Abstract
It is shown that most rare‐earth activators in lanthanum oxysulfide show anomalous thermal quenching effects similar to those previously reported for Eu3+. An analysis of the activation energies derivable from regions of unusually rapid thermal quenching indicates, for all ions observed to date, that the quenching processes represent strong interaction between the 4f electron levels of the rare‐earth ions and the conduction band of the host. It thus becomes possible to locate experimentally the positions of the rare‐earth levels relative to the host‐bands. This in turn allows a general explanation of the excitation and relaxation characteristics of the rare‐earth oxysulfide phosphor system.

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