More on the F‐Center Decay in Additively Colored NaF:LiF

Abstract

Accurate measurements of the F-center lifetime, z ( T ) , in additively colored NaF: LiF samples are presented as a function of the temperature, starting from 7.6 K. The anomalous behavior of the F-center decay in NaF is confirmed only as regards the behavior of the relative quantum yield ~~( 2 " ): the presence of another decay channel for the F-center electron, besides the jumping into the conduction band, is evidenced. The disagreement with data obtained by other authors is explained by the presence of aggregate centers in their samples. A z(!P)/r],(T) behavior contrary t o that presented by Stiles, Fontana, and Fitchen is observed. A critical review of models and guesses proposed in the literature is also presented in order to obtain a correct understanding of the nature of the relaxed excited state of the F-center.Dans ce travail on pr6sente des mesures accurBes, en fonction de la temperature, de vie moyenne du centre F, pour des Bchantillons de NaF:LiF additivement colorbs, & partir de 7,G K. Le eomportement irrBgulier du centre F in NaF est confirm6 seulement si on considhe le rendement quantique 7lr(T) : d'autre partie on a mis en evidence la pr6sence d u n autre canal de dBsexcitation pour 1'Bleetron du centre F, au deli du ,,jump" dans la bande de conduction. Le dbsaccord trouvB avee les donnBes obtenues par d'autres auteurs est expliqu6 par la presence de centres complexes dans leur Bchantillons. On a observB, egalement, un comportement t( T)/q,(T) contraire B celui present6 par Stiles, Fontnna et Fitchen. On presente, finalement, ime revision critique des modhles et des tentative8 d'interprbtation proposBs dans la litterature dam le but dobtenir une comprehension correcte de la nature de 1'Btat excite relax6 du centre F.

The absence of energy transfer between F and M centres in alkali halides: the case of NaF

“…As a further proof we recall that, as anticipated, the absolute quantum yield of the F luminescence decreases as a function of the absolute (initial) concentration as well as of the local F-centre concentration (an ageing effect which implies the shortening of the distance between F centres) but the F-centre luminescence lifetime is unchanged. This seems to be a general rule for the F centre in alkali halides: as regards the NaF case, we refer to the papers of Bosi et al [3 (see, for instance, figure 7), 21,22,24]. Nevertheless, we recall the existence of another effect: the change of the F-centre lifetime as a consequence of the interaction between F and α centres.…”

“…In figure 1 we show an example of the emission spectra we obtained when light excitation was carried out in the M 1 band, whereas in figure 2 we show some spectra obtained by light excitation in the M F bands. It can be stressed that in the last case F-centre luminescence is present at T < 150 K. An accurate study concerning the F-centre luminescence quantum yield and lifetime has been presented previously by one of us [22]; as anticipated, the M luminescence in additively coloured NaF:Li samples was recently investigated by Bosi and Gallo [19,20]. So we were able to separate the contribution of the F-centre luminescence from that of the M centre; a deconvolution program using two Gaussian functions was adopted.…”

“…The F-centre lifetime does not depend on the F-centre concentration, nor on the presence of the M centres (whatever M concentration is employed): this rule was found to be valid for any alkali halide investigated (see the papers of Bosi et al here cited). Moreover the probability of a non-radiative transfer 1/τ tr might introduce a competitive channel on the decay mechanism of the F centre after relaxation: as well known, the reciprocal of the F-centre lifetime 1/τ (T ) is the sum [22] of a radiative term 1/τ R (T ) and of a thermal de-excitation term (1/τ 0 ) exp(− E/kT ).…”

“…Another disadvantage is the presence of several different aggregate and ionized centres in the samples that make the luminescence studies even more laborious. Thus, Bosi et al [22] obtained accurate measurements concerning the anomalous behaviour of the F-centre decay in NaF as well as concerning the F − -centre formation [23] using NaF:Li additively coloured samples.…”