Dynamics of Cl (2Pj) atom formation in the photodissociation of fumaryl chloride (ClCO - CH = CH - COCl) at 235 nm: A resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) study

Article abstract of DOI:10.1021/jp1100279

The photodissociation dynamics of fumaryl chloride (ClCO-CH=CH-COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl (²P_3/2) and Cl* (²P_1/2). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, β_i. The TOF profiles for both Cl and Cl* are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(E_T), of Cl and Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast components of the Cl and Cl* channels are 14.9 ± 1.6 and 16.8 ± 1.6 kcal/mol, respectively. Similarly, for the slow components, the average translational energies of the Cl and Cl* channels are 3.4 ± 0.8 and 3.1 ± 0.8 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation process. The HCl product has been detected, using a REMPI scheme in the region of 236-237 nm. The observation of the molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent molecule relaxes to the ground state from where the molecular (HCl) elimination takes place.

Full text of DOI:10.1021/jp1100279

ARTICLE
pubs.acs.org/JPCA
Dynamics of Cl (2Pj) Atom Formation in the Photodissociation of
Fumaryl Chloride (ClCO - CH = CH - COCl) at 235 nm: A Resonance
Enhanced Multiphoton Ionization (REMPI) Time-of-Flight (TOF) Study
Monali Kawade, Ankur Saha, Hari P. Upadhyaya,* Awadhesh Kumar, Prakash D. Naik, and P.N. Bajaj
Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
ABSTRACT: The photodissociation dynamics of fumaryl
chloride (ClCO—CHdCH—COCl) has been studied
in a supersonic molecular beam around 235 nm using
resonance enhanced multiphoton ionization (REMPI)
time-of-flight (TOF) technique by detecting the nascent
state of the primary chlorine atom. A single laser has been
used for excitation of fumaryl chloride and the REMPI
detection of chlorine atoms in their spin-orbit states, Cl
(2P3/2) and Cl* (2P1/2). We have determined the transla-
tional energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination
channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy
distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, βi. The TOF profiles for both
Cl and Cl* are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the experimental
uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(ET), of Cl and
Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast
components of the Cl and Cl* channels are 14.9 ( 1.6 and 16.8 ( 1.6 kcal/mol, respectively. Similarly, for the slow components, the
average translational energies of the Cl and Cl* channels are 3.4 ( 0.8 and 3.1 ( 0.8 kcal/mol, respectively. The energy partitioning
into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the
chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation
process. The HCl product has been detected, using a REMPI scheme in the region of 236-237 nm. The observation of the
molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent
molecule relaxes to the ground state from where the molecular (HCl) elimination takes place.
acid [CH3COCOOH]5 using laser photolysis-laser induced
I. INTRODUCTION
fluorescence (LP-LIF) technique.
Numerous studies on the photodissociation of carbonyl chlo-
Photodissociation dynamics of carbonyl compounds always
has been an area of extensive research that has been pursued with
rides are available in the literature; very few of these are focused
both experimental and theoretical approaches. The existence of
on the photochemistry of fumaryl chloride. The UV photodisso-
close-lying excited states and their interactions is mainly a great
ciation of the most elementary acid chlorides, acetyl chloride
attraction for the researchers. Again, the compounds with two
(CH3COCl),6,7 and propionyl chloride (CH3CH2COCl), have
carbonyl groups (diketones), provide useful information for the
been previously studied in both gas and solid phases.8 Acetyl
study of intramolecular energy transfer, and interactions invol-
chloride on photodissociation at 236 nm produces acetyl and chlorine
ving remote carbonyl groups.1 It is very interesting to study and
radicals while propionyl chloride gives ketene (H2CdCdO) and
identify the mechanism of these types of interactions between the
HCl as well. Photodissociation and isomerization processes of
carbonyl groups. Besides these diketones, there is another class of
acryloyl chloride (CH2dCHCOCl), an enone similar to fumaryl
chloride, have been studied both experimentally9,10 and
theoretically.11 Apart from the Cl atom formation channel, it
photodissociates through two additional channels, one produces
ketene and HCl (same as in propionyl chloride), while the
other channel involves a 1,3-sigmatropic rearrangement, produc-
ing 3-chloro-1,2-propenone (H2ClCCHdCdO). Extensive UV
compounds, generally termed as R,β-enones, in which the
carbon-carbon double bond (-CdC-) is in conjugation with
carbonyl group (-CdO) giving rise to altogether a different
and interesting photochemistry. These R,β-enones undergo a
variety of photochemical rearrangements,2,3 and their photo-
chemistry, in general, has attracted much attention in recent
years. In our earlier studies, we investigated the dynamics of
formation of the OH radical in photodissociation of few enones,
such as acrylic acid [H2CdCHCOOH],3 propiolic acid,4 eno-
lic-acetylacetone [CH3C(O)CHC(OH)CH3],2 and pyruvic
Received: October 20, 2010
Revised:
January 12, 2011
Published: February 15, 2011
r
2011 American Chemical Society
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relative quantum yield of Cl*. To gain further insights, the
anisotropy parameters (βi) were also measured.
