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 vaca ncy initially present, when slow highly charged ions (Taq+, q = 39-48) capt ure a single electron in single collisions with rare gas atoms (He). This i s explained by the formation of bound doubly excited states through electro n correlation. An elaborate theoretical treatment shows that bound doubly e xcited states are mixed with states where a Rydberg electron is bound in th e core of a highly charged ion. It is striking that this occurs with a larg e probability (close to unity), and one needs to assume that higher Rydberg states are populated than predicted by the overbarrier model in order to e xplain the experimental results.