Direct ionization in the quasimolecule H-He

Doubly differential cross sections for electron ejection in H-He collisions have been measured in the incident atom energy range 2.3–10 keV. Analysis of the experimental data on the basis of the adiabatic theory of direct ionization has been made and parameters of the quasimolecule, namely, the energy of the initial 2pσ state coupled with the continuum, the effective charge, and the real and imaginary parts of the internuclear distance at which the ‘‘superpromoted’’ diabatic term crosses boundary of continuum, have been determined. The values of the initial-state energy and the effective charge agree well with the corresponding parameters of the Li united atom, whereas the dynamical characteristics ReR(E) and ImR(E) reveal considerable influence of electron correlations. The possibility for the development of an alternative method of quantitative spectroscopy of quasimolecules based on the analysis of experimental data on differential cross sections for direct ionization is discussed. © 1996 The American Physical Society.