II. EXPERIMENTAL SECTION
The experiment was performed, using a molecular beam time-
of-flight mass spectrometer system (MB-TOF-MS), as described
in our earlier studies on photodissociation of PCl3.24 Briefly, it
consists of a supersonic beam source and an ionization (beam -
laser interaction) region, which were differentially pumped, using
turbomolecular pumps (TMP), backed by rotary pumps. An
additional 4 in. TMP was used to pump the TOF tube and
mounted near the detector. Typical operating pressures in the
source and the ionization regions were 5 ꢀ 10-5 and 8 ꢀ 10-7
Torr, respectively. A pulsed supersonic free jet was generated,
using a solenoid valve. The free jet was skimmed off by a 1.9 mm
diameter conical skimmer generating a supersonic molecular
beam (MB), which was directed to the interaction region. The
pulsed valve was located ∼2 cm from the skimmer, and 7.5 cm
from the interaction region. The MB pulse profile and the
number density were characterized, using a fast ion gauge
(FIG), located beyond the ion optics. The fumaryl chloride
sample (95% purity, Aldrich) was used, without further purifica-
tion. Helium was bubbled through the sample maintained at
room temperature, and the mixture was expanded through the
nozzle at a stagnation pressure of 1000 Torr of He. It was ensured
that any interference to the measurements due to cluster photo-
fragmentation was absent by operating at a low stagnation pressure,
and using only the rising part of the molecular beam pulse.
Figure 1. Optimized geometries for various isomers of fumaryl chloride.
photodissociation studies of oxalyl chloride, (ClCO)2, a lower
homologue of fumaryl chloride, have been carried out.12-14 Suits
and co-workers12,13 investigated photodissociation dynamics of
oxalyl chloride near 235 and at 193 nm, with a photofragment
imaging technique, using REMPI detection of Cl and CO
moieties. They suggested that the photodissociation of
(ClCO)2 proceeds via an impulsive three-body mechanism
yielding translationally hot spin-orbit excited Cl* and ground
state Cl, together with translationally hot and rotationally excited
CO, and a translationally cold ClCO. The primary product COCl
undergoes subsequent dissociation to yield slow CO and Cl.
They also measured the translational energy distribution and
anisotropies of Cl, Cl*, and CO.
Fumaryl chloride, as shown in Figure 1, is a planar molecule
and has three stable isomers, out of which two isomers, namely,
trans-trans and cis-cis, belong to the C2h point group and the
third trans-cis isomer belongs to the Cs point group.15-18 The
relative abundances of these isomers are found to be 30%, 24%,
and 46%, respectively, at 480 K.18 A negligible effect of tempera-
ture on the relative abundance indicates only small energy
differences among the isomers. The complete IR and Raman
studies of liquid and crystal forms have identified two conformers
as trans-trans and cis-trans.19 It has a symmetric structure with
both carbonyl groups associated with an identical functionality.
The thermal decomposition of fumaryl chloride at 900 ꢀC gives
rise to various products, such as C3O(CdCdCdO), HCl, and
CO.20 Besides, fumaryl chloride has many synthetic applications,
mainly since it contains unsaturated carbon-carbon double
bonds, which can be used for in situ cross-linking. Jabbari et
al.21 have suggested that it can be copolymerized with a poly(ε-
caprolactone) for constructing a biocompatible, bioresorbable,
injectable, and self-cross-linkable polymer for bone tissue en-
gineering. Poly(ethylene glycol) fumarate, synthesized from
fumaryl chloride and polyethylene glycol in the presence of
propylene glycol, is used as a rigid coating for the superpara-
magnetic iron oxide nanoparticles, which are increasingly being
evaluated for clinical applications such as hyperthermia, drug
delivery, magnetic resonance imaging, transfection, and cell/
protein separation.22 Several ultrastructure cross-linked and
functional polymer thin films, for potential nonlinear optical
(NLO) application, or other applications requiring molecular
orientation, are synthesized from fumaryl chloride and maleic
anhydride.23
The detector system consists of a two-stage Wiley-
McLaren25 time-of-flight mass spectrometer (TOF-MS), with
extraction and acceleration regions. The system is mounted
vertically, perpendicular to the horizontal MB. The extraction
region consists of a repeller electrode and an extractor grid,
mounted 10 mm above the repeller electrode. The acceleration
region is defined by the extractor electrode and a grid held at the
ground potential, separated from each other by 10 mm. Both
these grids (5 cm ꢀ 5 cm) are constructed from stainless steel
mesh, with 90% transmission. To collect the total ions, the
extraction region was held at ∼300 V/cm, and the acceleration
region was held at ∼3900 V/cm. After passing through the
acceleration region, the ion packet passed through a 1035 mm
long field-free flight tube to the detector. Two deflector plates,
placed perpendicular to the detector axis (z axis), allowed the ion
packet to be translated in the (x, y) plane, to center it on the
detector. The typical field strength for the deflector plates was
2-6 V/cm. The ions were detected by a set of 18 mm dual
microchannel plates (MCP). A single compact voltage generator,
having multiple output voltage ports, was employed to power the
TOF ion optics, the deflection plates and the MCP detector.
