were determined from a Maxwellian fit and
mean translational energies of ͑2343Ϯ140͒ cmϪ1 31
,
͑3462
Ϯ245͒ cmϪ1 31
,
and ͑2728Ϯ175͒ cmϪ131,32 were found after
exciting cycloheptatriene at 248 and 193 nm and toluene at
193 nm, respectively. These mean translational energies are
about 50% higher than the experimental results of Lee’s
FIG. 6. On the left hand time-of-flight spectra of the phenyl radical following excitation of ͑a͒ toluene and ͑b͒ cycloheptatriene at 193 nm are shown. Together
with the experimental data calculated spectra are plotted. The SACM/PST distribution fits the data well. On the right hand the calculated SACM/PST ͑—͒
translational energy distributions for the maximum translational energies of ͑c͒ 21 160 cmϪ1 and ͑d͒ 32 160 cmϪ1 are shown.
J. Chem. Phys., Vol. 102, No. 12, 22 March 1995

¨
Ralf Frochtenicht: Photodissociation of toluene
4856
translational energies. One possible reason for the greater
mean translational energy after excitation at 193 nm could be
multiphoton excitation, which is much more important at
␭ϭ193 nm compared to 248 nm. A difficulty in the LIF
measurements presented in Refs. 31 and 32 comes from the
H atom detection. H atoms can be produced in different pro-
cesses. From the Doppler profile alone the origin of the H
atoms cannot be unambiguously identified. For example, the
benzyl radical can decay under H atom loss ͑Sec. III A and
Ref. 41͒. These H atoms start with the higher benzyl velocity
and lead to higher energy H atoms, so that the overall Dop-
pler profile broadens.
A possible explanation for the observed difference be-
tween SACM calculation and experiment for the benzyl
channel might be the existence of a small barrier in the exit
channel. A barrier of height EB can be simulated by a shift of
the distribution with EB to higher translational energies.57 It
was found that a barrier of less than 500 cmϪ1 height is
sufficient to reproduce the measured spectra with the SACM/
PST calculations.40 On the other hand, measurements of the
thermal recombination rate constant for benzylϩH→toluene
contradict this assumption.24 Further theoretical as well as
experimental studies are necessary to resolve this open ques-
tion.
In contrast to the benzyl channel, a good agreement be-
tween SACM/PST theory and experiment is found for the
reaction path R2 to phenyl and methyl. Mean translational
energies of E ϭ(900ϩ175) cmϪ1 after exciting toluene at
͗
͘
T
T
Ϫ90
193 nm and E ϭ(1240ϩ120
Ϫ150) cmϪ1 after exciting cyclo-
͗
͘
FIG. 7. Comparison of measured mean product translational energies of the
reaction toluene→benzylϩH with two calculations. Experimental results
from Refs. 38 ͑᭞͒, Refs. 31, 32 ͑ᮀ͒ and from the present work ͑᭹͒ are
shown together with a RRKM calculation ͑छ͒ ͑Ref. 32͒ and a SACM/PST
calculation ͑—without a barrier and --- with a barrier of about 500 cmϪ1͒.
heptatriene at 193 nm were measured. Besides the measure-
ments presented in this work, a further experiment of the Lee
group38 has been performed, which is in excellent agreement
with our results ͑see Fig. 8 and Table IV͒.
B. Branching ratio
group and this work ͑cf. Table III and Fig. 7͒.
As mentioned above, the SACM/PST calculation yields
mean translational energies which are about a factor of 2
smaller than the experimental results measured by Lee et al.
and in this work and about a factor of 2.7 compared to Ber-
sohn’s results. In contrast, a RRKM calculation of the three-
dimensional density of states ͑Eq. 6 and Table IV in Ref. 32͒
yield a mean translational energy of 1610 cmϪ1 at ETmax
ϭ 21 650 cmϪ1. Another RRKM calculation performed in
Lee’s group38 yield a mean translational energy of 970 cmϪ1
The branching ratio of two reaction channels can be di-
rectly expressed by the ratio of the specific rate constants and
is therefore as well a sensitive observable to be compared
with statistical models.
In this work, relative reaction efficiencies Q2 of 0.23
Ϯ0.05 after excitation of cycloheptatriene at 248 nm, 0.22
Ϯ0.05 after excitation of toluene at 193 nm and 0.21Ϯ0.05
after excitation of cycloheptatriene at 193 nm were deter-
mined. The major source for the given errors are due to
uncertainties in ionization cross section and fragmentation
probabilities, which were considered in the given errors.
Table V summarizes the experimental results of this work
and other groups with a SACM calculation fitted to a mea-
at ETmax ϭ 20 355 cmϪ1
.
