Direct charge transfer by X irradiation in the system CaF₂:Eu, Tm

Starting with CaF2 containing Eu2+ and Tm 3+ ions, we have employed optical techniques to observe the decrease in valence of the Tm ion under x irradiation. We find that the number of Tm 2+ ions produced corresponds closely to the number of Eu2+ ions which disappear, and we suggest that the underlying mechanism is a direct transfer of an electron between the two types of ions. © 1965 The American Institute of Physics.