Site and isotopic effects on the angular anisotropy of products in the photodissociation of ethene at 157 nm

We measured angular-anisotropy parameters beta(Et) of fragments from photolysis of ethene and four isotopic variants at 157 nm using photofragment translational spectroscopy and selective photoionization. The averaged beta value of products ranges from -0.17 to 0.10, depending on dissociation pathways. Angular distributions of atomic hydrogen produced from C2H4 and C2D4 are isotropic. For dissociation into C2H2 + H-2, beta has a small negative value whereas dissociation into C2D2 + D-2 has an isotropic angular distribution. The photolysis of dideuterated ethene reveals site and isotopic effects on the angular distributions of products; products H-2, HD, and D-2 from photolysis of 1,1-CH2CD2 have negative, nearly zero, and positive values of beta, respectively. Molecular hydrogen from photolysis of 1,2-cis-CHDCHD has a negative beta value and the anisotropy has a trend D-2 > H-2 > HD. Photolysis of 1,2-trans-CHDCDH produced a result similar to photolysis of 1,2-cis-CHDCHD for the angular anisotropy of molecular hydrogen except slightly more isotropic. A calculation of optimized geometries of ethene in the ground electronic state and pertinent transition structures enables a qualitative interpretation of the site and isotopic effects on the angular anisotropy of products. We deduce that the photoexcited state of ethene at 157 nm has a major character B-1(1u) that produces a transition dipolar moment parallel to the C=C bond.