To explain the reasons for these large discrepancies, we
can exclude experimental errors in this work due to cluster-
ing or multiphoton processes since many different expansion
conditions and low laser fluences were used ͑Ӷ5 mJ/cm2͒
without observing significant changes. In addition, such sys-
tematic errors would also affect for the second reaction chan-
nel, where experiment and calculation agree satisfactorily
͑see below͒.
sured branching ratio at Eexϭ52 700 cmϪ1 33
.
The group of Troe determined a relative reaction effi-
ciency of Q1ϭ1ϪQ2ϭ0.75Ϯ0.2 following laser excita-
tion of toluene at ␭ϭ193 nm from the benzyl yield via the
known absorption cross section of the benzyl radical.8,26 In
further experiments of this group, the methyl product ͑chan-
nel R2͒ was detected with resonant-enhanced multiphoton
ionization ͑REMPI͒ and nonresonant one photon ionization
of the methyl radical.33,34 The ion signal was converted into
a reaction efficiency by calibrating the product yield with a
photoreactions of a known yield of methyl radicals. For this
One noticeable point in the Doppler measurements31,32 is
the significant discrepancy between the mean translational
energies after excitation of toluene at ␭ϭ193 nm of ͑2728
Ϯ175͒ cmϪ1 and cycloheptatriene at ␭ϭ248 nm of ͑2343
Ϯ140͒ cmϪ1 31
.
Since the excitation energies Eex are nearly
identical in both cases one would expect similar product
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¨
Ralf Frochtenicht: Photodissociation of toluene
4857
FIG. 9. Relative reaction efficiency Q2 for the photodissociation of toluene
to phenyl and methyl. Experimental results of this work ͑᭹͒, Refs. 37, 38
͑ᮀ,᭝͒, Ref. 33 ͑᭞͒ and Ref. 34 ͑᭞͒ are shown together with a SACM/PST
calculation ͑—͒ ͑Ref. 33͒.
constant in the energy range between 50 000 and 65 000
cmϪ1. This is in contrast to results of the SACM theory
which predicts a smooth increase.
FIG. 8. Mean product translational energy of the reaction
toluene→phenylϩCH3 . Experimental results of this work ͑᭹͒ and from Lee
et al. ͑᭞͒ Ref. 38 are shown together with a SACM/PST calculation ͑—͒.
V. CONCLUSIONS
purpose the photodissociation of ethylbenzene to benzyl
ϩmethyl was used. The uncertainty in the methyl yield in
this reaction ͑75%–100%͒ determined their value as well as
the error in the relative yield to Q2ϭ0.17Ϯ0.06 after laser
excitation of toluene at ␭ϭ193 nm.33 New results in Refs. 34
and 40, 58 indicate, that the relative reaction efficiency for
the methyl channel in ethylbenzene is larger than 96%, so
that the published value can be recalculated to
Q2ϭ0.19Ϯ0.07. In addition, the methyl yield after laser
excitation of cycloheptatriene at ␭ϭ193 nm was investigated
with nonresonant one photon ionization at 121 nm,34 leading
to the same relative reaction efficiency at a 11 000 cmϪ1
higher internal energy;34 i.e., the relative efficiency is con-
stant in this considered energy range.
In this work the photodissociation of toluene to
benzylϩH and phenylϩCH3 after laser excitation was inves-
tigated in a molecular beam experiment. For this system the
advantages of the photofragment translational spectroscopy
together with selective ionization using low energy electrons
detection in forward direction were illustrated. The reaction
pathways, their branching ratio as well as the translational
energy distributions of the products were determined. Calcu-
lations based on statistical theories were used to reproduce
the measured spectra.
The primary advantage of the experimental method used
here is its universal detection ability for even very slowly
recoiling reaction products and the simultaneous reduction in
Molecular beam investigations in Lee’s group deter-
ϩ4.4 38
mined branching ratios of BRϭ6.5637 and BRϭ͑8.4
͒
Ϫ2.5
TABLE IV. Mean product translational energies for toluene→phenylϩCH3
after laser excitation of cycloheptatriene at ␭ϭ248 nm and
␭ϭ266 nm, respectively. The branching ratios can be con-
verted into relative reaction efficiencies of Q2ϭ0.13 and
Q2ϭ(0.11Ϯ0.04).
