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

The multiphoton dissociation processes of acetylene via a two-photon resonant predissociative state, v = 0 of ¹Σ_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, H₂ molecules, and C₂ molecules in the X ¹Σ_g⁺, a ³Π_u and A ¹Π^u states; among the latter, C₂ X ¹Σ_g⁺ molecules are formed by a sequential bond-rupture mechanism whereas some C ₂ in a ³Π_u, is formed by a concerted two-bond fission process. Other, minor dissociation channels due to three-photon processes, such as C₂ (d³Π_g) + 2H( ²S_1/2), C₂ (d³Π_g) +H₂(X ¹Σ_g⁺), C₂ (C ¹Π_g) + H₂ (X), C₂ (e ³Π_g) + H₂ (X), and C₂ (D ¹Σ_u⁺ + H₂ (X), were also detected. In the 2 + 1 concerted dissociation yielding C₂ (C ¹Π_g) + 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.