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.