The chlorine atoms were probed, using 2 þ 1 resonance
enhanced multiphoton ionization (REMPI) transitions in the
region 234-236 nm. The laser pulses were generated by a dye
laser (TDL 90, Quantel), using rhodamine 101 dye. The dye
laser was pumped by 532 nm light from the second harmonic of a
Nd:YAG laser (YG-981-C, Quantel), operating at 20 Hz. The
fundamental dye laser output was frequency-doubled in a KDP
crystal, and mixed with the fundamental output of the Nd:YAG
laser, to obtain an output in the range 230-236 nm. In all the experi-
ments reported in this work, the same laser beam was employed
as a pump as well as a probe, i.e., for both photodissociation of the
The present study is undertaken to understand the photo-
dissociation dynamics of fumaryl chloride (ClCO—CHdCH—
COCl), a molecule with two CdO groups and a CdC group in
conjugation, in a supersonic molecular beam around 235 nm.
The chlorine atom product in both the ground state (2P3/2) and
the spin-orbit excited state (2P1/2) has been detected using the
REMPI-TOF technique. We have also measured the transla-
tional energy distributions for both types of chlorine atoms and
parent molecule and ionization of the photoproducts, Cl(2P3/2
and Cl*(2P1/2) atoms. The laser beam was focused by a lens of
)
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200 mm focal length, and the distance of the lens from the center
of the molecular beam axis was varied, to obtain the best ratio of
on- and off-resonant signals. The spin-orbit ratio was deter-
mined from the ion intensities for the corresponding transitions,
after correction due to their two-photon oscillator strengths. For
this purpose, two lines were chosen, one at 42492.5 cm-1 and
another at 42516.1 cm-1, corresponding to 4p 2D3/2 r 3p 2P3/2
resonance wavelength, and found to be quadratic dependent.
This shows that, at 293.77 nm, the REMPI is 1 þ 1 type because
of one photon resonant transition, 1A1 f 1B2, followed by pumping
to the ionization continuum by absorption of a second photon. Such
measurement showed the instrumental response function to be well
described by a fixed Gaussian function in the time domain, with
fwhm of 27 ns at aniline mass. Under space focusing conditions, this
leads to a convolution function in the velocity domain, which is
linearly dependent on the extractor voltage Vex.
2
2
and 4p P1/2 r3p P1/2 transitions, respectively. The polariza-
tion of the resultant laser beam was rotated, using a double
Fresnel rhomb, and a polarizer ensured 99% polarization of the
laser beam entering the chamber. The laser power was mon-
itored, using a power meter, and was typically 50-100 μJ/pulse.
To obtain the TOF mass spectrum, the signal was sent to a digital
oscilloscope, which was interfaced to a PC. Subtracting the off-
resonant signals from the on-resonant signals effectively removed
the minor pump-oil related background contribution to the TOF
spectra, and also the contribution from a multiphotonic process.
A digital delay/pulse generator, with pulse resolution of 20 ps,
was employed as the master to trigger all the instruments for time
synchronization. The time delay between the trigger pulse
applied to the pulsed valve and its opening was obtained by
measuring the delay between the trigger pulse and the fast
ionization gauge (FIG) signal, employing a digital oscilloscope.
This delay is the sum of the time required to open the pulsed
valve from its trigger input and that for the molecular pulse to
reach FIG from its position of generation, i.e., the nozzle exit. By
measuring these time delays for different FIG positions with
respect to the skimmer, we estimated the flow velocity of the
molecular beam and used it to obtain the time required for the
molecular beam to reach the extraction region of the TOFMS. A
PC was used to control the scan of the dye laser via an RS232
interface, and collect data from SRS 245, through a GPIB
interface, using a control and data acquisition program.
The power dependence measurements were performed by
integrating the chlorine atom REMPI spectra, which were
obtained by sending the detector output directly to a boxcar
integrator, gated on the m/z 35 and m/z 37 peaks in the TOF
mass spectrum. The laser power was varied, and the boxcar
output was recorded at each power. The REMPI spectra and
survey scans were taken by recording the boxcar output as a
function of the laser wavelength and archiving the spectra on a
PC interfaced to the dye laser controller. TOF profiles were taken
for three different experimental configurations, vertical (laser
polarization II detection axis), horizontal (laser polarization ^
detection axis), and magic angle (laser polarization at 54.7ꢀ to
detection axis). Doppler broadening of the transitions was always
well within the laser bandwidth. For polarization experiments,
TOF profiles were averaged for a minimum of 10000 laser shots.
