Visible photodissociation of the CO2 dimer cation: fast and slow dissociation dynamics in the excited state

Abstract

Velocity and angular distributions of photofragment CO₂⁺ ions produced from mass-selected (CO₂)₂⁺ at 532 nm excitation were observed in an ion imaging experiment. The velocity distribution was assigned to two components, fast and slow velocity components, which was consistent with the previous study by Bowers et al. The anisotropy parameters of the angular distributions for the fast and slow velocity components were experimentally determined to be β_fast = 1.52 ± 0.14 and β_slow = 0.46 ± 0.10, respectively. In the theoretical approach, potential energy surfaces (PESs) of (CO₂)₂⁺ were calculated along two coordinates, the intermolecular distance and mutual orientations of the CO₂ monomers. In addition, molecular dynamics simulations were performed. The visible transition of the most stable staggered structure of (CO₂)₂⁺ was attributed to ²A_g ← ²B_u by an excited state calculation. On the PES of the state, a potential well was found in which the two CO₂ monomers lay side by side to each other, in addition to a repulsive slope along the intermolecular distance. The results of the simulations confirmed that the fragment CO₂⁺ ions with fast velocity and large anisotropy originated from the rapid dissociation of (CO₂)₂⁺ on the repulsive slope. Meanwhile, the fragment CO₂⁺ ions with slow velocity and small anisotropy were expected to emerge from statistical dissociation after large amplitude libration of CO₂ molecules which was caused by the potential well in the excited state PES.