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Theoretical analysis of decay processes of inner-hole states of atomic systems

Decay processes of inner-hole states in atomic systems have received considerable interest in recent years in connection with a discovery of the formation of hollow atoms during the collision between a highly-charged ion (HCI) and a solid surface. Atomic inner-hole states, including hollow states with a completely vacant inner-shell or more, are extremely highly-excited and short-lived discrete states embedded in continuum energy spectra. These states usually stabilize by emitting Auger electrons or x rays successively. The formation mechanism and decay processes of a hollow atom have been investigated through the analysis of these spectra with an appropriate model. A classical over-barrier model has effectively been used to explain the formation of a hollow atom due to the electron transfer from a neutral atom or a solid target to an incident HCI during the collision between them. On the other hand stabilization mechanism for a hollow atom has been studied with an appropriate cascade decay model in which the rate equation is solved to obtain populations in each excited state as a function of time. In this case one is required to prepare a large number of various atomic data such as energy level, transition rates for the Auger and radiative transitions to solve the equations. Moreover, as mentioned above, one always encounters a difficulty that a variational calculation for energies of inner-hole states in an atomic system cannot necessarily yield an upper bound of the corresponding exact energy because there exist infinite number of states below the state under consideration. So the calculation of the structure of an atomic system with inner-holes is still a challenging theme.