First direct proof of internal conversion between bound states

We present direct evidence for the process of internal conversion between bound atomic states (BIC) when the binding energy of the converted electron becomes larger than the nuclear transition energy. This process has been proposed as an explanation of the measured, unexpectedly short lifetime of the first excited state of ¹²⁵Te with charge state larger than 44⁺. We have detected the K_α x rays emitted in flight which follow the filling of the K-shell vacancy created by the bound internal conversion process, together with γ rays from Te ions in charge states ranging between 44⁺ and 48⁺. For Te⁴⁵⁺ and Te⁴⁶⁺, the comparison of the x-ray to γ-ray ratios with the theoretical calculations of the internal conversion coefficients including decay to bound atomic states, assuming Te ions in their ground electronic state, show poor agreement. The agreement becomes good if account is taken of BIC decay of excited initial states with different occupancies of the 2p_1/2 and 2p_3/2 subshells. In this situation, the half-life becomes sensitive to the precise initial state and simple specification of the charge state alone is no longer appropriate.