Unexpected x-ray emission due to formation of bound doubly excited states

A strong emission of characteristic M x rays is observed, without an M vacancy initially present, when slow highly charged ions (Ta(q+), q = 39--48) capture a single electron in single collisions with rare gas atoms (He). This is explained by the formation of bound doubly excited states through electron correlation. An elaborate theoretical treatment shows that bound doubly excited states are mixed with states where a Rydberg electron is bound in the core of a highly charged ion. It is striking that this occurs with a large probability (close to unity), and one needs to assume that higher Rydberg states are populated than predicted by the overbarrier model in order to explain the experimental results.