J. Phys. Chem. A 2010, 114, 37–44
37
Photodissociation of cis-, trans-, and 1,1-Dichloroethylene in the Ultraviolet Range: Characterization of Cl(2PJ) Elimination Linqiang Hua,†,‡ Xiaopeng Zhang,§,| Wei-Bin Lee,§,| Meng-Hsuan Chao,§,| Bing Zhang,†,‡ and King-Chuen Lin*,§,| State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China, Graduate School of the Chinese Academy of Sciences, Beijing 10039, P. R. China, Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan ReceiVed: July 23, 2009; ReVised Manuscript ReceiVed: October 11, 2009
By using photofragment velocity imaging detection coupled with a (2 + 1) resonance-enhanced multiphoton ionization technique, the elimination channel of spin-orbit chlorine atoms in photodissociation of cis-, trans-, and 1,1-dichloroethylene at two photolysis wavelengths of 214.5 and 235 nm is investigated. Translational energy and angular distributions of Cl(2PJ) fragmentation are acquired. The Cl(2PJ) fragments are produced by two competing channels. The fast dissociation component with higher translational energy is characterized by a Gaussian distribution, resulting from a curve crossing of the initially excited (π, π*) state to nearby repulsive (π, σ*) and/or (n, σ*). In contrast, the slow component with a lower translational energy is characterized by a Boltzmann distribution, which dissociates on the vibrationally hot ground state relaxed from the (π, π*) state via internal conversion. cis-C2H2Cl2 is found to have a larger branching of Boltzmann component than the other two isomers. The fraction of available energy partitioning into translation increases along the trend of cis- < trans- < 1,1-C2H2Cl2. This trend may be fitted by a rigid radical model and interpreted by means of a torque generated during the C-Cl bond cleavage. The anisotropy parameters are determined, and the transition dipole moments are expected to be essentially along the CdC bond axis. The results are also predicted theoretically. The relative quantum yields of Cl(2PJ) have a similar value for the three isomers at the two photolysis wavelengths. I. Introduction Photodissociation dynamics of dichloroethylenes (DCEs) has attracted wide attention experimentally1-11 and theoretically12,13 in the past decades. The absorption spectra of the three DCE isomers in the 140-260 nm range have been reported since the 1970s,1 yielding at least five product channels hV
C2H2Cl2 98 C2HCl + HCl
(1)
hV
C2H2Cl2 98 C2H2Cl + Cl
(2)
hV
C2H2Cl2 98 C2HCl2 + H
(3)
hV
C2H2Cl2 98 C2Cl2 + H2
(4)
hV
C2H2Cl2 98 C2H2 + Cl2
(5)
Among them, channels 1 and 2 were observed in the VUV and UV range,4,6,11 while the remaining channels were only in the VUV range.1-3,5,7-10 * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: 886-2-23621483. † Wuhan Institute of Physics and Mathematics. ‡ Graduate School of the Chinese Academy of Sciences. § National Taiwan University. | Academia Sinica.
