Study on the energy terms of a system with two metastable helium atoms

The behaviour of sup(1)Σsub(g)sup(+) terms of the system of two He metastable atoms (MA) at small internuclear spacings is investigated. The problem is solved under the assumption that inner electrons of a quasimolecule form a ''rigid'' system with the electron shell of He2++, and the outer ones move in the averaged field of nuclei and inner electrons. The total electron energy is composed of the electron energy of. the He2++ quasimolecule and the energy of outer electrons which is found by a variational method. The accuracy of the given model is quite sufficient for obtaining a unique solution to the problem. Calculations show that for the energy of 2s-electron of He the model applied gives -4.11 eV at an effective charge of Z=1.19. The experimental energy values for the same electron in a singlet or triplet state are -3.97 and -4.76 eV, respectively. The dependence of the sup(1)Σsub(g)sup(+) term of two He MA on internuclear spacing is obtained. Analysis of the terms shows that two He MA do not form a stable molecule because in the minimum region the total energy curves intersect with an entire band of terms. As a result, besides the previously known autoionization channel, at small spacings there opens a channel of inelastic collisions with the release of a large amount of energy (ΔE approximately 20 eV)