Photodissociation dynamics of C₂H₂, C₂D₂, and C₂HD at 121.6 nm

The product translational energy distribution P(E_t) and the recoil anisotropy parameter β(E_t) for acetylene (and isotopic variants) photodissociation at 121.6 nm have been obtained by detecting the H(D) atom fragment using the Doppler-selected time-of-flight (TOF) technique. A strong propensity toward the formation of the C₂H(Ã ²Π) fragment was revealed. The resolution of this technique is sufficiently high to resolve the vibrational structures of a small polyatomic molecule. The vibrational spectroscopic constants of the C-H and C-C stretching modes for the C₂H(Ã ²Π) state were obtained for the first time, and the mode-specific vibrational state distributions were deduced. It is conjectured that two distinct dissociation pathways, arising from the Rydberg-valence surface intersections, are involved in the title processes. The branching of these two pathways is governed by the initial absorption step.