DCEs may feasibly release chlorine atoms after photodissociation,14 thereby causing potential impact on ozone depletion. Research on the Cl dissociation channel is actively focused by using radiation at 193 and 157 nm. For instance, Moss et al.2 looked into both HCl and Cl elimination in the 193 nm photolysis of DCEs using infrared emission spectroscopy. In the study of Cl elimination at 193 and 157 nm using photofragment transitional spectroscopy (PTS), Umemoto et al.3 and Sato et al.9 found the translational energy distributions composed of two components and suggested the probable dissociation pathways. Applying Doppler spectroscopy, Mo et al.4 analyzed Doppler profiles of the Cl-fragment velocity distribution at 193 nm, confirming these two processes to the Cl(2PJ) production. One produced an isotropic distribution, while the other produced an anisotropic distribution. They also obtained angular distributions of the Cl(2PJ) photofragments at 235-237 nm in transand 1,1-DCE, yielding the anisotropy parameters for the fast dissociation components. Using resonance-enhanced multiphoton ionization (REMPI) technique, Gordon and co-workers5 determined the relative quantum yields of Cl(2PJ) for three DCE isomers upon irradiation of 193 nm. They also employed magic angle Doppler spectroscopy to measure the speed distribution functions of Cl and Cl*. Bimodal energy distributions were found for both spin-orbit states,8 of which Cl* had more kinetic energy than Cl. Applying an ion imaging technique, Suzuki et al.6 obtained the angular and speed distributions of the Cl(2PJ) photoproducts in trans-C2H2Cl2 at 193, 210, and 235 nm. They then determined Cl/Cl* branching ratios, anisotropy parameters, and fraction of translational energy partition and suggested probable photodissociation pathways. When the photolysis wavelengths were tuned to a near-UV range of 222-304 nm,
10.1021/jp907030e 2010 American Chemical Society Published on Web 12/18/2009
38
J. Phys. Chem. A, Vol. 114, No. 1, 2010
Chandra et al.11 determined the relative quantum yields of Cl*. The cis-C2H2Cl2 was found to have larger quantum yield than the other two isomers for all the excitation wavelengths studied except at 222 nm. Thus far, the Cl(2PJ) elimination in the photodissociation of DCEs with long UV wavelengths is less studied thoroughly. In attempt to supplement information in this wavelength range, we have investigated the photodissociation of three DCE isomers at two wavelengths, 214.5 and 237 nm, using velocity-map ion imaging combined with the (2 + 1) REMPI. This technique has been proven to be powerful, but less employed except for trans-DCE.6 With this technique, the Cl(2P3/2) and Cl*(2P1/2) images are acquired following the photodissociation, and the corresponding state-resolved speed and angular distributions of the fragments may be extracted simultaneously. Then, translational energy distributions, anisotropy parameters, and branching ratios of spin-orbit ground and excited states may be determined. Finally, the dissociation pathways producing Cl(2PJ) fragments and comparison among these isomers are discussed. II. Experimental Section The photofragment velocity imaging apparatus has been described elsewhere.15 It consisted of a source chamber and a main chamber, both of which were pumped to a low background pressure of ∼2 × 10-7 torr. Liquid samples of cis-, trans-, and 1,1-C2H2Cl2 were bought (Sigma-Aldrich) and used as received. Each was carried by helium gas at 1.3 atm through a pulsed valve with 0.6 mm diameter orifice operating synchronously with the laser pulses at 10 Hz and expanded into the source chamber. After passing through a 1 mm diameter skimmer and a collimator, the molecular beam was intersected perpendicularly by a linearly polarized laser beam in a two-stage ion lens region. The skimmer was mounted 30 mm downstream from the nozzle to divide the source chamber from the main chamber. The electrostatic ion lenses installed in the main chamber consisted of a set of repeller, extractor, and ground electrodes, each with a central hole of 2, 16, and 16 mm diameter, respectively. The biased voltages were optimized with the aid of simulation using a standard ion optics trajectory program (Simion 6.0).16 In two-color experiments, a Nd:YAG laser (Quanta-Ray, Spectra-Physics) pumped dye laser (ScanMate, Lambda Physik) operating at 10 Hz was used as photolysis source. Its output was frequency-doubled to emit at 214.5 nm with the energy of ∼20 µJ/pulse. The UV radiation was then linearly polarized perpendicular to the flight tube direction and focused at the skimmed beam with a 200 mm focal-length lens. A 308 nm XeCl excimer laser (LPX 200, Lambda Physik) pumped dye laser (PD 3000, Lambda Physik) was guided in the opposite direction to probe the atomic fragments. Its output was frequency-doubled to emit at ∼235 nm with the energy controlled from 60 to 180 µJ/pulse for different samples. The probe radiation was adjusted to be linearly polarized perpendicular to the flight tube and then focused with a 200 mm focallength lens. The Cl* and Cl fragments were ionized at the wavelengths of 235.21 and 235.34 nm, corresponding to the 4P 2P1/2 r 3P 2P1/2 and 4P 2D3/2 r 3P2P3/2 transition,17 respectively, using a (2 + 1) REMPI technique. The time delay between the photolysis and probe beams was adjusted less than 20 ns. In contrast, a simpler one-color experiment contained only the radiation at ∼235 nm, which played both roles for dissociation of the precursors and subsequent ionization of the atomic fragments. The resulting chlorine ions were extracted and accelerated into a 36 cm long field-free drift tube along the molecular beam
Hua et al.
