Production of electronically excited CH via the vacuum ultraviolet photodissociation of ethylene and the possible role of the ethylidene isomer

摘要

The visible fluorescence of CH fragments (A (2)Delta-->X (2)Pi and B (2)Sigma(-)-->X (2)Pi transitions) formed in the vacuum ultraviolet photodissociation of ethylene in the 11.7-21.4 eV energy region, was recorded. Two formation thresholds for each excited fragment, CH* (A) or CH* (B), were identified and associated with two dissociation channels namely CH* CH* + CH3 and CH* + CH + H-2. Unlike previous studies of the dissociation dynamics on the ground-state potential energy surface, neither of these channels exhibit an energy barrier within the experimental uncertainty, even in the latter case of molecular H-2 elimination. It is proposed that both channels pass via an ethylidene intermediate (H3CCH:), an isomer never previously experimentally detected and whose existence has been debated in theoretical publications. The observed behavior, at the excitation energies used in the present work, also suggests that fast isomerization and internal conversion to excited states of ethylene precede fragmentation. Above 18.5 eV, that is around the ionization limit C2H4+ (D B-2(1u)), dissociative ionization starts to compete with neutral dissociation into excited CH fragments giving rise to a substantial decrease in the neutral fragment signal. (C) 2003 American Institute of Physics. References: 36