Energy transfer from donor-acceptor pair states is known to be an efficient excitation channel for 4f-4f emission, especially if the rare-earth centers themselves are the relevant donors. Information on this type of mechanism can be obtained in the case when it is possible to observe, parallel to the 4f-4f emission, luminescence arising from radiative deexcitation of the pairs.

We have found these two kinds of emission, under electron-beam excitation, in ZnS:Tm crystals which, according to earlier site-selective studies performed by us, contain predominantly donor-type rare-earth centers. Characteristic differences between the two kinds of emission are found in the dependence of intensities on pumping rate; these arise from partial blocking of the energy transfer due to depopulation of the 4f ground levels. By quantitative studies of the correlation between those intensities we demonstrate that the same types of initial states are involved in the energy transfer and the pair transitions.