The forward convolution method that we have employed has
been discussed in our previous publication24 as well as
elsewhere,28,29 and will be only briefly outlined. In the REMPI-
TOF-MS technique, the component of the photofragment that
speeds along the TOF-MS axis, which defines the lab frame Z
axis, is measured. This speed component results from the
averaging of the angular distribution, as described,30 over the
photofragment speed distribution, g(v), and is given as28,29,31,32
ꢂ
ꢀ ꢁꢃ
Z
¥ gðvÞ
vz
fðvz;χÞ ¼
1 þ βP2ðcos χÞP2
dv ð1Þ
2v
v
jvzj
where vZ is the velocity component along the Z axis, v is the recoil
speed of the fragments, β is the anisotropy parameter given by
β = 2ÆP2(cos θm)æ. θm is the molecular frame angle between the
molecular transition dipole moment and the photofragment
recoil direction, P2(cos χ) is the second-order Legendre poly-
̂ ^z in the above equation is the projection
nomial, and cos(χ) = ε
3
of the pump laser electric field, ε̂, on the detector axis, ^z, which is
also defined as the angle between the dissociation laser polariza-
tion and the Z axis. In the present work, we have used a procedure
of noncore sampling data, in which it is assumed that the nature
and the shape of the TOF profiles for chlorine photofragment is
independent of the probe polarization. In general, this assump-
tion holds good. But, the presence of atomic v j correlation
3
might make this assumption only approximate. However, these
correlations are weak enough to be neglected.33,34
In the case of
several contributing channels, eq 1 must be summed over the
photofragment speed distribution, gi(v), and anisotropy, βi, of
each channel i. The dependence of f(vz,χ) on β can be eliminated
by either measuring the data with the magic angle of χ = 54.7ꢀ,
where P2(cos χ) is zero, or coadding normalized profiles with χ =
90ꢀ and χ = 0ꢀ, giving a 2:1 weightage. Both the approaches yield
isotropic f(vz,χ) profiles, and the total center of mass (c.m.)
speed distribution is obtained by differentiation of eq 1 at χ =
54.7ꢀ and is given as
d
gðvÞ ¼ - 2v
fðv;54:7ꢀÞjv ¼v
ð2Þ
z
dvz
The measurements in this work were recorded as TOF spectra,
i.e., in the time domain I(t,χ). Under space focusing conditions, a
simple linear transformation gives the signal in the velocity domain,
I(vZ,χ), according to, vz = (qVex(t-t0))/m, where q and m are the
charge and mass of the photofragment, respectively, Vex is the electric
field in the extraction region, and t0 is the mean time-of-flight.35
Application of eq 1 on the magic angle TOF profile I(vZ,54.7ꢀ)
yields an estimate of the total c.m. speed distribution g(v) and, in
turn, some indication of the form of the individual speed distribu-
tions gi(v). These are usually modeled with the functional form36,37
III. RESULTS AND ANALYSIS
A. Analysis. We have measured TOF profiles of Cl and Cl* at
different laser polarizations. The translational energy distribution
and the anisotropy parameter for both Cl and Cl* are determined
from these TOF profiles, using a commonly used forward
convolution (FC) technique, as described in our earlier
publication.24 In this method, the knowledge of the instrumental
response function is required, and the same is determined by
studying the REMPI of aniline beam. The REMPI spectrum of
aniline recorded at 34029 cm-1 (293.77 nm) matches very well
with that reported in the literature.26,27 The aniline molecular
ion signal was measured as a function of laser intensity, at the
ai
bi
giðvÞ ¼ ðfTÞi ð1 - ðfTÞiÞ
ð3Þ
where (fT)i is the fraction of the available energy channeled into
translational modes, Etirans/Eaivail, and ai and bi are adjustable
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the translational energy distributions and the anisotropy param-
eters are invariant over the laser fluences used.
B. Spin-Orbit Branching Ratio. Figure 3 shows typical
Doppler profiles of Cl(2P3/2) and Cl*(2P1/2) atoms produced
in the dissociation of fumaryl chloride on excitation at 235 nm,
which are wavelength scans in the region 235.336 and 235.205
nm, respectively. The relative quantum yield of chlorine atom
fragments in different spin-orbit states was determined by
normalizing the integrated intensity, i.e., peak areas, S(Cl) or
S(Cl*), of the respective 2þ1 REMPI transitions with respect to
the laser intensity, and the ratio of the two-photon absorption
coefficients. The measured area of 2þ1 REMPI lines is propor-
tional to the actual product ratio, with a factor of k, which is the
relative ionization probability for Cl and Cl*, f(Cl)/f(Cl*),
ꢁ
ꢁ
NðCl Þ
SðCl Þ
¼ k
ð4Þ
Figure 2. Dependence of the observed Cl(2P3/2) atom REMPI signal
from fumaryl chloride photolysis on the laser intensity. The slope of the
fitted linear log-log plot is 3.1 ( 0.2.
NðClÞ
SðClÞ
where N(Cl) and N(Cl*) designate the number density of Cl and
Cl* produced. S(Cl) and S(Cl*) are obtained by integrating the
measured ion signal intensity over the proper range containing
the Doppler width and the probe laser bandwidth. The measure-
ments were repeated at different laser light intensities, giving
similar relative signal intensities. For fumaryl chloride photo-
dissociation, the ion integrated signal intensity ratio, S(Cl*)/
S(Cl), has been measured to be 0.39 ( 0.06. From the measured
integrated intensity ratio, one can easily obtain the product ratio,
using the above eq 4, by taking a value of 0.85 ( 0.10 for k.38 The
relative quantum yields, Φ(Cl) and Φ(Cl*), can be determined
from the product ratio, and Φ(Cl*) can be expressed as
ꢁ
NðCl Þ
ꢁ
ΦðCl Þ ¼
ð5Þ
ꢁ
NðCl Þ þ NðClÞ
The value of Φ(Cl*) calculated for fumaryl chloride dissociation
is found to be 0.24 ( 0.03. On statistical grounds one expects a
quantum yield, Φ(Cl*) = 0.33. As described in the later section,
the low quantum yield for the Cl* can be attributed to the fact
that the C-Cl bond fission via the lowest-recoil-kinetic energy
mechanism mainly produces Cl atoms in the Cl(2P3/2) state; the
same is true for the lowest-kinetic-energy C-Cl bond fission
channel in photodissociation of many other alkyl chlorides.
