Importance of Thomas single-electron transfer in fastp-He collisions

Abstract

We report experimental angular differential cross sections for nonradiative single-electron capture in p-He collisions (p + He → H + He + ) with a separate peak at the 0.47 mrad Thomas scattering angle for energies in the 1.3-12.5 MeV range. We find that the intensity of this peak scales with the projectile velocity as v −11 P . This constitutes the first experimental test of the prediction from 1927 by L. H. Thomas [Proc. R. Soc. 114, 561 (1927)]. At our highest energy, the peak at the Thomas angle contributes with 13.5% to the total integrated nonradiative single-electron capture cross section.

Transfer excitation reactions in fast proton-helium collisions

“…These findings contradict the measurements reported by Fischer et al [22], who found the minimum at θ p,lab = 0.35 mrad for 1.3 MeV and have a narrower distribution in general. To exclude the most obvious possible source of error we have cross-checked that the calibration of our momenta is accurate within 1%, as described in Sec.…”

“…The data presented here (similar to [33]) show a significantly different scattering angle distribution than [22]. A notorious problem in most COLTRIMS measurements which might lead to differences in the results from different works is the calibration of the momentum.…”

“…The broad contribution, peaking at larger scattering angles (1.5 mrad at 630 keV and 1.0 mrad at 1200 keV), may well be a result of the well-known Thomas-process (for details see Kim et al [33] and references therein). This classical double-scattering process ideally predicts a sharp peak at 0.47 mrad and was found to be around this value for very high energies [20][21][22]. At the rather low energies presented here, this peak might be shifted towards larger θ p,lab due to an additional nucleus-nucleus scattering (N-N).…”

“…Especially at higher impact energies E p > 100 keV/u, where the final electronic state determination in experiments is challenging or often impossible, the influence of excitation has not been investigated [20][21][22][23]. Projectile scattering angle distributions, which are final state selective, are rather rare [1,24,[26][27][28][29][30][31][32].…”

Angular scattering in fast ion–atom electron transfer collisions: projectile wave diffraction and Thomas mechanisms

“…The experimental procedure used for the H + + He measurements has been reported in ref. [9] and here we give a description of the pure doubleelectron capture He 2+ + He measurements, focusing on some special precautions and steps that had to be taken in this particular case.…”