Single electron capture from hydrogen excited states by bare ions impact

P. N. ABUFAGER, H. F. BUSNENGO, A. E. MARTÍNEZ Y R. D. RIVAROLA

Santa Fe, USA

Conferencia; 22rd International Conference on Photonic, Electronic and Atomic Collisions; 2001

The single electron capture reaction from excited H(2s) and H(2p) targets by H+, He+2 and Li3+ ions, at intermediate and high collision energies, is theoretically studied. These reactions are of interest in the interpretation of the physical phenomena that takes place in controlled fusion plasmas and astrophysical gases[1,2] The calculation of total and partial cross sections is carried out by using three different distorted wave models: the Continuum Distorted Wave[3] (CDW), the Continuum Distorted Wave - Eikonal Initial State$^4$ (CDW-EIS) and the Continuum Distorted Wave - Eikonal Final State$^5$ (CDW-EFS) aproximations. These models treat in a different way the distortions in the entrance and exit channel. Whereas the CDW approximation corresponds to the use of continuum factors in both channels, the CDW-EIS and CDW-EFS models replace the continuum factor of the entrance or exit channel, respectively, by an eikonal phase. In this sense we will speak of symmetric and asymmetric approximations. A prescription was proposed [5] to determine the asymmetric character of a given collision system and to select the distorted wave model that should be most appropiate to study it. An asymmetry parameter $alpha$ was defined. If alpha < 1 ( alpha > 1) the CDW-EIS (CDW-EFS) model should be employed and the CDW approximation should be more suitable when alpha is iqual to 1. The validity of this prescription has been tested for electron capture processes from the ground state of H, by comparison with available experimental data, providing reliable indications. In this work we apply this prescription to the study of the reaction of electron capture from excited states of H, and we analize its validity. Final states up to n_f = 4 were considered in the calculation of the total cross sections. A combinated calculation is also presented, obtained by using the alpha parameter to determine the distorted wave model more appropriate to describe the electron capture into each final shell. Results are compared with other theoretical predictions$^6$ provided by the Eikonal Impulsive (EI) approximation, Close Coupling (CC) and Classical Trajectory Montecarlo (CTMC) calculations. To our knowledge, at present there is no experimental results for the reactions under consideration. The combined calculation shows a good agreement with the CC calculations in the lower energy range and with the results provided by the EI model and each distorted wave approximation for the higher energies, when capture form H(2s) initial states is analyzed. In the case of capture from H(2p) initial states, some discrepancies were found in the higher energy range. These discrepancies are related to the different asymptotic velocity dependence$^5$ of the theoretical models. From comparison of our results with other theoretical cross sections, we can conclude that the prescription appears also to provide adequate predictions in the case of capture processes from excited initial states of monoelectronic targets. This work was partially supported by the ANPCyT (BID 1012/OC-AR PICT N$^circ$ 03-04262) and the CONICET. References 1. Seaton M.J., Rep. Prog. Phys. 23, 313 (1960) 2. Osterbrok D.E., Ann. Rev. Astron. Astrophys. 2, 95 (1964) 3. Cheshire I.M., Proc. Phys. Soc 84, 89 (1964) 4. Crothers D.S.F. and McCann, J. Phys. B 16, 3229 (1983) 5. Busnengo H.F. Mart´{i}nez A.E. and Rivarola R.D., Phys. Scr. 51, 190 (1994) 6. Montanari C.C., Gravielle M.S. and Miraglia J.E., Phys. Scr. 56, 279 (1997) and references therein.