The multiphoton dissociation of acetylene. I. Long-lived intermediates, sequential and concerted processes of dissociation

Abstract

The multiphoton dissociation processes of acetylene via a two-photon resonant predissociative state, v = 0 of 1Σg+, 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, H2 molecules, and C2 molecules in the X 1Σg+, a 3Πu and A 1Πu states; among the latter, C2 X 1Σg+ molecules are formed by a sequential bond-rupture mechanism whereas some C 2 in a 3Πu, is formed by a concerted two-bond fission process. Other, minor dissociation channels due to three-photon processes, such as C2 (d3Πg) + 2H( 2S1/2), C2 (d3Πg) +H2(X 1Σg+), C2 (C 1Πg) + H2 (X), C2 (e 3Πg) + H2 (X), and C2 (D 1Σu+ + H2 (X), were also detected. In the 2 + 1 concerted dissociation yielding C2 (C 1Πg) + H2 (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) μs. A long-lived intermediate C2H was also found in the 2 + 1 sequential dissociation by TRPFES. © 1991 American Institute of Physics.