In CsCdBr3 the trivalent Pr ions substitute the divalent Cd ions. Charge compensation leads to Pr vacancy centers. By selective excitation and fluorescence spectroscopy and by annealing the crystals, three different Pr centers could be identified. The first one is the well-known symmetric pair center Pr'+-Cd + vacancy-Pr'+, the second one is the asymmetric pseudopair center Pr'+-Pr +-Cd + vacancy, and the third one finally is a Pr'+ single ion with a Cs+ vacancy nearby. Energy-level schemes of all three centers were deduced from polarized site-selective spectra including the symmetries of the crystalfield states. Only the Pr'+-Pr'+-Cd'+ vacancy center showed upconversion emission from 'Po and 'P, under pulsed 'D2 excitation.keeping in mind that the highest phonon frequencies in CsCdBr3 are of the order of 160 cm ' (Ref. 18) or, maybe, 200 cm ' because the crystal consists of heavy ions only. Doping with 1% Pr + did not change the known Raman spectra' as found experimentally. ' Therefore we ' proposed another process to be responsible for the upconversion: cooperative energy transfer

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The spectroscopy of pairs of rare-earth ions is demonstrated for Pr3+ :CsCdBr3• For reasons of charge compensation only two definite types of Ρr3+ pairs are formed independent of concentration. Only the pair with the Pr3 + ions as direct neighbors shows upconversion. The dynamics of this upconversion process is studied by time-resolved spectroscopy and discussed in the energy leve1 schemes of the single ions, and the pair. Coupled pairs of 3d and 4f ions offer a number of new physical effects as compared to the single ions which are of interest for basic research as well as for applications. We are mainly interested in optical effects of coupled ions in incoherent as well as in coherent radiation fields. In incoherent fields the following effects have been found: 1. Cooperative absorption and emission of photons [1]. This effect means that two ions can absorb or emit a photon of the sum energy where hvA and hYB are energy differences on the ions A and B, respectively. The cooperative absorption or emission are caused by the ion-ion coupling, and in terms of perturbation theory they end on or originate from a virtual level.2. Upconversion by cross relaxation [2]. In this case two excited ions accumulate their excitation energies on one of them transferring it to a higher excited state, whereas the other ion relaxes down to a lower state. In this case we get the emission of a photon from a real level.(889)

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