Figure 1. Time-of-flight mass spectrum of 1,1-C2H2Cl2 fragmentation upon irradiation at 235.34 nm. Cl+ ions were resonantly ionized through a (2 + 1) REMPI scheme, while the remaining peaks were produced from a nonresonant process.
direction. The ion-cloud expansion was mapped onto a twostage microchannel plate (MCP) and a phosphor screen (FM3040, Galileo). The MCP could be gated within a minimum duration of 250 ns for mass selection. Only 35Cl+ was selected to construct the image. The ion imaging on the phosphor screen was recorded by a charge coupled device (CCD) camera (200XL4078, Pixelfly). The laser wavelength was scanned back and forth within the range of Doppler broadening to cover all the velocity components of the selected fragments. All the ion signals without gate restriction may be acquired by a photomultiplier tube, instead of the CCD camera, and then transferred to a transient digitizer for display of the time-of-flight (TOF) mass spectrum. III. Results and Discussion A. Time-of-Flight Mass Spectrum. After absorbing one photon in the wavelength range of 210-235 nm, C2H2Cl2 may feasibly release a Cl atom or HCl radical, as given in eqs 1 and 2. Figure 1 shows a TOF mass spectrum of 1,1-C2H2Cl2 fragmentation upon irradiation at 235.34 nm, yielding m/e ) 12, 26, 35, 37, and 62 corresponding to C+, C2H2+, 35Cl+, 37Cl+, and C2H2Cl+, respectively. Among them, 35Cl+ and 37Cl+ were resonantly ionized through a (2 + 1) REMPI process, while the remaining peaks were produced from nonresonant process. The channel of HCl elimination has been identified with various techniques in this wavelength range.3-5,9 However, we failed to find any significant HCl+ signal, in part because the weaker peak might be buried in the neighboring REMPI signals. It is alternatively probable that HCl may further undergo predissociation18 by successively absorbing two photons at ∼235 nm. Figure 2 shows a power dependence measurement of the Cl fragment obtained at 235.34 nm in the one-color experiment. The measurement yielded a straight line with slope of 3.0 ( 0.2, indicating that the ions were obtained via a process of onephoton dissociation of C2H2Cl2, followed by (2 + 1) REMPI. The final ionization step is partially or completely saturated. The results are consistent with the values obtained between 3.3 and 3.8 by Gordon and co-workers5 at different photolysis wavelengths. The Cl+ contribution from secondary photodissociation of remaining moieties like C2H2Cl, C2HCl, and HCl can be neglected, because at least one additional photon is required to undergo this process. B. Translational Energy Distribution. As shown in Figure 3, the raw ion images of Cl(2P3/2) and Cl*(2P1/2) dissociation
Characterization of Cl(2PJ) Elimination
J. Phys. Chem. A, Vol. 114, No. 1, 2010 39 symmetry around the polarization axis of the photolysis laser. The corresponding three-dimensional spatial distributions of the fragments are reconstructed by the basis-set expansion method (BASEX).19 The speed distribution P(V) of the fragment, extracted from the reconstructed ion image, may be converted to the centerof-mass translational energy distribution P(ET) as follows
P(ET) ) P(V)
dV dET
mCl 1 2 ET ) (mCl + mC2H2Cl) × × VCl 2 mC2H2Cl Figure 2. Plot of Cl(2P3/2) signal as a function of laser fluence at 235.34 nm. The open circles are the experimental data, while the solid line is a linear least-squares fit, yielding a slope of 3.0 ( 0.2.
