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(¹A_u)/B(¹B_u) 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.