Coulomb and Coulomb-Stark Green-function approach to adiabatic Rydberg energy levels of alkali-metal–helium systems

The adiabatic Σ electronic terms of highly excited states of the He–alkali-metal atom systems are determined as the roots of a transcendental equation whose only required inputs are the alkali-metal quantum defects, the $e^{\mathrm{-}}$-He s-wave phase shifts and, of course, the internuclear distance R. The results obtained for relatively low excited states are in reasonable agreement with more elaborate calculations. It is found that the notion of adiabatic quantum defects requires R(a.u.)≲2n, n being the principal quantum number. The method is generalized to the case where a small external electric field scrE is present. Then the energy levels strongly depend on the angle between scrE and the internuclear axis. Numerical applications to He-H are presented.