PHOTODISSOCIATION DYNAMICS OF C2H2, C2D2, AND C2HD AT 121.6 NM

The product translational energy distribution P(E-t) and the recoil an isotropy parameter beta(E-t) for acetylene (and isotopic variants) pho todissociation at 121.6 nm have been obtained by detecting the H(D) at om fragment using the Doppler-selected time-of-flight (TOF) technique. A strong propensity toward the formation of the C2H((A) over tilde (2 ) Pi) fragment was revealed. The resolution of this technique is suffi ciently high to resolve the vibrational structures of a small polyatom ic molecule. The vibrational spectroscopic constants bf the C-H and C- C stretching modes for the C2H((A) over tilde (2) Pi) state were obtai ned for the first time, and the mode-specific vibrational state distri butions 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.