The Thomas' double scattering mechanism: differential cross sections in H++He collisions at high energies

P. N. ABUFAGER, A. E. MARTÍNEZ, R. D. RIVAROLA AND P. D. FAINSTEIN

Stockholm

Conferencia; 23rd International Conference on Photonic, Electronic and Atomic Collisions; 2003

In this work we apply the CDW-EIS model for multielectronic targets[1,4], to the study of differential cross sections (DCS) for single electron capture processes by impact of protons on helium atoms, at high energies. Numerical bound and continuum wavefuctions are employed using a Hartree-Fock-Slater potential to describe the active electron-residual target interactions. DCS can always be divided into three approximate angular regions where different mechanisms operate, giving rise to dissimilar features[2]: the small angles region corresponding to forward scattering and that extends up to the position of an interference minumum between the single and the double scattering amplitudes; the double scattering region centred on the Thomas$^3$ angle and where the electron is captured after undergoing two binary collisions (a first one with the projectile and a second one with the target nucleus); and the large angle scattering region where a rapid monotonic decrease is predicted. Of particular interest in high collision energies is the second angular region because the Thomas two step process becomes dominant. It is well known that the structure around the Thomas angle is very sensitive to the theoretical descriptions used [2]. Moreover, in the case of multielectronic targets, an adequate description of the target potential becomes crucial because during the second binary collision the active electron is moving mainly under the effect of it. At high impact velocities the electron moves with a wavelength much smaller than the atomic radius, acting as a probe of the target potential We performed an exhaustive study of the DCS for H+ + He(1s) collisions, at different energies. Present results are compared to those obtained with a former version of the CDW-EIS approximation for electron capture[4]. It takes into account different distorsion charges Z_D to represent the active electron-target interaction in the outogoing channel as an effective Coulombic potential. Theoretical results are also compared to experimental data[5]. The new calculations are in excellent agreement with the experiments in all the angular regions, improving remarkably the description of the Thomas peak. The comparison with the case Z_D=2 shows that the ejected electron penetrates deeply in the residual target during the second elastic collision, giving a further evidence of the two-step Thomas' mechanism[3]. 1. P.N. Abufager, A.E. Martinez, R.D. Rivarola P.D. Fainstein, see contributed paper to this conference. 2. L. J. Dube; J Phys. B 16 1783 (1983) 3. L. H. Thomas; Proc.R.Soc. (Londres) A 114 561 (1927) 4. Martinez, A. E., Deco, G. R., Rivarola, R. D. and Fainstein, P.D., Nucl. Instrum. Methods B 34, 32 (1988). 5. E.Horsdal-Pedersen el al., Phys. Rev. Lett., 50, 1910 (1983)