Interactions of Excited Lithium Atom with Molecular Hydrogen. II. Properties of Wave Functions in the Potential Crossing Region

To elucidate the electronic mechanism of the quenching of electronically excited Li atoms by H2 molecules, CASSCF calculations were made regarding the Li–H2 system with various geometries. Special attention was payed to the electronic structure of the system in the crossing, or pseudocrossing, region of the potential energy surfaces. Simple CASSCF functions do not have a satisfactory spacial symmetry when the upper and the lower states are almost degenerate in energy, and need to be refined by the addition of a nonorthogonal CI. In the crossing region, electron density maps have shown that the Li atom is bonded more strongly to the nearer H atom than to the farther H atom in the ground state, whereas the opposite is the case for an excited state. Such properties of the wave function lead naturally to a picture of a nonadiabatic process occurring in the crossing region. The pseudocrossing of the first excited and the ground state potential energy surfaces has also been found for all geometries midway between the C2v and C∞v symmetries.