C. Translational Energy Distribution and Anisotropy Param-
eter. The measured Cl atom TOF profiles were analyzed to
determine the kinetic energy distribution and average kinetic
energy of the fragments in the fumaryl chloride photodissocia-
tion at ∼235 nm. The TOF profiles for the Cl and Cl* atoms
were converted to the velocity domain. Figures 4 and 5 show
background subtracted TOF spectra, recorded for the laser
polarization parallel, at the magic angle ∼54.78ꢀ and perpendi-
cular to the detection axis, for the Cl and Cl* fragments,
respectively. We analyzed the TOF data, using a forward con-
volution procedure, as described in the earlier section. Here, an
initial photofragment speed distribution, g(v), and the anisotropy
Figure 3. Profiles of Cl and Cl* atoms produced in the 235 nm laser
photolysis of fumaryl chloride used for the determination of their relative
quantum yields.
parameters. By taking into account an adjustable anisotropy para-
meter βi and weight for each decay channel, we simultaneously
calculate f(vZ,χ) for the geometries χ = 0ꢀ, 54.7ꢀ, and 90ꢀ.
Convolution with the instrument response function yields simulated
TOF profiles, which can be compared with the experimental results.
The parameters are then adjusted until a satisfactory agreement with
the experimental data is achieved. Once the photofragment speed
distributions have been determined, these may be used to obtain the
corresponding translational energy distributions.
Figure 2 shows the power dependence of the one-color
REMPI signals for transitions corresponding to Cl (2P3/2).
The signal is linear in a log-log plot over the range of powers
used in the present study. For both the types of chlorine atoms,
the lines exhibit a slope of ∼3.1 ( 0.2, which is consistent with
one-photon dissociation of fumaryl chloride, followed by 2 þ 1
REMPI of the chlorine atoms, assuming that the ionization step is
saturated. Apart from the power dependence studies, we also
systematically monitored the shape and the width of TOF
profiles of Cl atoms at various laser intensities. All the experi-
ments were performed in the intensities, which are much lower
than the intensity at which the shape and width of the TOF
profiles were invariant. This experimental condition ensures that
parameter β are assumed. As atomic v j correlations are known
3
to be very weak, we can safely assume the TOF profiles to be
independent of the probe polarization. The TOF profiles are
calculated for the three experimental configurations, convoluted
with the instrumental response function determined, as de-
scribed in an earlier section, and are compared with the experi-
mental results. The translational energy distributions, P(ET),
determined from the data in Figures 4 and 5, for the Cl and Cl*,
are depicted in Figures 6 and 7. Inspection of Figures 6 and 7
reveals that the P(ET) consists of two components. For the Cl
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Figure 5. REMPI-TOF profiles of Cl(2P1/2) produced from the 235 nm
photodissociation of fumaryl chloride. The circles are the experimental
data and the solid line is a forward convolution fit. Three panels, namely,
upper, middle, and lower panels, correspond to horizontal, magic angle,
and vertical experimental geometries, respectively.
Figure 4. REMPI-TOF profiles of Cl(2P3/2) produced from the 235 nm
photodissociation of fumaryl chloride. The circles are the experimental
data and the solid line is a forward convolution fit. Three panels, namely,
upper, middle, and lower panels, correspond to horizontal, magic angle,
and vertical experimental geometries, respectively.
atom, the faster component, centered at ∼14.4 kcal/mol, con-
sists of 67 ( 5% of the total fragments while the slower
component, centered at ∼2.7 kcal/mol, consists of the remaining
33 ( 5%. Similarly, for Cl*, the faster component, centered at
∼16.8 kcal/mol, consists of 87 ( 5% of the total fragments while
the slower component, centered at ∼2.4 kcal/mol, consists of the
remaining 13 ( 5%. It was not possible to determine indepen-
dently the anisotropy parameter for the two components, and
hence, the anisotropies were assumed to be identical for each
channel. The TOF profiles for both Cl and Cl* are independent
of laser polarizations, implying the β is well characterized by a
value of ∼0.0, within the experimental uncertainties. The calcu-
lated TOF profiles are displayed by the solid line, with each
component shown by a dotted curve. The difference in the TOF
profile for Cl and Cl* in Figure 4 and 5 arises mainly due to the
different nature of fT, their nonstatistical branching ratio and the
ratio of fast to slow component.
D. HCl Formation Channel. The multiphoton ionization
spectrum of fumaryl chloride at 235 nm clearly shows peaks at
m/e 36 and 38, corresponding to H35Cl and H37Cl, respectively.
