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The energetics of the unimolecular reactions €2^^ > CgHg'*' + CH3 and C^H-y'*' + H have been reinvestigated by the molecular beam photoionization method. At nozzle expansion conditions where the concentrations of (CgH^)^ and heavier clusters produced in the beam are high, the appearance energies (AE) for CgHg'"' and C^H^* were found to shift to energies lower than the expected thermochemical thresholds for the formation of C3H5"'" and C^H-y"*" from ethylene dimers. This indicates that ethylene trimers and heavier clusters can give rise to the same product ions as ethylene dimers. The AE's for CgH^'"' and C^H-y""" from (C2H4)2 measured in an ethylene beam, v^ich mainly consists of C2H4 and (C2H4)2, are 10.21+0.04 eV (1214+5 Â) and 10.05±0.04 eV (1234+5 Â), respectively. Taking into account the error estimates of the thermochemical thresholds for the formation of C^Hc;"'" and C^H-y"*" from (C2H4)2f the latter values are in accordance with the conclusion of previous studies that the barriers for the reverse reactions of the ionmolecule reactions + C2H4 are negligible. From the observed ionization energies of (€28^)2 (9.84+0.04 eV) and C^H^ (10.505+0.004 eV) and the estimated binding energy of (C-^^)2 (0.02 eV), the bond dissociation energy for C2H4 is calculated to be 15.8+1 kcal/mol.

Vacuum ultraviolet photochemistry: I. Unimolecular decomposition studies of small hydrocarbon clusters by molecular beam photoionization mass spectrometry; II. Photofragment dynamics study of carbon disulfide by high resolution molecular beam laser photofragment time-of-flight mass spectrometry