(6) (7)
where ET is the total translational energy, mCl and mC2H2Cl are the mass of Cl and C2H2Cl fragments, and VCl is the velocity of the chlorine fragment. Figure 4 shows the P(ET) results of chlorine dissociation from the three DCE isomers at 235 and 214.5 nm. Each distribution of Cl(2P3/2) and Cl*(2P1/2) at 235 nm appears to have two components, but the lower translational energy component in the latter fragment becomes smaller. The bimodal distribution for Cl(2P3/2) still exists with a smaller branch of the lower P(ET) component at 214.5 nm. In contrast, it is hard to discern this lower component in Cl*(2P1/2). The Cl or Cl* distributions with higher translational energy component can be fitted with a Gaussian curve, whereas the lower Cl or Cl* distributions are fitted with a Boltzmann distribution. The Cl*(2P1/2) distribution at 214.5 nm is fitted with only a Gaussian function, but the small hump of the lower component is neglected. The signal-to-noise of the small hump is poor and cannot be further analyzed. The available energy Eavl during the photodissociation may be evaluated by
Eavl ) EhV - D0 - Eel + Eint
(8)
where EhV is the photon energy; Eel is the electronic energy equal to 0 kcal/mol for Cl(2P3/2) and 2.52 kcal/mol for Cl*(2P1/2). D0 denotes the dissociation energy of the C-Cl bond, which is 89.24 kcal/mol for cis-C2H2Cl2, 88.74 kcal/mol for transC2H2Cl2, and 89.64 kcal/mol for 1,1-C2H2Cl2.20 Eint is the internal energy of the parent molecule. Its value is considered to be zero, since the rotational and vibrational excitations are negligible in a supersonic molecular beam. The fraction fT of the translational energy partition, defined as a ratio of the average translational energy to the available energy, can be determined by
fT ) Figure 3. (left half) Raw and (right half) reconstructed ion images of Cl(2P3/2) and Cl*(2P1/2) fragments resulting from cis-, trans-, and 1,1C2H2Cl2 at two photolyzing wavelengths of 235 and 214.5 nm. Each raw image was accumulated by at least 20 000 shots. The laser is linearly polarized along the vertical direction.
from cis-, trans-, and 1,1-DCE are acquired at 235.34 and 235.20 nm, respectively, following either one-color or two-color experiments. Each image was accumulated over 20 000 laser shots, and the background was removed by subtracting a reference image collected at off-resonance wavelength under the same conditions in the one-color experiments. For the twocolor experiments, the reference images, as obtained with either photolyzing or ionizing pulse alone, were both subtracted. The obtained raw image is a two-dimensional projection of the threedimensional speed and angular distributions with cylindrical
〈ET〉 Eavl
(9)
Given the above-mentioned data, some dynamical properties of the DCE isomers at the wavelengths studied can be determined as listed in Table 1. They include available energy Eavl, average transitional energy of the higher and lower components, and the fT value for the higher component. The fT values in Table 1 increase along the trend of cis< trans- < 1,1-C2H2Cl2 at either photolysis wavelength. This consequence may be realized from Figure 5. The figure shows the selected Cl atom that will be dissociated along the C-Cl bond axis and the position of the center-of-mass of the organic radical. When the Cl atom flies away, a restoring force should impose a torque on the remaining fragment. Obviously, cisC2H2Cl2 receives the largest torque during the photodissociation. Thus, a substantial extent of the available energy may be partitioned into the rotational degree of freedom, and the subsequent fraction into translational energy becomes less. In contrast, 1,1-C2H2Cl2 has the smallest torque, since the
40
J. Phys. Chem. A, Vol. 114, No. 1, 2010
center-of-mass of the organic radical is close to the C-Cl bond axis and thus the least rotational energy is deposited after dissociation. The available energy partitioning into translation can be predicted by using two radical limits of the impulsive model.21-23 In the spectator model, the carbon which is bound to the Cl atom behaves as a spectator during the Cl recoiling. The fraction of the translational energy partition is then estimated by
Hua et al.