The results for the relative reaction efficiencies of chan-
nel R2 obtained with the different experimental techniques of
about 10% to 20% are in a surprisingly good accordance by
taking into account the uncertainties in the different calibra-
tion procedures. The results of this work together with the
results from Ref. 34 indicate a reaction efficiency which is
obtained from photofragment translational spectroscopy measurements.
a
EmT ax
E
͘
T
͗
͑cmϪ1
͒
͑cmϪ1
͒
Used distribution
SACM/PST
Reference
This work
ϩ60
Ϫ90
16 240
͑900
͒
ϩ2
͑RRK with qϭ16
SACM/PST
͑RRK with qϭ20
͒
͒
Ϫ1
ϩ120
Ϫ150
27 240
15 214
͑1240
͒
This work
38
ϩ3
Ϫ2
ϩ240
Ϫ285
ϩ2,5
Ϫ2,0
͑845
͒
RRK with qϭ12
aMaximum translational energy EmaxϭEexϪE0 .
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130.70.241.163 On: Tue, 23 Dec 2014 03:05:31

¨
4858
Ralf Frochtenicht: Photodissociation of toluene
TABLE V. Comparison of measured and calculated relative reaction efficiencies Q2 for toluene→phenylϩCH3 .
Eex
Relative efficiency Q2
͑10Ϫ2
͑cmϪ1
͒
͒
Observation method
Reference
51 590
52 240
63 240
52 730
52 730
52 730
63 510
53 500
50 900
23Ϯ5
22Ϯ5
21Ϯ5
Photofragment translational spectroscopy
Photofragment translational spectroscopy
Photofragment translational spectroscopy
UV Absorption of benzyl
REMPI of Methyl
REMPI of Methyl
VUV 1 Photon ionization
Photofragment translational spectroscopy
Photofragment translational spectroscopy
This work
This work
This work
8
33
33
33, 34
37
100Ϫ͑75Ϯ20͒
17Ϯ6
19Ϯ7a
19Ϯ7b
13
10.6
ϩ3.9
Ϫ3.4
38
aCorrected for the methyl reaction efficiency from ethylbenzene determined in recent experiments to be
Q
EthBϭ0.96Ϯ0.04 ͑Refs. 34 and 40͒.
bCalculated from the corrected value of Ref. 33 by using the constancy of the branching ratio in this energy
range ͑Ref. 34͒.
experimental requirements ͑no UHV detection chamber or
rotatable detector required͒ compared to standard photofrag-
ment translational spectroscopy. Therefore, this method can
be used for investigations of a widespread range of unimo-
lecular processes. It is especially advantageous for processes
in which products arise which are difficult to investigate via
laser spectroscopy, e.g., large, internally hot products and/or
with unknown spectroscopy. Examples are the photoinduced
dissociations of large molecules or isomerization reactions.17
In a forthcoming publication the investigation of laser in-
duced unimolecular reactions of methyl as well as alkylsub-
stituted benzenes using this technique will be presented.58
Due to conservation of momentum mA–uAϭϪmB–uB , and
since ETϭ(1/2)mA–u2Aϩ(1/2)mB–u2B, where mA , mB , and
uA , uB are the masses and velocities in the center-of-mass
system of A and B, respectively, the c.m. velocity can be
expressed by
mB
1
uAϭ
ͱ
2•
•ET.
͑A2͒
mA mAB
The velocity of the product molecules in the laboratory
frame w is the vector sum of the reactant velocity v and the
product c.m. velocity u. In the experiment w is measured by
recording the flight time for a certain distance. The goal is to
extract from the measured time-of-flight spectra in the labo-
ratory frame the c.m. translational energy distributions by a
forward convolution formula.40,46,47 To do so, a trial product
translational energy distribution PE(ET) is assumed and
transformed into a c.m. velocity distribution Q(u). Q(u) is
then convoluted with the initial reactant velocity distribution
ACKNOWLEDGMENTS
Peter Toennies introduced me into the reaction dynamics
of polyatomic molecules. I thank him for his stimulation all
the time as well as his continuing encouraging support. This
work profited from fruitful discussions with Matthias Damm,
Osram GmbH, Thomas Glenewinkel-Meyer, University of
Wisconsin, Horst Hippler, University of Karlsruhe, Stefan
P( ) ͑determined from the reactant time-of-flight distribu-
v
tion͒ and the apparatus function ͑finite detection angle and
length of the ionizer͒ to yield the velocity distribution on a
certain angle in the laboratory frame, which is directly con-
verted to a time-of-flight distribution TЈ(t). TЈ(t) is com-
pared with the measured time-of-flight spectra T(t). This
procedure is repeated until a good agreement between theory
and experiment is achieved.
¨
Schlemmer, University of Chemnitz, Jurgen Troe, University
¨
of Gottingen and Matthias Hartmann, Marius Lewerenz,
¨
¨
¨
Horst-Gunter Rubahn, Max-Planck-Institut fur Stromungs-
forschung, which are gratefully acknowledged. Partial finan-
cial support was provided by the DFG Sonderforschungs-
bereich 93 Photochemie mit Lasern.
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