No abstract

Photolysis of acetylene has been performed by vacuum-ultraviolet excitation with the synchrotron radiation via the Rydberg states converging to the first ionization potential ͑IP͒ at 11.4 eV. Only the visible fluorescence of the ethynyl radical was observed in the à 2 ⌸-X 2 ⌺ ϩ system. Excitation of several Rydberg states of acetylene over a large energy range between 9 and 11.4 eV allowed us to observe for the first time the evolution of this continuum with increasing Rydberg excitation. Intensity calculations based on accurate ab initio potential energy surfaces of C 2 H were performed by using a one-dimensional model accounting for the large-amplitude motion of the H atom around the CC bond and for the overall rotation of the radical. These calculations successfully reproduce the observed visible continuum ͑maximum at 500 nm and blue side cutoff at 400 nm͒ and bring new information on the distribution of the internal energy deposited in the fragment. For most excited Rydberg states, predissociation occurs in a rather low time scale, leaving the C 2 H fragment in the à state, vibrationally hot, mostly with significant excitation in the bending mode around the isomerization barrier.