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

We present here first results on the coincident detection of 0 degrees elec trons with Balmer cc 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 pred icted recently. It is still not possible to confirm the presence or not of such resonances, however the obtained electron spectrum has a pronounced cu sp shape. A cusp in these collisions can only be produced by dipolar intera ction between the detached electron and the Pl(n = 3) atom due to the perma nent electric dipole momentum induced by linear Stark effect in the excited hydrogen atom by the field of the slow outgoing electron. In a previous ex periment for H(n = 2) we have observed the dipolar cusp, but in this case i t is strongly affected by the presence of the P-1(o) shape resonance lying above the H(n = 2) threshold by 20 meV. At the H(n = 3) threshold the reson ant channel seems to have negligible contribution to the electron spectrum, therefore the dipolar cusp appears clearly. (C) 1999 Published by Elsevier Science B.V. All rights reserved.