J . Phys. Chein. 1994, 98, 13447-13451
13447
Photodissociation of trans-Dichloroethylene: State-Resolved Speed and Angular Distributions of C1 Atoms Toshinori Suzuki,**f$* Kenichi Tonokura,t Lizla S. Bontuyan,t and Nobuhisa Hashimoto' Institute for Molecular Science and The Graduate University for Advanced Studies. Myodaiji, Okazaki 444 Japan Received: July 25, 1994; In Final Form: October 19, 1994@
The elimination of atomic chlorine in the ultraviolet photodissociation of trans-dichloroethylene was studied by measuring speed and angular distributions of C1(2P,; J = l/2, 3/2) atoms. Two components were observed in the translational energy distributions, similar to those observed by Umemoto et al. using photofragment translational spectroscopy. One component has a strong anisotropy and a Gaussian-shaped translational energy distribution, while the other has a weak anisotropy and a Boltzmann-like distribution. The former is ascribed to rapid C-Cl bond rupture due to surface crossing between the (JC,X*) and (n,a*) or (n,o*) states, while the latter to thermal decomposition via internal conversion to the ground electronic, state. The wavelength dependence and Cl*/Cl branching ratios of the two dissociation channels were also investigated.
Introduction
Efficient intramolecular electronic and vibrational energy transfer (relaxation) is an inherent property of polyatomic molecules and plays an essential role in their dynamics. Photodissociation experiments may be suitable for studying intramolecular relaxation, since the extent of energy randomization, the branching ratios, and the time scales of the energy transfer can be probed by measuring the intemal state, translational energy, and angular distributions of the photofragments. The ultraviolet photodissociation of vinyl chloride, trans-, cis-, and 1,l-dichloroethylene (DCE),1-5 is an example of a system involving intramolecular relaxation. Umemoto et al. measured the translational energy distribution of C1 atoms by photofragment translational s p e c t r o ~ c o p yand ~ ~ observed ~ two C1 elimination channels. In order to examine these channels in more detail, this work extends their investigation by using two-dimensional (2D) ion imaging.*-l0 In this work, (1j the entire velocity (speed and angular) distributions of the C1 atoms are determined, (2) the wavelength dependence of the relative yield of the two dissociation channels is examined, and (3j the difference between the velocity distributions of C1 atoms in the two spinorbit states ( 2 P ~J; = l/2 or 3/2jis examined. This paper focuses mainly on experiments with trans-DCE, although results on cisDCE will be briefly mentioned. More detailed analyses and discussion on vinyl chloride, trans-, cis-, and 1,l-dichloroethylene, will be reported elsewhere. Experimental Section
A supersonic beam 1 mm in diameter was introduced into a Wiley-McLaren time-of-flight (TOF) mass spectrometer' in the direction parallel to the electric field vector. Sample gases, 10% seeded in He, were expanded with a stagnation pressure of 2 atm relative to vacuum. At 79 mm downstream from the nozzle, the molecular beam intersected with the photolysis and probe laser beams which were counterpropagated perpendicular to the molecular beam. The output of an ArF laser (193 nm) was polarized using a pile-of-plates polarizer. The probe beam served as the photolysis beam for the photodissociation at 235
' Institute for Molecular Science.
* The Graduate University for Advanced Studies.
@
Abstract published in Advance ACS Abstrucrs, November 15, 1994.
