State-selective dissociation of acetylene isotopomers

Rovibrational excitation combined with promotion of C2HD/C2H2 molecules to the excited electronic trans-bent states A 1Au/B 1Bu and photofragment ionization are used to generate action spectra and Doppler profiles. Rovibrational states of C2HD and C2H2 are photodissociated by approximately 243.1 nm photons that also probe the H/D fragments. The yield of both H/D photofragments is greatly enhanced upon rovibrational excitation of both isotopomers. In the photolysis of C2HD several couples of rotational transitions, terminating at the same upper rotational state, stand out from the rotational contour and the H/D branching ratio is rotationally state dependent. In C2H2 photolysis the transition intensities of the combination bands that involve high stretch and low bend excitation, (1410020) and (20311)0), are close to that of the fourth overtone of the C-H stretch, (2030000). In addition, in C2H2 some couples of rotational transitions stand out form the rotational contour, and the R(13) line of the (2030000) state shows anomalous intensity. The mechanism for the photoproducts preference in C2HD and for intensity enhancement in C2H2 is discussed.