UV photodissociation of oxygen dimer cation O4+ studied by an ion imaging technique and theoretical calculations

Abstract

The photodissociation reaction of dimer cation of oxygen, O4+, in the ultraviolet region was investigated by velocity-map ion imaging experiments and theoretical calculations. Photofragment O2+ ion was obtained as an image, and its shape had a distribution divided along the polarization direction of the dissociation laser. From the angular distribution, we calculated the anisotropy parameter β, and its value (β = 1.6) is somewhat less than the axial recoil limit. The radial distribution showed that only a small fraction of the available energy after photoexcitation was partitioned to kinetic energy, and the fragments were highly rotationally excited. We also conducted complete active-space self-consistent field calculations and identified the precursor O4+ as a doublet rectangular isomer. The calculations also revealed a repulsive potential energy surface (PES) of the excited state, indicating that the dissociation lifetime is relatively fast. To explain the fragment’s rotational excitation during the rapid dissociation process, we considered two of its rotational degrees of freedom: in-plane corotation and out-of-plane counter-rotation. Topology of the excited state potential energy surface shows that wave packet motion on the excited state PES induced the excitation of librational motion of fragments, resulting in the in-plane corotation and out-of-plane counter-rotation.