Photodissociation mechanisms of the CO_2 ~(2+) dication studied using multi-state multiconfiguration second-order perturbation theory

摘要

Employing the multi-state multiconfiguration second-order perturbation theory (MS-CASPT2) and complete active space self-consistent field (CASSCF) methods, the geometries, relative energies (T_v') to the ground state (X~3Σ_g ~-), adiabatic excited energies, and photodissociation mechanisms and corresponding kinetic energy releases for the lower-lying 14 electronic states of the CO_2 ~(2+) ion are studied. The T_v' values are calculated at the experimental geometry of the ground state CO_2 molecule using MS-CASPT2 method and highly close to the latest threshold photoelectrons coincidence and time-of-flight photoelectron photoelectron coincidence spectrum observations. The O-loss dissociation potential energy curves (PECs) for these 14 states are drawn using MS-CASPT2 partial optimization method at C _(∞v) symmetry with one C-O bond length ranging from 1.05 to 8.0 ?. Those 14 states are confirmed to be correlated to the lowest four dissociation limits CO~+(X~2Σ~+) + O ~+(~4S_u), CO~+(A~2Π) ~+ O+(~4Su), CO~+(X~2Σ ~+) ~+ O~+(~2Du), and CO +(X~2Σ~+) + O~+(~2P_u) by analyzing Coulomb interaction energies, charges, spin densities, and bond lengths for the geometries at the C-O bond length of 8.0 ?. On the basis of these 14 MS-CASPT2 PECs, several state/state pairs are selected to optimize the minimum energy crossing points (MECPs) at the CASSCF level. And then the CASSCF spin-orbit couplings and CASPT2 state/state energies are calculated at these located MECPs. Based on all of the computational results, the photodissociation mechanisms of CO_2 ~(2+) are proposed. The relationships between the present theoretical studies and the previous experiments are discussed.