To explore the origin of the HCl mass peak, the laser wavelength
was scanned over its REMPI region in the range 236-237
nm.39-41 In our experiments, we did find 2þ1 REMPI lines of
HCl in the TOF spectra in the range of 236-237 nm, for the
Q(J) branch of the V 1Σþ (0þ) rr X 1Σþ (0þ) (12,0) band
system.39 Although the signal was weak due to the predissociative
nature of this particular transition,39 we were able to carry out the
power dependence studies. The predissociative nature of this
transition was also established in our experiment, since we
observed the corresponding Cl and H atom ion signals at the
Figure 6. Center-of-mass recoil translational energy distribution de-
rived from Figure 4 for Cl(2P3/2) in the photodissociation of fumaryl
chloride at 235 nm. The dashed lines indicate the speed distributions for
the fast and slow components of the chlorine atom formation channel;
the solid line shows the sum. The vertical arrow indicates the maximum
available energy for Cl(2P3/2) elimination channel.
resonant wavelength corresponding to the REMPI lines of HCl
molecules. A linear log-log plot for the power dependence
studies shows a slope of ∼2.9 ( 0.2, which is consistent with one-
photon dissociation of fumaryl chloride forming HCl followed by
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transition, with oscillator strengths exceeding 0.4, is the first
π-π* transition and is termed as S2. Topological analyses of the
electron density notably highlight the delocalized nature of the
OdC—CdC—CdO chain, whereby the central CdC bond
possesses the largest electron density, although less than the pure
π-bonds. The C—C bonds consequently possess larger electron
populations than the expected σ-bonds. The nature of π and π*
orbitals in this transition is having a mixed character involving
mainly the CdC and the CdO π electrons. The weaker n-π*
transitions at lower energy is termed as S1 and it mainly involves
the transition from the nonbonding orbitals of both Cl and O to
the π* orbitals, which have a mixed character as discussed earlier.
At 235 nm, we believe that fumaryl chloride is excited to the S2
state. This S2 state adiabatically correlates only with highly
excited photoproducts, and that is not feasible in a single-photon
excitation in the present case. Therefore, it is assumed that
fumaryl chloride from the S2 states crosses over to the nearby
states, mostly the n-σ* state, from where the C-Cl bond cleaves
forming Cl atoms. Also, it can undergo rapid internal conversion
to the ground state, from where various other dissociation
channels can occur, in addition to the Cl atom elimination.
B. Translational Energy Release and Anisotropy Param-
eter. Several processes have been proposed as primary dissocia-
tion pathways in fumaryl chloride photoexcitation at 235 nm, and
these are given as follows:
Figure 7. Center-of-mass recoil translational energy distribution de-
rived from figure 5 for Cl(2P1/2) in the photodissociation of fumaryl
chloride at 235 nm. The dashed lines indicate the speed distributions for
the fast and slow components of the chlorine atom formation channel;
the solid line shows the sum. The vertical arrow indicates the maximum
available energy for Cl(2P1/2) elimination channel.
its detection with the 2þ1 REMPI scheme, similar to the
dynamics of Cl atom formation. Laser polarization dependent
studies on the HCl TOF profile to elucidate the translational
energy distribution could not be done due to weak signal, as
discussed earlier. The above set of experiments establishes the
formation of HCl in the photodissociation of fumaryl chloride at
235 nm as a primary channel.
—
—
—
—
ClOC CHdCH COCl f ClOC CHdCH CO þ Cl
ΔH ¼ 83 kcal=mol
ð6Þ
ΔH ¼ 100 kcal=mol
ð7Þ
—
f ClOC CHdCH þ COCl
—
f ClOC CHdCdCO þ HCl
ΔH ¼ 38 kcal=mol
ð8Þ
IV. DISCUSSION
Reactions 6 and 7 involve the cleavage of single bonds C-Cl and
C-C, respectively, while reaction 8 is the HCl molecular
elimination channel. The Cl atom can be produced directly from
reaction 6 as a primary product and also from the subsequent
dissociation of highly energetic COCl radical (reaction 9)
formed in reaction 7 as a secondary product. The ΔH values
are taken as similar to that of acryloyl chloride from refs 42 and
43.
A. Nature of Excitation at 235 nm. As discussed earlier,
fumaryl chloride contains one -CdC- and two -CdO groups
in conjugation, which make it bichromophoric, similar to fumaric
acid (HOCO—CHdCH—COOH) and acryloyl chloride
(H2CdCHCOCl). Hence, we expect that fumaryl chloride also
has a similar type of absorption spectrum with two bands
corresponding to n-π* and π-π* transitions. Ab initio molec-
ular orbital (MO) calculations were performed to investigate the
nature of the excited electronic states of fumaryl chloride. We
optimized ground state geometries of various isomers, as shown
in Figure 1, employing time dependent (TD) density function
theory (DFT) calculation, using the cc-pVDZ set of basis sets.
