A joint theoretical and experimental study of the excitation of the autoionizing (2s 2 ) 1 S, (2p 2 ) 1 D and (2s2p) 1 P states of helium by 100 keV proton impact is presented for the first time. The role of the three-body Coulomb interaction in the final state between the ejected electron, the scattered proton and the recoil helium ion is emphasized. Calculations have been carried out with inclusion of the three-body Coulomb interaction and within an expansion of a two-electron excitation amplitude in powers of projectile-target interaction up to the second order. A new parametrization is proposed to describe resonance profiles distorted by the Coulomb interaction in the final state (CIFS). New high-resolution (up to 68 meV) measurements of electron emission spectra made it possible to resolve the near-lying (2p 2 ) 1 D and (2s2p) 1 P resonances and reveal an evident distortion of the resonance profiles by CIFS for forward electron ejection angles below 40 • . Processing of the experimental spectra has been done both with the new parametrization, with allowance for CIFS, and with the Shore formula. Considering the complexity of the problem, reasonable agreement is achieved between experiment and theory.

We have measured and calculated doubly differential cross sections for ionization of atomic hydrogen using 75-keV proton impact for fixed projectile energy losses as a function of scattering angle. This collision system represents a pure three-body system and thus offers an accurate test of the theoretical description of the few-body dynamics without any complications presented by electron correlation in many-electron targets. Comparison between experiment and several theoretical models reveals that the projectile-target nucleus interaction is best described by the operator of a second-order term of the transition amplitude. Higher-order contributions in the projectile-electron interaction, on the other hand, are more appropriately accounted for in the final-state wave function.

Excitation cross sections of the ͑2s 2 ͒ 1 S, ͑2p 2 ͒ 1 D, and ͑2s2p͒ 1 P autoionizing states of helium, produced in collisions with 100-keV protons, have been measured for the first time. Using a high resolution electron spectroscopy together with a recently proposed parametrization of autoionizing resonances distorted by Coulomb interaction in the final state makes it possible to extract from electron spectra total cross sections as well as magnetic sublevel populations. These new experimental data are briefly compared with out theoretical calculations. [S0031-9007 (97)04892-8] PACS numbers: 34.50.FaTwo-electron excitation of the helium atom by proton impact is currently a benchmark test for the theories of multielectron transitions which try to understand the excitation mechanisms involved in heavy particle collisions [1]. Doubly excited states of helium lie above the singleionization threshold and decay primarily by autoionization. Since the excitation and subsequent nonradiative decay of autoionizing states is coherent with direct ionization, the resonant and direct ionization amplitudes should be added together, rather than cross sections of both processes [2]. This results in at least two effects. First, the resonances in ionization cross sections are asymmetric. They are commonly described by Fano's formula [2]. Second, the intensity of an autoionizing resonance is not proportional to the population of the resonant state [3]. The total resonant yield can provide only a lower estimate for the excitation cross section at asymptotic collision velocities [4]. This means that information on two-electron excitation is hidden in the resonance profiles. Therefore, comparing the calculation results for two-electron excitation with available experimental data (electron spectra or resonance parameters) requires not only an adequate theoretical description of the double excitation itself, but also a realistic description of the direct ionization, including the interference of resonant and direct ionization. Moreover, Coulomb interaction in the final state (CIFS) between the scattered charged particle, the ejected electron, and the recoil ion considerably influences the resonance profiles [5][6][7][8]. Under the conditions of strong CIFS the shapes of the resonant lines can be very different from the familiar Fano's one [5,7] complicating the comparison of experimental and theoretical results, and hence the description of three-body Coulomb interaction of charged particles must be as accurate as the description of twoelectron excitation [8]. The theoretical excitation cross sections reported in this paper are part of our complete calculation [8] where all these problems are solved. From another point of view, if we are able to extract excitation cross sections from experimental data instead of resonance yields a comparison with less complex calculations becomes also possible. Indeed, several approximate theoretical models [9][10][11][12][13] have been developed in the past to discuss the excitation mechanisms; they ca...

The transfer ionization process offers a unique opportunity to study radial and angular electron correlations in the helium atom. We report calculations for the multiple differential cross sections of the transfer ionization process p + He → H + He ++ + e −. The results of these calculations demonstrate the strong sensitivity of the fully differential cross sections to fine details of electron correlation in the target atom. Specifically, angular electron correlation in the ground state of helium results in a broad peak in the electron emission spectra in the backward direction, relative to the incoming beam. Our model explains some of the key effects observed in measurements of multiple differential cross sections using the COLTRIMS technique.

No abstract