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 C 2 H ( Ã 2 Π) 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 C 2 H ( Ã 2 Π) 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.