Mechanisms for Energy Shifts in Atomic K X-Ray Spectra

Abstract

It has been known for 15 years that the energies of K x-rays from the heavier elements to a small extent depend on the isotopic and chemical composition of the source. As discussed in the reviews by Boehm (1975) and by Sumbaev (1976), the isotope shifts originate in the nuclear finite-size effect and serve to give information on nuclear radii, while the chemical shifts originate in differences in the screening for different chemical components of the same element. It has been found by Hahn et al (1976) that the energies of tantalum K x-rays depend on the excitation mechanism: those originating from photoionization of tantalum have about 1 eV higher energy than those from the electron-capture (EC) decay of 181W. The experiment was interpreted as demonstrating a selective shake-off of 4f electrons in the beta decay process; it was soon pointed out (Borchert et al 1977, Freedman 1977) that there are strong theoretical reasons to expect outer-shell shake-off to be very weak in the EC process and that the sign of the CalTech measurement, in fact, was consistent with that expected for shake-off in the photo-ionization process. The quantitative explanation of this measurement still remains as an interesting puzzle, but it is now clear that all four of the mechanisms discussed in the following must play a role.