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

Dissociation pathways following phololysis 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 (A) 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 H-2 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 all easier relaxation of the excited state into pyramidalised states.