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 à ¹A_u 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 à ¹A_u 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 à ¹A_u 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 à ¹A_u state, leading to the dominant non-adiabatic predissociative photofragmentation into C₂H ( ²Σ⁺) + H.