fT ) 〈Et〉 /Eavl ) µa /µf
(10)
where µa is the reduced mass of the atoms C and Cl and µf is the reduced mass of the organic radical and Cl. Accordingly, fT is calculated to be 0.40 for all the DCE isomers. In contrast, the rigid model in the cleavage of the C-Cl bond takes into account the energy partitioning into the rotation and translation, but not the vibration of the organic radical. A fT value of 0.6, 0.8, and 0.9 was thus predicted previously for cis-, trans-, and 1,1-C2H2Cl2, respectively.3 As listed in Table 1, the fT values
Figure 4. Transitional energy distributions derived from the corresponding images shown in Figure 3. In each figure, open circle represents the signal intensity in arbitrary units, dotted line denotes the simulated Boltzmann component, dashed line denotes the simulated Gaussian component, and solid line is the sum of the Boltzmann and Gaussian components. The Cl*(2P1/2) distribution at 214.5 nm is fitted with only a Gaussian function.
Characterization of Cl(2PJ) Elimination
J. Phys. Chem. A, Vol. 114, No. 1, 2010 41
TABLE 1: Kinetic Data in the Photodissociation of Three Dichloroethylene Isomers wavelength (nm) 235
DCE cistrans1,1-
214.5
cistrans1,1-
a
Cl Cl* Cl Cl* Cl Cl* Cl Cl* Cl Cl* Cl Cl*
E(hν) (kcal/mol)
Eavl (kcal/mol)
〈ET(fast)〉a (kcal/mol)
〈ET(slow)〉a (kcal/mol)
fT (fast)
121.49 121.56 121.49 121.56 121.49 121.56 133.29 133.29 133.29 133.29 133.29 133.29
32.25 29.80 32.75 30.30 31.85 29.40 44.05 41.53 44.55 42.03 43.65 41.13
12.5 12.5 20.4 18.9 22.7 21.2 13.5 13.8 25.4 23.8 26.6 23.3
4.3
38.8% 41.9% 62.3% 62.4% 71.3% 72.1% 30.6% 33.2% 57.0% 56.6% 60.9% 56.6%
8.8 10.8 3.6 8.8 9.7
〈ET(fast)〉 and 〈ET(slow)〉 are the average translational energy of the fast and slow component.
The anisotropy parameter may provide information on the transition dipole moment in the molecular frame and the symmetry of excited states. Its relationship with the orientation of the transition dipole moment may be expressed by27
β ) 2P2(cos χ) Figure 5. Chlorine atom in the dashed square selected to be released. The dot point marks the center-of-mass of the remaining organic radical. The arrow denotes the direction of the transition dipole moment in each isomer.
(12)
are 0.40, 0.62, and 0.72 obtained at 235 nm and 0.32, 0.57, and 0.59 at 214.5 nm for cis-, trans- and 1,1-C2H2Cl2, respectively. The trend for the observed fT is consistent with the rigid model at both wavelengths. Despite failure in this work, the spectator model, which takes into account vibrational excitation, was found to fit better in the photodissociation of alkyl halides such as C2H4Br224 and C3H6Br225 The resulting moieties are substantially vibrationally excited following the photodissociation, and thus this model can be applied suitably. In the photolysis of C2H2Cl2, the organic radicals gain much less vibrational energy partition. However, vibrational excitation to some extent should be considered, since the observed fT values are generally smaller than those predicted by the rigid radical model. C. Fragment Angular Distribution. The fragment angular distribution I(θ), determined from the reconstructed ion image, may be characterized by anisotropy parameter, β, as expressed by26
