Non-adiabatic dissociation of rovibrationally excited acetylene

Abstract

The photofragmentation dynamics of acetylene, C2H2, was explored via vibrationally mediated photodissociation. Direct near infrared (NIR) excitation efficiently prepared rovibrational states in the region of three C–H stretch quanta (∼9640 cm−1), subsequently ∼243.1 nm UV1 photons promoted the pre-excited C2H2 molecules to the A 1Au state and dissociated them and finally the ensuing H atoms were probed by UV2 photons via (2+1) resonantly enhanced multiphoton ionization. UV dependent action spectra, monitoring the H photofragment yield vs. the UV1 dissociating laser wavelength displayed sharp peaks depending on the combined energy and the initially excited rovibrational state. These spectra indicate that the rovibrational transitions belonging to the (1112000) combination band, containing trans-bend mode excitation, are excessively enhanced over those of the (0030000) state with three C–H stretch quanta, due to favorable Franck–Condon (FC) factor. The UV absorption from these states sampled particular rovibronic levels of the potential well on the upper A 1Au state, disclosing transitions to FC active vibronic modes, involving couplings between torsion and cis-bend with CC stretch and trans-bend modes. These results suggest that the initial state preparation samples the bound rather than the purely repulsive region of the A 1Au state and that the rovibronic structure is the essential factor in affecting the absorption cross-section. They also indicate that the provided energy is insufficient to overcome the barrier on the A 1Au state, leading to the dominant non-adiabatic predissociative photofragmentation into C2H ( 2Σ+)+H.