Electric dipole moments of H( n =2) induced in H sup + -He and H-He collisions

Electric dipole moments of H({ital n}=2) induced in H{sup +-}He and H-He collisions are investigated at incident energies of 5--25 keV. For these collision systems, an energy-dependent forward-backward asymmetry (dipole moment) of the emitted Lyman-{alpha} radiation as a function of an external electric field is observed. In H{sup +-}He collisions, the measured dipole moment is positive, corresponding to an electron trailing behind the proton. The same analysis applied to the H-He system shows the electron riding in front of the proton. The observed differences between H and H{sup +} impact are partly explained with the help of a simple model which assumes different cross sections for excitation into the different Stark states.