Transverse momentum transfer distribution in ionization of helium by fast and highly charged ions

P. A. MACRI; V. D. RODRÍGUEZ

Rosario, Argentina

Conferencia; 24th International Conference on the Physics of Electronic, Photonic and Atomic Collision (XXIV ICPEAC) Rosario, Argentina (2005); 2005

tRANSVERSE MOMENTUM transfer DISTRIBUTION IN IONIZATION OF hELIUM BY FAST AND HIGHLY CHARGED IONS Pablo A. Macri1 and Vladimir D. Rodr¨ªguez2 1) Instituto de Astronom¨ªa y F¨ªsica del Espacio, 1428 Buenos Aires, Argentina2) Departmento de F¨ªsica, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina The continuum distorted wave-eikonal initial state has demonstrated to be a successful theoretical tool to predict the electron distributions of atoms ionized by fast ions [1]. Since the introduction of the reaction microscopy technique [2] which allows measuring simultaneously the electron and the recoil target ion momenta, the incorporation of the projectile-residual target interaction in the theoretical framework has become an ineludible task to be made. For many electrons atoms, the not ionized electrons screen the pure Coulombic interaction between the nuclei making this task very involved. The simplest approach consists considering a pure Coulomb interaction where target nuclear charge is replaced by a constant effective charge given by the binding energy of the active electron. This approach can not explain, however, the double hump structure founded in recent experiments [3]. With this goal, in [4] a transfer momentum dependent target charge was proposed, which interpolates between the limits of total and zero screening. In this work, the effect of the passive electrons on the total projectile¨Cresidual target ion interaction is considered through a static Hartree-Fock potential VS (previously considered in a first Born approximation in [5]) and a dynamic polarization potential VP. Although for target residual ions in its ground state the dipole polarizability is not expected to be large (for He+, a = 9/32 au) the full interaction must be scaled by a factor ZP2. For highly charged projectiles, as considered here, the effect of introduced by the polarization potential could be therefore important. In Fig. 1 we present doubly differential cross section (DDCS) as a function of transverse momentum transfer ¦Ç at fixed electron energies Ee, for single ionization of He by 3.6 MeV/u Au53+ impact. We compare results using an affective charge Zeff = 1.35 with calculations employing VS and VS + VP potentials. For an electron energy Ee= 50 eV the use of Zeff gives the general tendency of the experiments although it lacks to show the bump around ¦Ç = 2 au. Surprisingly, the use of the more precise VS makes this comparison worst. On the other hand, the use of both VS + VP enables to recover the general description and even insinuates a bump at a larger ¦Ç. This can be confirmed for Ee=130 eV where the use of Zeff or VS fails to follow the experiments. We see that the inclusion of VP improve the results at low ¦Ç and reproduce de bump at higher transverse transfer momentum. Figure 1: DDCS as a function of ¦Ç at fixed Ee, for single ionization of He by 3.6 MeV/u Au53+ impact. Doted line: Zeff = 1.35, dashed line: VS, full line VS + VP calculations. Circles: experiments Ref. [3]. References [1] P. D. Fainstein et al, J. Phys. B 21 2989 (1988) [2] for a review, see J. Ullrich et al, Rep. Prog. Phys. 66 1463 (2003) [3] R. Moshammer et al, Phys. Rev. Lett. 87 223201 (2001) [4] V. D. Rodr¨ªguez, Nuc. Instr. Meth. B 205 498 (2003) [5] H. Fukuda et al, Phys. Rev. A 44 1565 (1991)