Vibrationaly mediated photodissociation of C₂H₄ ⁺

We report the vibrationally mediated photodissociation dynamics of C ₂H₄⁺ excited through the B̃²Ag state. Vibrational state-selected ions were prepared by two-photon resonant, three-photon ionization of ethylene via (π, 3s) and (π, 3p) Rydberg intermediate states in the wavelength range 298-349 nm. Absorption of a fourth photon led to dissociation of the cation, and images of the product ions C ₂H₃⁺ and C₂H₂⁺ were simultaneously recorded using reflectron multimass velocity map imaging. Analysis of the multimass images yielded, with high precision, both the total translational energy distributions for the two dissociation channels and the branching between them as a function of excitation energy. The dissociation of ions that were initially prepared with torsional excitation exceeding the barrier to planarity in the cation ground state consistently gave enhanced branching to the H elimination channel. The results are discussed in terms of the influence of the initial state preparation on the competition between the internal conversion to the ground state and to the first excited state.