Distribution of Rotational States in Half-Collisions: 193-nm Photolysis of Dichloroethylene Isomers

The photodissociation of cis-, trans-, and 1,l -dichloroethylene (DCE) was studied by a pump-and-probe technique, using a 193-nm excimer laser to excite the parent molecule and time-of-flight resonance-enhanced multiphoton ionization to detect the products. We report here the nascent rotational state distributions of HCl(u”=O,1,2) and also the relative yields and intensity dependencies of HCl, H, CK2P3p), C1(2P1/2), HCP, and C1+. Our finding that the rotational state distributions of HCl for all three isomers are very similar leads us to conclude that the nascent distribution is determined far along the reaction coordinate and is insensitive to the isomeric form of the parent molecule, regardless of whether photoelimination results from a threeor four-center transition state. We also observed qualitatively different behavior for HCl(u”=O) and HCl(u”=1,2). As in our previous study of vinyl chloride, we found that HCl(u’30) has a Boltzmann-like rotational state distribution, with temperatures in this case near 1400 K, whereas the distribution for u” = 0 is described by a biexponential function with a low J” temperature around 300 K and a high J” temperature around 10 000 K. We propose that the dichotomy between these two distributions is due to microscopic rather than chemical branching. The relative yields of HCl and C1+ for trans-DCE are approximately double those of cisand 1,l-DCE. This result is consistent with the interpretation that the yield-determining step is faster than cis-trans isomerization. The yields of all the other products are the same for cis and trans. For 1,l-DCE the yields of H, Cl(*P1p), and HCl+ are much greater than for cisand trans-DCE, while for C1(2P3p) the 1 ,1-DCE yield is somewhat smaller. The relative yields of H and C1(2P3p) can be explained by the stabilities of the organic fragments, while the HCl+ yields can be explained statistically. All of the neutral fragments were found to be produced by singlephoton processes, while HCl+ and Cl+ require three and four photons, respectively. Ladder climbing mechanisms are proposed for the two ionic fragments.