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...

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Probabilities for adiabatic or near-adiabatic state transformation within a highly excited shell of Li(n = 25) were studied experimentally and theoretically for a time dependent electric field, E⃗(t), and a constant magnetic field, B⃗. The fields were sufficiently weak and the time dependence slow enough such that only states belonging to the chosen shell were involved. The studies show that the dynamics are governed by the approximate hydrogenic character of the system in most cases, but for some specific time dependences it is influenced strongly by core interactions as expressed through the quantum defects, δl. The s-state is effectively decoupled from the rest of the n = 25 manifold due to a very large quantum defect. However the quantum defects of the p, d and f states are shown to play a decisive role in the dynamics. The core interactions lead to avoided crossings, non-adiabatic state transformations, and possibly even phase-interference effects. When a resonance condition pertaining to the hydrogenic character of the system is fulfilled, a linear Stark state is transformed completely into a circular Stark state oriented along E⃗f.

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Electron capture cross sections for coherent elliptic Rydberg states have been measured as a function of eccentricity. The ion impact velocity was parallel to the minor axis of the associated, classical elliptic orbit and both clockwise and anticlockwise orbital motion of the electron were studied. The incoming ions were 2.5 keV 23Na+ and the Rydberg target, with principal quantum number n=25, was formed by laser excitation of Li atoms with subsequent adiabatic switching of crossed, external electric and magnetic fields. A strong dependence on eccentricity and sense of orbital motion was found.