Non-adiabatic dissociation of rovibrationally excited acetylene

The photofragmentation dynamics of acetylene, C ₂H ₂, 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 UV ₁ photons promoted the pre-excited C ₂H ₂ molecules to the à ¹A _u state and dissociated them and finally the ensuing H atoms were probed by UV ₂ photons via (2 + 1) resonantly enhanced multiphoton ionization. UV dependent action spectra, monitoring the H photofragment yield vs. the UV ₁ 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 (1112 ⁰0 ⁰) combination band, containing trans-bend mode excitation, are excessively enhanced over those of the (0030 ⁰0 ⁰) 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 C≡C 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 C ₂H (X̃ ²Σ ⁺) + H.