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Abstract

Energy transfer from donor-acceptor pair states is known to be an efficient excitation channel for 4f-4f emission, especially if the rare-earth centres 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 the radiative de-excitation of the pairs. The authors have found these two kinds of emission, under electron-beam excitation, in ZnS:Tm and ZnS:Sm crystals which, according to earlier site-selective studies performed by them, contain considerable concentrations of donor-type rare-earth centres. Characteristic differences between the two kinds of emission are found in the dependences 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 and measurements of optical excitation spectra of 4f-4f emission the authors demonstrate that the same types of initial state are involved in the energy transfer and the pair transitions.