Magnetic circular dichroism of colour centres in CaF2 : Na

Abstract

2014 Le dichroïsme circulaire magnétique (D.C.M.) de différents centres colorés dans CaF2: Na coloré additivement a été mesuré à différentes températures, dans le domaine 1,65-195 K. Les bandes d'absorption 612, 435 et 382-385 nm sont associées à des D.C.M. paramagnétiques importants, ce qui prouve qu'elles proviennent très probablement de centres à nombre impair d'électrons. 612 nm ne peut donc pas être seulement une transition du centre F2A orienté [1, 0, 0] comme on l'admet habituellement. D'autre part, la bande 490 nm ne possède qu'un D.C.M. diamagnétique. Nous discutons quelques identifications possibles pour les principales bandes d'absorption des centres colorés dans CaF2 : Na, mais il faut, pour arriver à des conclusions sûres, d'autres travaux utilisant des techniques complémentaires.

Colour Centres in Alkali Metal Doped SrF2

“…[a], we explain these MCD signals by assuming that a single s-p like transition is widely split by the crystal field so as to give rise to the 328, 388, and 632 nm bands (or 326, 382, and 612 nm in CaF,:Na+). This assumption is much better supported by our present data than by those of [4]. As the MCD apparatus has been considerably improved in the U V region, we observe an important positive wing of MCD a t small wavelengths (it was considerably understimated on Fig.…”

“…The agreement between both methods is very good, which makes us quite confident in our interpretation of the ''F2A'' centre spectrum as a single, very widely split, s-p like transition. For CaF,:Na+, one also notices a satisfactory agreement with the result of Doualan et al [4] who used the unsophisticated rigid shift approximation for the MCD of the isolated red band.…”

“…The preceding section demonstrates, as already [4], [7], [21] did, that ''F2-&'' centres are CC's with an odd number of optical electrons, which can therefore not be really F~A .…”

“…As the MCD apparatus has been considerably improved in the U V region, we observe an important positive wing of MCD a t small wavelengths (it was considerably understimated on Fig. 2 b of [4]), so that (3a) is much better verified: the relative difference between the positive and negative contributions to the integral (2 c) is -35 yo for CaF, : Na+ and -25 yo for CaF, : Li+. This is still not zero, but the difference is explainable by the inadequacy of our present MCD apparatus to yield good results below = 330 nm, while the third absorption band of " F~A " centres in CaF,: A+ extends to much shorter wavelengths.…”

“…On the other hand, there is an obvious similarity between the CC's in CaF,: Li+ and CaF,: Na+ as already pointed out by several authors [l to 31. I n our laboratory, magnetic circular dichroism (MCD) experiments showed that the very conspicuous FzA centre4) in CaF, : Na+ does not deserve its name since it has a paramagnetic ground level [4]. As its genuine ixicroscopic structure does not seem t o have been yet reliably determined, we shall call this CC from here on " F Z A " .…”