THE MULTIPHOTON DISSOCIATION OF ACETYLENE .1. LONG-LIVED INTERMEDIATES, SEQUENTIAL AND CONCERTED PROCESSES OF DISSOCIATION

The multiphoton dissociation processes of acetylene via a two-photon resonant predissociative state, upsilon = 0 of 1-SIGMA-g+, have been studied by three techniques: time-resolved photofragment excitation spectroscopy (TRPFES), laser-induced fluorescence (LIF) of the C2 fragments, and dispersed emission. We found that the major dissociation products are H atoms, H-2 molecules, and C2 molecules in the X1-SIGMA-g+, a3-PI-u and A 1-PI-u among the latter, C2 X1-SIGMA-g+ molecules are formed by a sequential bond-rupture mechanism whereas some C2 in a3-PI-u is formed by a concerted two-bond fission process. Other, minor dissociation channels due to three-photon processes, such as C2 (d3-PI-g) + 2H(2S1/2), C2(d3-PI-g) + H-2(X1-SIGMA-g+), C2 (C1-PI-g) + H-2(X), C2 (e3-PI-g) + H-2(X), and C2 (D1-SIGMA-u+) + H-2(X), were also detected. In the 2 + 1 concerted dissociation yielding C2 (C1-PI-g) + H-2(X), a long-lived intermediate C2H2, likely a cis isomer or other conformer in which the hydrogen atoms are relatively close to each other, was revealed by TRPFES; its zero-pressure lifetime was estimated to be (8 +/- 1)mu-s. A long-lived intermediate C2H was also found in the 2 + 1 sequential dissociation by TRPFES.