Action spectra vs rovibrational absorption spectra: a tool for photodissociation dynamics investigation

Rovibrational excitation enables molecules to reach regions of the ground p otential energy surface that are unavailable to vibrationless ground state species. Consequently, the photodissociation outcome may be affected via th e Franck-Condon factor, which effectively influences the electronic excitat ion, depending on the initially prepared rovibrational state. Here we focus on the rovibrationally mediated photodissociation of C2HD/C2H2 and compare action spectra to rovibrational absorption spectra to study the photodisso ciation dynamics. Rovibrationally excited C2HD (in the 15637-15735 and 1525 6-15337 cm(-1) regions) and C2H2 tin the 15480-15723 cm(-1) region) are pro moted to the excited electronic trans-bent states (A) over tilde(1)A(u/)(B) over tilde(1)B(u) and photodissociated by similar to 243.1 nm photons that also interrogate the H/D fragments. The yield of both H and D photofragmen ts is greatly enhanced upon rovibrational excitation of both isotopomers sh owing both vibrational and rotational state dependencies. The mechanism for photoproduct propensity in C2HD and for intensity enhancement and photodis sociation pathways in C2H2 is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.