where P2(cos χ) is the second-order Legendre polynomial and χ is the angle between the recoil velocity of Cl atom and the transition dipole moment. The higher P(ET) component from the C-Cl bond cleavage at 155 nm takes place within ∼200 fs,10 faster than a molecular rotation period. Thus, given the assumption of negligible rotational motion, the χ angles are obtained in Table 3. The transition dipole moment is found to be along the CdC bond axis for cis- and 1,1-C2H2Cl2, whereas it tilts with a small angle relative to the CdC bond axis for trans-C2H2Cl2 (Figure 5). For comparison, a theoretical calculation based on the GAUSSIAN 03 package was carried out.28 The TD-B3LYP method was adopted to optimize the geometry of ground states of the three isomers, while the TD-HF method was used to calculate the transition dipole moment. The 6-311G++(d, p) basis set was adopted in both methods. The calculated results are also listed in Table 3. The experimental findings are consistent with the calculated values except for trans-C2H2Cl2. A slightly larger χ obtained in the experiment might be due to the effect of molecular rotation or the structure change of the excited state during the photodissociation. D. Quantum Yield. Given the ratio of ion signals measured for Cl(2P3/2) and Cl*(2P1/2), the corresponding branching ratio can be estimated by the following equation
I(θ) ) (4π)-1[1 + βP2(cos θ)]
N(Cl*) S(Cl*) )k N(Cl) S(Cl)
(11)
where P2(cos θ) is the second-order Legendre polynomial and θ is the angle between the laser polarization direction and the recoil velocity of fragments. The β value may be obtained by least-squares fit to the angular distribution. The results are listed in Table 2. The trans- and cis-C2H2Cl2 yield positive β in the Cl(2PJ) channels, while the 1,1-C2H2Cl2 fast component gives negative β. The trans-C2H2Cl2 results at 235 nm are essentially consistent with those reported by Suzuki et al.6 but are larger than 0.65 and 0.61 for Cl and Cl*, respectively, obtained by Mo et al.4 The discrepancy rises in the different conditions for the molecular beam setup. Mo et al. employed an effusive beam with higher molecular temperature such that the resultant β may be reduced to some extent due to the effect of molecular rotation. As listed in Table 2, the β values show consistent magnitudes between these two photolysis wavelengths, implying that an identical upper state is excited.
(13)
where N denotes the population of chlorine atoms in the spin-orbit quantum state, S the REMPI intensities measured individually, and k the calibration factor especially related to the ionization efficiency. The REMPI intensity ratio of S(Cl*)/ S(Cl) was measured by integrating the individual area over the entire mass peak. Since the Cl and Cl* atoms are probed at similar wavelengths of 235.34 and 235.20 nm, respectively, the influence of laser energy and detection efficiency on the spin-orbit branching ratio may be neglected. The calibration factor of 1.06 ( 0.17 obtained by Regan et al.29 is adopted herein. They determined this factor by measuring the Cl* branching fraction in the UV photodissociation of HCl using H atom Rydberg TOF technique and then comparing to the results obtained using the REMPI method. The relative quantum yields of Φ(Cl*) evaluated for the three isomers are listed in Table 4. The values do not change much as the wavelength decreases
42
J. Phys. Chem. A, Vol. 114, No. 1, 2010
Hua et al.
TABLE 2: Anisotropy Parameters at Different Wavelengths Obtained in This Work and by Other Groups cis-C2H2Cl2 wavelength (nm) 235
fast slow fast slow fast slow fast slow
214.5 235a 210a
trans-C2H2Cl2
Cl
Cl*
Cl
Cl*
Cl
Cl*
0.50 ( 0.1 0.33 ( 0.1 0.17 ( 0.1 0.41 ( 0.1
0.20 ( 0.1
1.02 ( 0.1 0.51 ( 0.1 0.93 ( 0.1 0.32 ( 0.1 1.14 ( 0.2 0.33 ( 0.1 1.14 ( 0.2 0.19 ( 0.1 0.65 ( 0.08
0.83 ( 0.1
-0.25 ( 0.1 0.10 ( 0.1 -0.18 ( 0.1 0.14 ( 0.1
-0.16 ( 0.1
0.02 ( 0.1
235-237b a
0.89 ( 0.1 1.01 ( 0.2 0.24 ( 0.1 1.22 ( 0.2 0.21 ( 0.1 0.61 ( 0.06
-0.15 ( 0.01
-0.04 ( 0.1
-0.09 ( 0.01
b
By Suzuki et al. in ref 6. By Mo et al. in ref 4.
