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((1)A(u))/B(B-1(u)) and f ragment 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 (1410(0)2(0)) and (2 031(1)0(0)) are 0.5 +/- 0.1 and 0.7 +/- 0.1, respectively, times that of the fourth overtone of the C-H stretch (2030(0)0(0)). These intensi ties are more than an order of magnitude higher than in the absorption spectrum and result from better Franck-Condon overlap between combina tion bands and the upper bent-electronic states. (C) 1998 Elsevier Sci ence B.V. All rights reserved.