Experimental Test of Higher-Order Electron-Capture Processes in Collisions of Fast Protons with Atomic Hydrogen

Abstract

We present measurements of the angular distribution of fast hydrogen atoms formed by electron capture of 2.8-and 5.0-MeV protons in atomic hydrogen. In the angular region of the Thomas peak (0.47 mrad) the experimental results obtained with this pure three-body collision system are in reasonable agreement with a strong-potential Born calculation and the impulse approximation, but not with other higher-order theories. PACS numbers: 34.70.+eAt present there is great interest in electron capture in ion-atom collisions at high projectile velocity. High velocity here means that the projectile velocity is much higher than the Bohr electron orbital velocities in the initial and final states. Because of the rapid transfer of the electron from the target bound state to the fast-moving projectile bound state, a large change of electron momentum and energy is required which must be transferred to a third particle, which can be, in the case of nonradiative capture, the target nucleus. This process is theoretically attractive, 1,2 since higher-order terms of the electronnucleus interaction become important in order to mediate this energy and momentum transfer. Formally, higherorder terms can be described by perturbative series expansions like the Born series. In the last few years different approaches to these series expansions have been developed, e.g., second-order Born (B2), 3 " 5 strongpotential Born (SPB), 6 " 9 impulse (IA), 10 " 12 continuum distorted-wave (CDW), 13 ' 14 and eikonal approximations (EA). 15 The higher-order terms in the capture amplitude can be interpreted as multiple scattering of the electron at the target and projectile potentials. In this view the secondorder term is a double scattering for the capture of the electron, where the electron is first scattered at the projectile and then at the target nuclear potential as described first classically by Thomas. 16 This leads to a peak at a forward angle 9*°* (m/M p )sin60° (m is the electron mass; M p is the projectile mass) in the angulardifferential capture cross section. The shape and absolute magnitude of the differential cross section may be determined not only by first-and second-order, but also by higher-order terms in the series expansions mentioned above. 12 However, in such treatments and at high collision velocities the description of the asymptotic states even to first order is a difficult task. 17 The ultimate experimental test of these theories can only be performed with a pure three-body collision system. We have therefore studied the three-body collision system /?-H by measuring the angular-differential electron-capture cross sections at 2.8 and 5.0 MeV.Among the different experimental approaches to test higher-order contributions to the electron-capture amplitude were studies of the electron cusp from capture into projectile continuum states. In these experiments, some signature of higher-order terms could already be identified. 18 ' 19 In total cross sections of electron capture the higher-order terms might dominate the first-order term ...

Double charge transfer cross sections in collision of protons with helium atom

“…The post-prior discrepancy in the BCCIS-4B method is well within 15-20% in the entire range of energy as the collision is nearly symmetric. These results show the steeply decreasing dependence on the scattering angle but neither oscillating structures characteristic of interference effects nor peaks reminiscent of the Thomas peak found for single capture at large energies [42,43] have been observed. cm sr W ± / / as a function of the scattering angle q º q lab (mrad) at incident energy E lab = 100 keV for the double charge exchange reaction H + + He(1s 2 ) ® H -(1s 2 ) + He 2+ .…”

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

“…Here M P is the projectile mass and the corresponding cross section is expected to have a nonrelativistic asymptotic dependence on the projectile velocity, v P , of v −11 P [1]. The first experimental observation of a structure at the Thomas angle was made by Horsdal-Pedersen et al [16] for collisions with helium and by Vogt et al [17] * schmidt@fysik.su.se for an atomic hydrogen target by direct measurements of the projectile deflection angles. However, until the present study no experimental investigations of the velocity dependence of the related cross section have been reported.…”

Dynamics of ionization processes studied with the COLTRIMS method—new insight into e–e correlation

“…The electron transfer to the projectile can proceed via different reaction channels: (a) electron-electronThomas TI (eeTTI) Palinkas et al, 1989;Briggs and Taulbjerg, 1979;Ishihara and McGuire, 1988;McGuire et al, 1989McGuire et al, , 1995Shakeshaft and Spruch, 1979); (b) nucleus-electron-Thomas TI (nTTI) (Thomas, 1927, Horsdal-Pedersen et al, 1983Vogt et al, 1986); and (c) kinematical capture (KTI) (Brinkman-Cramer type). While eeTTI always leads to a transfer ionization where the second electron is ejected, the KTI and nTTI processes are accompanied by ionization of the second electron either by shake-off via e-e correlation or by an independent binary collision of the proton with the second electron.…”