A review of the relevant data concerning the decay properties of the Cu þ fluorescence in alkali halides is presented. A critical analysis of the experiments reported in literature shows that the problem of the correct and complete interpretation of the Cu þ fluorescence was made complicated in the past essentially because of historical reasons. The validity of the decay models was ascertained by the authors by simultaneously detecting both lifetime and quantum yield during each set of measurements as a function of the temperature.

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Summary. --Lifetime and quantum yield measurements concerning the fluorescence of Cu + impurity centre in KI were performed in the range 19-300 K. The emission mechanism was shown to involve two excited states in thermal equilibrium, one of them being metastable and lying close under the emitting level, ~e. according to the Pedrini model for Cu § fluorescence in alkali halides. Monovalent copper impurity in alkali halides has long been an active area of research. An interesting feature is that in several host matrices Cu + (as well as other ions) do not occupy the position of the ion it substitutes: in other words, it goes ,,off-centre,. Forecast for off-centre displacement was obtained by one of the authors[l-3], but essentially it concerned the ground-state configuration of monovalent impurity ions (Li t, F-, CI-, Na t, Ag t, Cu t) in alkali halide crystals as well as of Mn 2 t impurity ion in some oxides. In principle, the position in the lattice of Cu § in the relaxed excited state (RES) could be different from that in the ground state. For this reason, in the past Piccirilli and Spinolo[4] carried out lifetime measurements as a function of the temperature T concerning the fluorescence of the substitutional Cu § ion in several alkali halides (/.e. those not containing Li and F): it was assumed that the involved 3d94si--, 3d 1~ forbidden transition is made possible by odd lattice vibrations that dynamically mix the initial and final wave functions with different-parity wave functions. These authors, on the basis of the temperature behaviour of the lifetime as well as of the absorption oscillator strength, tried to determine[4] the change on off-and on-centre configuration of the Cu + in its RES. Nevertheless, these authors subsequently apologized [5] for an instrumental error in their lifetime measurements at low T. Consequently, Bertolaccini et al. repeated the experiment [6] by using a better technique, but restricting the investigation to NaC1, NaBr, KBr and RbBr. They noticed [6] that in some cases the lifetime dependence as a function of T is similar to that observed for the oscillator strength of Cu + in an on-centre configuration (i.e. involving a coth function) and, what is more, suggested the possibility of a change from an on-centre (at low T) to an off-centre configuration (at high T).In 1978, Pedrini suggested [7] that the temperature dependence of the Cu + 1199

The fluorescence of the Cut impurity centre in KBr was investigated in the range 19-300 K in pariicular. lifetime and quantum yield data were recorded in the same sets of measurements. The de-excitation mechanism is interpreted in term of the Pedrini model: two excited states in thennd equilibrium are involved in the emission process. one of which is metastable and lies close below the emitting level. d