State-selective dissociation of acetylene isotopomers

Rovibrational excitation combined with promotion of C₂HD/C₂H₂ molecules to the excited electronic trans-bent states ùA_u/B̃¹B_u and photofragment ionization are used to generate action spectra and Doppler profiles. Rovibrational states of C₂HD (in the 15637 - 15735 cm^-1 region) and C₂H₂ (in the 15480 - 15723 cm^-1 region) are photodissociated by ∼ 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 (5v₁) (five quanta of C-H stretch) several couples of rotational transitions, terminating at the same upper rotational state (J'), 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 (2031¹0⁰), 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 from 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.