PHOTODISSOCIATION OF ETHYLENE AT 193 NM

The photodissociation of ethylene at 193 nm was studied by measuring the product translational energy distributions for the H+C2H3 and H-2+C2H2 channels. In agreement with previous workers, it was determined that atomic and molecular elimination occur in relatively equal amounts. Using 1,1 D2CCH2 and 1,2 cis HDCCDH, it was shown that both acetylene and vinylidene are formed and that the acetylene/vinylidene ratio is approximately 2/3 in the molecular elimination. This H-2 elimination channel has a translational energy distribution peaked at around 20 kcal/mol, indicating that it is a concerted process with a substantial exit barrier. It was found that the H atom elimination channel is best described as a simple bond rupture occurring after internal conversion of the electronically excited molecule to the vibrationally excited ground state ethylene. Some of the primary C2H3 product has sufficient internal energy to spontaneously decompose to H+HC=CH. At higher laser intensity a large fraction of the C2H3, however, absorbs another photon and fragments to H+H2C=C: (1A1 and 3B2.