Combination bands versus overtone stretch excitation and rotational effects in vibrationally mediated photodissociation of acetylene

• Published in: The Journal of chemical physics 1998-11, Vol.109 (20), p.8959-8967
• Main Authors: Schmid, R. P., Ganot, Y., Bar, I., Rosenwaks, S.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.477567

Rovibrational excitation combined with promotion of C2H2 molecules to the excited electronic trans-bent states Ã 1Au/B̃ 1Bu and photofragment ionization are used to generate action spectra, H Doppler profiles, and time-of-flight mass spectra. Rovibrational states of C2H2 in the 15 480–15 723 cm−1 region are photodissociated by 243.135 nm photons that subsequently tag the H fragments. The H photofragment yield is greatly enhanced upon rovibrational excitation. In the action spectra, the 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), while in the absorption spectra the intensities of the combination bands are much weaker. In addition, the effect of rotation on fragment yield and dissociation channels is demonstrated. Several pairs of rotational transitions (sharing similar J′) stand out from the rotational band contour and the R(13) line of (2030000) shows an anomalously high intensity. The origin of the intensity enhancement for the combination bands is the better Franck–Condon (FC) overlap of states containing bend excitation with the dissociative wave functions. The source of the high intensity for the pairs of rotational transitions is the interaction with zero-order dark states that also give rise to a better FC factor, whereas in R(13) excitation an additional bent state is prepared, followed by two photon ultraviolet absorption, leading to a different photodissociation pathway.