nm. In the one-color photodissociation-detection experiments, the wavelengths for the detection of C1 and C1* atoms need to be similar so that the initial photoexcitation of the parent molecule is not changed. This was achieved by employing [2 -I- 11 REMPI detection of chlorine atoms using the resonances of *D3/2 2P3/2 (235.336 nmj and 2P1,2 2P1/2 (235.205 nm). The time delay between the photolysis and probe laser pulses was kept within 20 ns. The ions generated by REMPI were accelerated to 5 keV and projected into a pair of microchannel plates (MCP) backed by a phosphor screen. A high-voltage pulse was applied to the MCP as the 35Cl+arrived, raising the gain to 106-107. The 35Cl+ signal was thus discriminated from the scattered light, 37Clf, and other background ions with different mle. The transient image on the screen was captured by a CCD camera and was accumulated in the 16-bit memory of an image capture board. The entire apparatus was operated at 25 Hz. The vacuum pressures were (1-5) x Torr in the beam source and 1 x lo-' Torr in the main chamber when the pulsed valve was on. The probe laser beam was focused by an axisymmetric lens cf= 250 mmj, while the photolysis beam was unfocused. The laser intensities used were < 3 mJ cm-*/pulse (193 nmj, < 2 &/pulse (210 nm), and 0.03-0.2 mJ/pulse (235 nm). The rotational temperature of chloroethylenes in the molecular beam was estimated to be 10-30 K from the rotational analysis of A - X transitions of acetylene and NO measured with the same apparatus. No clusters in the molecular beam were detected by quadrupole mass analysis, although the possibility of some clustering cannot be completely excluded because of fragmentation upon electron impact ionization. The images observed experimentally are 2D projections of the 3D distributions of photofragments. A velocity map of the photofragments is obtained from a section of the original 3D distribution containing the axis of cylindrical symmetry by an inverse Abel transform on the 2D projection. l2
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Results and Discussion
Figure 1 shows the raw images of C1 observed in the photodissociation of trans-dichloroethylene (DCE) at 193 and 235 nm. The photolysis wavelength at 193 nm is on the peak of the strong ,z* x band of trans-DCE, while 235 nm is around the onset of the band.2 The difference between the C1
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0022-3654/94/2098-13447$O4.50/0 0 1994 American Chemical Society
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13448 J. Phys. Chem., Vol. 98, No. 51, 1994
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(a) C1(2P3n) at 193 nm, (b) C1*(2Pln) at 193 nm, = l/2 or 9 2 ) were selectively ionized by [2 + 13 the left and right sides of the images is due to rse Abel transform. Each image was accumulated
nergy and strong anisotropy and (2) low translational energy and weak anisotropy. Clearly, the angular distribution cannot be expressed by a single anisotropy parameter for the entire speed distribution. The two components in the velocity distributions are seen more clearly in the velocity contour maps obtained by the Abel transform of the images displayed in Figure 1. Two of the maps are shown in Figure 2. Two components in the velocity
distributions are evident, especially in the map for 193 nm photodissociation (Figure 2a). More quantitative analyses were performed on the translational energy and angular distributions in the center-of-mass frame obtained from these velocity maps. The translational energy distributions, &Et), are shown in Figure 3. The P(Et) in the 193 nm photolysis have shoulders in the high-energy side, similar to those observed in the time-of-flight (TOF) data of Umemoto et a2.I The observed P(Et) were well reproduced by combinations of Maxwell-Boltzmann and Gaussian functions, although the separation into these two functional forms should be considered to be approximate. In addition to the experiments at 235 and 193 nm, the photodissociation was also performed at 210 nm, which is on the shoulder of the n* n absorption band.* Best-fit parameters are summarized in Table 1. The
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J. Phys. Chem., Vol. 98, No. 51, 1994 13449
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expected to be overlapping in the UV region, the x* x transition has a much larger photoabsorption intensity (emax 10 000) than the others2?l7and is considered to be responsible for both of the Boltzmann and Gaussian components. It is important to note that the #? obtained for the Gaussian and Boltzmann components are different approximately by a factor of 4 (Table 2). This suggests that the Boltzmann component originates from the same transition as the Gaussian component but is produced via a long-lived intermediate. The anisotropy of photofragment distribution is diminished by rotation of the parent molecule during dissociation. The reduction is caused by a tangential velocity imparted by molecular rotation and/or by the loss of alignment of photoexcited molecule before dissociation. In our experiments, the tangential velocity6 is negligible (
