Decay of fluorescence emission of KI: Cu+

Abstract

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

Review of Emission and Lifetime Data Concerning Cu+ Fluorescence in Alkali Halides

“…In the tables and figures of our previous papers [21][22][23][24] (in which more experimental details are described) we presented every data concerning lifetime, temperature, fitting values, quantum yield in order to facilitate the reader understanding (in the case of interest in a different theoretical analysis of matter).…”

“…Indeed in Ref. [16] it [24] off-centre in both GS and RES was assumed that A 2 ¼ 2t À1 (300 K) and that A 2 (300 K) ¼ A 2 (4.2 K) (i.e. A 2 is independent of temperature) and D was often calculated on the basis of tðTÞ data obtained by other authors.…”

On the decay of KCl:Cu+ fluorescence

“…Furthermore, we want to complete, in this way, the study concerning the properties of Cu § in potassium halides: in fact, the validity of the Pedrini model was verified by Ramasesha and Payne in KF [9], and, very recently, by us in KBr [10] and in KI [11]. Incidentally, we note that in the past other alkali halide crystals were analysed by Mack and Van Sciver [12].…”