There are three isomers, namely, cis-cis, cis-trans, and trans-
trans, for fumaryl chloride. Theoretical calculations predict that
the relative stabilities of all three isomers differ by less than 0.5
kcal/mol. Thus, these isomers are indistinguishable, given the
experimental uncertainty of present study and the accuracy of the
theoretical methods employed here. Vertical excitation energies
were obtained for various transitions, to understand the nature of
excitation at 235 nm. Although the calculated vertical transition
energies are red-shifted compared to the experimental results in
case of fumaric acid,42 the nature of transition and the orbitals are
accurately predicted using this method. Orbitals participating in
the different electronic transitions were visualized for better
understanding of the process. The vertical excitation of fumaryl
chloride is associated with a strong π-π* transition at higher
energy and a weaker n-π* at lower energy. The strong
COCl f CO þ Cl
ΔH ¼ 8 kcal=mol
ð9Þ
In fumaryl chloride, two types of P(ET) were observed for the C-
Cl fission: one producing fragments with high kinetic recoil
energies and the other producing fragments with low recoil
energies. The P(ET) for Cl(2P3/2) derived from the forward
convolution fit to the high-translational energy C-Cl fission
channel peaks near 14.4 kcal/mol and extends to 30.0 kcal/mol.
Similarly, the low translational energy C-Cl bond fission channel
peaks at 2.7 kcal/mol and extends to 10 kcal/mol. The presence
of two types of P(ET) in C-Cl bond fission is analogous to other
systems involving a π-π* transition, such as acryloyl
chloride,9,43,44 allyl chloride,45,46 and 2-chloropropene.47 The high
translational energy chlorine atom channel, most likely, arises from
an electronic predissociation via a state repulsive in the C-Cl
bond, as seen in the above systems. It is possible that the low
translational energy channel results from the C-Cl fission follow-
ing internal conversion to the ground electronic state. Another
possibility of low energy Cl atom formation may arise from
reaction 9. However, the formation of the COCl radical cannot
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compete with the Cl formation channel due to its high endother-
micity on the ground potential energy surface. So the only
possible route for the formation of COCl, and hence, formation
of Cl from it, is from a higher excited state such as, S1, T2, and T1.
But this route for the COCl formation is also ruled out in acryloyl
chloride, a molecule with photochemical behavior similar to that
of fumaryl chloride, on the basis of theoretical studies by Cui
et al.11 The nature of P(ET) of the slow Cl atom obtained in the
present studies for fumaryl chloride also rules out this route on
the basis of energy consideration. So, the only formation pathway
for the slow Cl atom is attributed to the dissociation process from
the ground electronic state, after internal conversion. This
proposition is further supported by the fact that we did observe
the molecular HCl elimination channel, which is conclusively
from the ground state of fumaryl chloride.
2.4 kcal/mol (spin-orbit energy) for these channels. A low fT
value suggests that the kinetic energy released for the slow
component can be explained better in light of a statistical model.
The statistical model assumes partitioning of the available energy
democratically throughout the molecule and therefore neglects
the effects of specific dynamical interactions of the departing
fragments. A statistical dissociation process is predominant for a
long-lived photoexcited parent molecule, allowing partitioning of
the excess energy statistically among the available degrees of
freedom of the products. This may be applicable in a process
involving a rapid internal conversion to the ground electronic
state, followed by the subsequent slow dissociation. Under these
circumstances, in a large molecule with many low frequency
modes, a relatively small amount of the excess energy is parti-
tioned into translational motion of the products. For this kind of
dissociation process, a priori calculations50,51 were adopted,
which give the fT value of 0.07, matching very well with the
experimental value of 0.08. This implies that the slow component
in the translational energy distribution is mainly due to the Cl
atom, which arises from the ground state potential energy
surface, after internal conversion via some curve crossing me-
chanism. This mechanism is further confirmed by observation of
the molecular HCl elimination channel, as described earlier.
The higher experimental fT value as compared to that calcu-
lated using the impulsive model obtained in the dissociation of
fumaryl chloride for the fast chlorine atom channel prompted us
to apply the hybrid model, employed by North et al.52 for
reactions with barrier. In this model, the Eavail for the products
is divided into two parts, namely, the excess energy above the exit
barrier (Estat) and at the exit barrier energy (Eimp). The partition-
ing of Estat and Eimp is treated by the statistical and modified
impulsive models, respectively. The energy partitioned into each
fragment is then obtained by adding contributions from each of
these two models. For this type of dissociation with barrier, ET
does not change much with Eavail. Similar results were obtained in
our earlier studies on various saturated and unsaturated car-
boxylic acids, and also on the enone system.2,3 Photodissociation
studies on acryloyl chloride by Butler and co-worker9,44 at 193
and 235 nm show similar translational energy release for the Cl
atom. At 193 nm dissociation, ET was obtained to be 27 kcal/mol.
Similarly, at 235 nm dissociation, a value of 23 kcal/mol was
obtained. Here, it can be clearly seen that the change in the Eavail
from 65 kcal/mol (for 193 nm) to 39 kcal/mol (for 235 nm) did
not change the ET value considerably for the Cl atom dissociation
channel in acryloyl chloride. Also, the theoretical calculation by
Cui et al.11 clearly shows the presence of an exit barrier in the
S1(1n-π*) state, for the Cl atom dissociation channel. So, we
strongly believe that the dissociation from the S1 state with an
exit barrier cannot be ruled out in the photodissociation of
acryloyl chloride in addition to dissociation from the repulsive
state with (n-σ*) nature. With same analogy, we propose that in
fumaryl chloride the initially prepared S2(1π-π*) state crosses
over to various states, namely, S1(1n-π*), (n-σ*), and the
ground state. Finally, the Cl atom can be formed from all these
states with different values of ET.
