Ab initio study of spectroscopic and radiative characteristics of ion-pairstates of the Cl-2 molecule

Electronic structure and radiative characteristics of low-lying ion-pair st ates of Cl-2 converging to the Cl+(P-3, D-1)+Cl-(S-1) limits are studied. A b initio calculations of potential energy curves for the valence and ion-pa ir states and dipole moments for transitions between them are carried out e mploying the multireference single- and double-excitation configuration int eraction (MRD-CI) method, including spin-orbit coupling. It is shown that t he lowest two pairs of the Omega =0(u)(+), 1(u) ion-pair states arise from an avoided crossing between the (3)Sigma (-)(u) and (3)Pi (u) parent Lambda -S states, which leads to notably anharmonic shapes of the corresponding p otential curves and their mixed Lambda -S nature. This causes significant r adial coupling, resulting in the strongly perturbed character of the 0(u)() and 1(u) states observed experimentally. In contrast, their gerade counte rparts run parallel to one another and exhibit much less perturbation. Spec troscopic properties of the computed adiabatic curves are in very good agre ement with the available experimental data. Dipole moments have been calcul ated for parallel ion-pair-valence state transitions and radiative lifetime s have been obtained for the adiabatic ion-pair states. A reanalysis of the experimental bound-free emission spectra from the D0(u)(+)(P-3(2)) state [ N. K. Bibinov , Chem. Phys. 254, 89 (2000)] is given. (C) 2001 American Ins titute of Physics.