engleski

Correlation effects in atomic total absorption cross sections

Recent measurements of the X-ray mass attenuation coefficients for molybdenum [Martin D de Jonge et al, Phys Rev A 71, 032702 (2005)] and for tin [Martin D de Jonge et al, Phys Rev A 71, 032702 (2007)] are claiming very high accuracy (order of tenth of a percent) for the total absorption cross section, which goes beyond the theoretical accuracy (order of percent). Obtaining a theoretical accuracy of 0.1% is a challenge, since no existing calculations claim this accuracy. In trying to describe the total absorption cross section and understand it at this high level of accuracy, we focus on high energies, where processes are atomic and in circumstances when the dominant process is photoeffect. It is clear that to get this accuracy we need to include correlations, since independent particle approximation (IPA) approaches are not sufficient. As a first step we consider the situation in helium, where significant work has been done in understanding the processes of single ionization with excitations and double ionization which, together with the dominant single ionization without excitation, determine the total absorption cross section. Unlike the dominant process, ionization with excitation and double ionization can not at all be understood within the IPA picture, while they give important contributions to total absorption, and in the case of helium contribute of order of 10% even at very high energies. In the case of higher Z helium-like ions the relative contributions of excitations and of double ionization decreases, while for outer shells of neutral atoms they will be important. By considering all these processes and effects one may study the differences from the IPA type calculations for the K-shell, which gives the dominant contribution to the total absorption cross section. For helium and helium-like ions we compare our fully correlated calculations [based on T. Surić and R. H. Pratt J Phys B 37, L93 (2004)] for total absorption cross sections (which include single ionization without excitation, single ionization with excitation and double ionization) with IPA calculations using Kohn-Sham and Dirac-Slater potentials, for which excitation and double ionization contributions are included in an implicit sum rule.

Correlation effects; photoabsorption

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