Dispersion forces and long-range electronic transition dipole moments of alkali-metal dimer excited states.

Abstract

A formalism of degenerate perturbation theory is presented, in which the initial basis set of functions is fully adapted to the perturbation. This full adaptation is obtained by imposing a simple condition which leads to an iteration procedure free of singularities. The formalism includes as a special case nondegenerate perturbation theory. By using it we calculate the dispersion coefficients of alkali-metal dimers for molecular states which dissociate into one atom in the ground state and the other in one of the first two S, P, or D excited states. The dispersion forces are extracted from the first- and second-order energy corrections. Model potentials are used in order to describe the motion of the valence electron in the field of the alkali-metal positive-ion core. Using the first-order wave-function correction, we investigate the leading terms of the long-range expansion of the electronic transition dipole moments. An extension of the Dalgarno-Lewis method is developed in order to handle the radial matrix elements which involves a reduced Green's function for real and complex energies. The results are compared with previous computations.