Collisional Excitation and Decay of theP1Shape Resonance ofH−

Abstract

The ^P shape resonance of H~ has been populated in collisions between H~ and Xe. We report on the decay H-C^P)-*H^(« = 2)4-^", which for the first time is measured by high-resolution electron spectroscopy. New information on the angular distribution of ejected electrons is obtained by use of a Shore parametrization to describe the cross section. The Shore parameters are related to matrix elements of the transition operator. The obtained resonance energy and width are in good agreement with recent theoretical calculations as well as other experimental results.

Coincidence study of electron transfer to the continuum process in collisions at intermediate energies

“…Measurements of non-coincident spectra with high energy and angular resolution failed to show the presence of any additional resonance structures in the vicinity of the cusp top. In contrast, such structure was reported in studies for the H(n = 2) + e − system in H − detachment collisions (Andersen et al 1986, Duncan and Menendez 1989, Pennent et al 1991. This fact may led us to the conclusion that for the present case of H 0 projectiles no low-energy shape resonance states are populated during the collision.…”

“…In particular, many of the possible final-state channels converging to H(n 2) have attractive dipole fields strong enough at large distances to produce a cuspshaped peak with a 1/v behaviour (Liu and Starace 1989). Other channels can lead to the appearance of low-lying shape resonances, such as the (2s2p) 1 P o resonance observed around the cusp in H − detachment collisions (Andersen et al 1986, Duncan and Menendez 1989, Pennent et al 1991. These experiments provided evidence that a cusp is associated with an outgoing neutral H projectile.…”

Structure and dynamics of negative ions

“…The parameters of the 1 P resonance were extracted from the best fit to the measured electron energy spectrum. The energy and width of the resonance were found to be 18.5(1.0) meV and 23(2) meV, respectively, in agreement with theory and those of Andersen et al [212]. It seems, however, that the major objective of the experiment was to investigate the origin of the cusp peak since it implied that there existed a finite non-resonant cross section at threshold.…”

“…Several groups have focused on the 1 P shape resonance in the H − ion, which is situated ∼17 meV above the H(n = 2) parent state. Andersen et al [212] used a H − ion beam energy of 100 keV, which was sufficient to produce a kinematic shift in the energy of the electrons emitted in the decay from 17 meV to over 50 eV. The H − ion beam was collimated and passed through a gas cell that was 4 mm long.…”