Neutral excited radicals formed by ethylene photodissociation in the 8–24 eV region

Abstract

Dissociation pathways following photolysis of ethylene C₂H₄ in the 6.2–24.8 eV energy range, and leading to excited fragment production are investigated. The dispersed visible fluorescence of the products has been recorded for a large range of excitation energies, allowing for the determination of experimental energy thresholds for the different pathways. It is found that the C₂H(Ã) radical is the dominant fragment, emitting in the visible range. It is formed with a considerable amount of vibrational excitation energy, as was previously reported in the photodissociation study of acetylene C₂H₂. From the dissociation threshold measurements, several barrierless processes are evidenced. In particular, the excited CH*(A or B) radical is formed via an intermediate isomer, ethylidene, with no excess energy. More surprisingly, all channels involving one H₂ molecular elimination are observed without any barrier, unlike the dissociation processes occurring on the ground state potential energy surface (PES) of C₂H₄. Most probably, this observation originates from an easier relaxation of the excited state into pyramidalised states.