Experimental Observation of the Thomas Peak in High-Velocity Electron Capture by Protons from He

Abstract

Experimental angular distributions are reported for electron capture by protons of 2. 82, 5.42, and 7.40 MeV from He. A clear peak mda/dB appears near the Thomas angle of 0.47 mrad for the higher two bombarding energies, supporting the widely held belief that the double-scattering mechanism plays an important role for high-velocity electron capture.PACSnumberss 34.70. + e In 1927 Thomas 1 gave a classical treatment of the capture by a fast point projectile of a bound electron whose orbital velocity (v e ) is much less than the velocity of the projectile (v p ). The process he described involved a double scattering whereby the nearly free electron is first scattered off the projectile at a laboratory angle of 60°, for which it attains a velocity equal to that of the projectile, and then elastically off the target nucleus to redirect this velocity vector in the direction of the projectile. It is now widely understood that any quantum treatment of high-velocity capture must take this process into account. 2 In a perturbation expansion, it corresponds to a second-order Born process 3,4 in which the projectile-electron and target-electron potentials each act once, and indeed the second-order Born term dominates over the first-order Born in the limit of high v p . 5 Several high-velocity theoretical treatments 6 " 15 have now been given which include second-and higher-order terms and which have been quite successful in accounting for experimental total cross sections. By contrast, the first-order Born results are well known to be too large by a factor typically near 3.A differential cross-section measurement provides a much cleaner way to isolate the Thomas scattering mechanism than do the total cross sections. In the classical two-collision treatment, the projectile is scattered to the Thomas angle, 6 = V3~ra/2M, (where m and Mare electron and projectile masses, respectively). In the quantum treatment, the corresponding process is revealed by a peak (or shoulder) in the differential cross section at this angle. 2,3,6 ' 12,16 " 18 This feature becomes increasingly marked as v p is increased. 6,16 The frequently proposed 17 ' 19 experimental detection of such a peak would provide evidence that the fundamental physical process has been correctly identified. Further, as recently discussed by Briggs, Greenland, and Kocbach, 19 the exact shape of the angular distribution inside the Thomas peak is quite sensitive to the relative contributions of first-and higher-Born terms in a perturbation expansion, and thus experimental measurements of differential cross sections in the high-velocity region provide much more sensitive tests of the theoretical treatments than do total-cross-section measurements. This paper reports experimental detection of a peak in the angular distribution for high-velocity electron capture by protons from He. This peak occurs near 0.47 mrad and is interpreted as the first experimental detection of the Thomas peak in electron capture.In the experiment we measured the differential cross section for electr...

Transfer excitation reactions in fast proton-helium collisions

“…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 dependence of energy loss in proton-helium collisions

“…(ii) At larger angles (8) [14]. However, the influence of electron capture to our mean energy-loss data is of minor importance at energies above 100 keV.…”

Charge transfer in proton–helium collisions from low to high energy

“…Most of these works are also motivated by the description of features in the differential cross section [13,68,70] that have been observed in recent experiments on charge transfer and transfer ionization processes [71][72][73]. In particular, the Thomas classical double scattering process [1] leads to a peak in the differential cross section at small scattering angles [15,74,75] that is very sensitive to the theoretical method.…”