Enhanced action spectra of combination bands of acetylene via vibrationally mediated photodissociation and fragment ionization

Vibrational excitation combined with promotion of acetylene molecules to the excited electronic trans-bent states A(1Au)/B(1Bu) and fragment ionization are used to generate action spectra. Rovibrational states of acetylene in the 15480–15723 cm−1 region are accessed with high sensitivity and photodissociated by 243.135 nm photons that also tag the H fragments. Transition intensities of the combination bands that involve high stretch and low bend excitation (1410020) and (2031100) are 0.5±0.1 and 0.7±0.1, respectively, times that of the fourth overtone of the C–H stretch (2030000). These intensities are more than an order of magnitude higher than in the absorption spectrum and result from better Franck–Condon overlap between combination bands and the upper bent-electronic states.