(ricevuto il 21 Febbraio 1991; revisionato il 18 Luglio 1991; approvato il 29 Luglio 1991) Summary. --A review of acquisitions and of unresolved problems concerning Fcentres in alkali halides is presented. The main conclusions are a) the temperature dependence of the bands is mainly due to the thermal expansion for the absorption process and to electron-phonon interaction for the emission process: for the latter half-width and Huang-Rhys factor values are related to the transverse optical frequency ones; b) the lifetime VR of the relaxed excited state is not intrinsic at low temperature because of the interaction between F-and a-centres always present in real samples; c) even though the absolute quantum yield decreases when F-centre concentration increases, the relative quantum yield vr is unaffected by change on F-centre concentration, strongly evidencing a nonhomogeneous distribution of F-centres in real samples; d) a temperature-dependent fine structure in zR (an Vr) values is observed but only when temperature control of the samples is better than a few hundredth of a degree: this effect has been interpreted in terms of interactions which involve F-, a-and F'-centres. PACS 61.70 -Defects in crystals. PACS 61.70.Dx -Colour centres.

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Luminescence measurements show an increase in the F-centre lifetime in KBr, corresponding to a critical cc-centre concentration, after F-F' conversion. An inhibition of the luminescencc quenching effect, related t o the F-centre density, is also observed. The results are discussed in respect of the Crandall photoconductivity data and explained on the basis of an interaction mechanism that involves the electron capture cross-section by a-centres and the mean distance between F-centres.Par moyen des mesures de luminescence on a remarque que le temps de vie moyenne des centres F dans le KBr augmente en correspondance d'une concentration critique des centres a, aprhs conversion F-F'. On a aussi observe qu'il y a un effect d'inhibition de l'extinction (qui depend d r la densite des centres F) de la luminescence. On discute les resultats experimentaux, en les comparant avec les donn6s de photoconductivit6 de Crandall, e t on les interprete sur la base d'un mkchanisme d'interaction qui entraine la section de capture d'hlectrons par les centre a e t la distance moyenne entre les centres F.

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