Ab initio study of spectroscopic and radiative characteristics of ion-pair states of the Cl2 molecule

Abstract

Electronic structure and radiative characteristics of low-lying ion-pair states of Cl2 converging to the Cl+(3P, 1D)+Cl−(1S) limits are studied. Ab initio calculations of potential energy curves for the valence and ion-pair states and dipole moments for transitions between them are carried out employing the multireference single- and double-excitation configuration interaction (MRD-CI) method, including spin–orbit coupling. It is shown that the lowest two pairs of the Ω=0u+, 1u ion-pair states arise from an avoided crossing between the Σu−3 and Πu3 parent Λ−S states, which leads to notably anharmonic shapes of the corresponding potential curves and their mixed Λ−S nature. This causes significant radial coupling, resulting in the strongly perturbed character of the 0u+ and 1u states observed experimentally. In contrast, their gerade counterparts run parallel to one another and exhibit much less perturbation. Spectroscopic properties of the computed adiabatic curves are in very good agreement with the available experimental data. Dipole moments have been calculated for parallel ion-pair–valence state transitions and radiative lifetimes have been obtained for the adiabatic ion-pair states. A reanalysis of the experimental bound–free emission spectra from the D0u+(3P2) state [N. K. Bibinov et al., Chem. Phys. 254, 89 (2000)] is given.