The partitioning of the available energy into various degrees of
freedom of the fragments is mainly governed by the nature of the
dissociative potential energy surface. Very often, the disposition
of the available energy into the fragment translational and the
internal degrees of freedom can be predicted, using simple
models. These models generally fall into two categories: im-
pulsive and statistical. It is well-known that the energy partition-
ing for a dissociative event on a repulsive surface is well described
by an impulsive model. So, an impulsive model24,48,49 has been
used in this case, to calculate theoretically the translational
energy released to the products. In this model, the distribution
of energy among the product states is governed by the dissocia-
tive event, i.e., by the repulsive force acting during the breaking of
the parent molecule into the products. For example, in the
present case, by using only conservation of momentum and
energy, and the impulse assumption, one finds that the fraction of
the available energy (Eavail) released as translational energy is
given by
!
μC-Cl
μClCOCHCHCO-Cl
ET
¼
Eavail; and fT ¼ ET=Eavail ð10Þ
where μC-Cl is the reduced mass of the C and Cl atoms,
μClCOCHCHCO-Cl is the reduced mass of the ClCOCHCHCO
and Cl, Eavail is the available energy, and fT is the fraction of the
available energy going into the translational modes of the
fragments. In the case of fumaryl chloride, the ratio of reduced
masses is 0.32. The available energy is given by
Eavail ¼ Ehν - D00ðClCOCHCHCO - ClÞ - ESO
ð11Þ
where Ehν is the photon energy (122.0 kcal/mol), D00
(ClCOCHCHCO-Cl) is the C-Cl bond dissociation energy,
and ESO is the spin-orbit energy of chlorine (2.4 kcal/mol). The
bond dissociation energy is taken as 83.0 kcal/mol, similar to that
for acryloyl chloride.44 Thus, Eavail for the Cl and Cl* channels are
39.0 and 36.6 kcal/mol, respectively. The experimental average
translational energy (ET) for the fast component is found to be
14.9 ( 1.6 and 16.8 ( 1.6 kcal/mol for Cl and Cl* channels,
respectively, giving the fT values of 0.38 and 0.46 for the Cl and
Cl*, respectively. The experimental determined fT so obtained is
slightly higher than the value of 0.32 predicted, using the
impulsive model.
In the analysis of the recoil anisotropy, β, for the Cl fragment,
we note that in the limit of an instantaneous dissociation process,
β = 2ÆP2(cos θm)æ, where θm is the molecular frame angle
between the molecular transition dipole moment and the photo-
fragment recoil direction, and P2(cos χ) is the second-order
Legendre polynomial. In a photodissociation process, following
excitation via the electric transition dipole moments, namely,
Now, coming to the slow component in the translational
energy distribution of the Cl atom, the average energies deter-
mined are found to be 3.4 ( 0.8 and 3.1 ( 0.8 kcal/mol, for Cl
and Cl*, respectively. These values give the same fT of 0.08,
for both Cl and Cl*, even though the available energies differ by
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parallel or perpendicular, would be expected to give a β value of
þ2 or -1, respectively. The anisotropy measured in the present
work is ∼0.0, which indicates an isotropic dissociation process.
This value of β does not give any indication of the nature of the
transition dipole moment in fumaryl chloride dissociation. Gen-
erally, an impulsive dissociation is accompanied with an aniso-
tropic distribution of the photoproducts, since the dissociation
lifetime is much shorter than the rotation period of the molecule.
However, the anisotropy in an impulsive dissociation can be
reduced,53,33 or wiped out,54 due to several factors, such as mixed
initial transition with both parallel and perpendicular compo-
nents, longer dissociation lifetime, dissociation not from a single
geometry rather from a range of geometries etc. In fumaryl
chloride, for C2h geometry, the π-π* transition, which has Bu
symmetry, the transition dipole moment is mainly along the x
axis and y axis. The same is true for the n-π* transition. So, in
this case even if there is a crossover from the initially prepared
π-π* state to the n-π* state, the transition dipole moment is
not changed. However, during this crossover period the molecule
may rotate. The absence of recoil anisotropy in the present
studies is expected mainly due to the mixed transition, as
discussed, and the relatively longer dissociation lifetime, since
the dissociation is not from the initially prepared state rather
from the crossover state. In addition, the different geometries of
fumaryl chloride also contribute to the β value of ∼0.0.
that in fumaryl chloride the initially prepared S2(1π-π*) state
crosses over to various states, namely, S1(1n-π*), (n-σ*), and
the ground state, and the Cl atom can be formed from all these
states, with different values of ET.
’ AUTHOR INFORMATION
Corresponding Author
*Electronic mail: haripu@barc.gov.in.
’ ACKNOWLEDGMENT
We thank Drs. S.K. Sarkar and T. Mukherjee for their constant
guidance and keen interest throughout this work. We also
acknowledge help rendered by Mr. Yogesh Indulkar during the
initial experiments.
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In summary, fumaryl chloride generates a chlorine atom upon
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