Dynamics of photodissociation of ethylene and its isotopomers at 157 nm: Branching ratios and kinetic-energy distributions.

We investigated the photodissociation of ethylene and its isotopomers at 157 nm in a molecular-beam apparatus using photofragment translational spectroscopy combined with synchrotron-based photoionization. The time-of-flight TOF spectra of all photofragments H, H2, C2H2, C2H3, and their deuterium isotopic variants were recorded, from which kinetic-energy distributions P(Et) and branching ratios were obtained. Most C2H3 spontaneously dissociates to C2H2+H and only C2H3 with small internal energy survives. The C2H2 fragment due to H2 elimination is observed leading the C2H2 fragment due to 2H elimination in TOF distribution because the former process has more kinetic-energy release. An analogous result is observed for C2D4 photolysis. That elimination of molecular hydrogen is site-specific and is revealed from photolysis of three dideuterated ethylene isotopomers, in which an isotopic effect plays a significant role. Observations of C2D2+2H and C2H2+2D product channels in the photolysis of 1,1-CH2CD2 provide evidence for migrations of H and D atoms. A comparison with previous experimental and theoretical results is made. [ABSTRACT FROM AUTHOR]