TABLE 3: Angle between the Transition Dipole Moment and the Recoil Velocity of Fragment Determined Experimentally and Theoretically wavelength (nm) a
235 214.5a theoryb
cis- C2H2Cl2 Cl
trans- C2H2Cl2
Cl*
45 ( 3° 51 ( 3° 51 ( 3° 54 ( 3° 55°
Cl
1, 1- C2H2Cl2
Cl*
35 ( 3° 37 ( 3° 36 ( 3° 36 ( 3° 30°
Cl
Cl*
60 ( 3° 58 ( 3° 58 ( 3° 54 ( 3° 57°
a
b
1,1-C2H2Cl2
Only the fast translational energy component was considered. Calculations using Gaussian 03 software.
TABLE 4: Relative Quantum Yields of Cl* in Photodissociation of C2H2Cl2 wavelength (nm)
cis-C2H2Cl2
trans-C2H2Cl2
1,1-C2H2Cl2
235 214.5
0.27 ( 0.03 0.27 ( 0.03
0.23 ( 0.03 0.21 ( 0.03
0.30 ( 0.03 0.25 ( 0.03
from 235 to 214.5 nm. Φ(Cl*) is 0.27, 0.23, and 0.30 at 235 nm for cis-, trans-, and 1,1-C2H2Cl2, respectively, as compared to 0.39, 0.25, and 0.29 obtained by Das and co-workers.11 The latter two are consistent. As for cis-C2H2Cl2, they obtained relatively higher Φ(Cl*) at most near-UV wavelengths, but this trend is not found in this work. To thoroughly understand how the spin-orbit ground state and excited state chlorine fragments are produced, information on the related potential energy surfaces must be required. The behavior of predissociation of DCEs (details in next section) is similar to the cases of aryl halides.30-32 For instance, for the predissociation channel of iodobenzene at 266 nm,32 a bound singlet S2(ππ*, B2) state is first excited, followed by coupling to the repulsive states of T2(nσ*, B1), T5(nσ*, B2), and/or S1(nσ*, B1) prior to dissociation. The singlet S2 correlates with the spin-orbit excited state I*(2P1/2), while the repulsive T2(nσ*, B1) and T5(nσ*, B2) states correlate to the ground state I(2P1/2). The curve crossing (for instance, S2 f T5) should make the final state carry a mixed property of both parallel and perpendicular transition. Calculations of simplified one-dimensional potential energy curves along the C-Cl bond dissociation should be useful in understanding the details of the branching ratios of Cl*/Cl and wavelength dependence. E. Photodissociation Pathway. The absorption spectra of the DCE isomers1,11 show a broad distribution peaking at 200-210 nm that corresponds to a π* r π excitation localized on the CdC bond. This transition band denoted as (π, π*) state possesses a substantially large oscillator strength, despite overlapping with some electronic states including singlet and/ or triplet states associated with π f σ* and/or n f σ* transitions. The (n, σ*) band of the C-Cl chromophore is expected to appear below 200 nm as in vinyl chloride.11 According to the calculations of Umemoto et al.,3 the additional
C-Cl chromophore complicates the DCE electronic states, resulting in one (n, σ*) and two (π, σ*) states lying below the (π, π*) state. As shown in Figure 4, the P(ET) distributions of Cl(2PJ) dissociation at either 214.5 or 235 nm are composed of a Gaussian and a Boltzmann component except for Cl*(2P1/2) at 214.5 nm. The higher P(ET) component is expected to follow a curve crossing from the initially excited (π, π*) state to a repulsive (n, σ*) and/or (π, σ*) state.3,4,6 As an electron is ejected to the antibonding orbital σ* of C-Cl, the bond breaks rapidly such that a large amount of available energy may be partitioned into the translation. Since the dissociation lifetime is so short as compared to the rotational time scale, the recoiling fragments are characteristic of anisotropy. Is it probable for a direct dissociation from the repulsive (n, σ*) state at the photolysis wavelengths studied? In most alkyl halides,33,34 the C-X bond usually breaks directly from the repulsive (n, σ*) state upon irradiation of a UV light. Nevertheless, the probability for such an initial excitation to the (n, σ*) state should be negligible in this work. The n f σ* transition accounts for
