Contribution of the H(n=3) + e− detachment channel in H−/rare gas collisions

Abstract

We present here first results on the coincident detection of 0° electrons with Balmer α photons produced in H−/rare gas (Ar and Ne) collisions at 5 keV. One of the goals was to observe possible excitation and decay of H− shape resonances lying above the H(n=3) threshold that have been predicted recently. It is still not possible to confirm the presence or not of such resonances, however the obtained electron spectrum has a pronounced cusp shape. A cusp in these collisions can only be produced by dipolar interaction between the detached electron and the H(n=3) atom due to the permanent electric dipole momentum induced by linear Stark effect in the excited hydrogen atom by the field of the slow outgoing electron. In a previous experiment for H(n=2) we have observed the dipolar cusp, but in this case it is strongly affected by the presence of the 1Po shape resonance lying above the H(n=2) threshold by 20 meV. At the H(n=3) threshold the resonant channel seems to have negligible contribution to the electron spectrum, therefore the dipolar cusp appears clearly.