Color centers in the ionized and in the triplet state ?Z 2 centers in ytterbium-doped KCl crystals

Some observations on the optical absorption bands in LiF:Mg, Ti

“…Furthermore, subsequent illumination at 280 nm should, according to Okhura's (1964) model for Z-centres, result in a decrease in the F-band and an increase in the Z3-centre absorption (e.g. Paus and Strohm 1979). When this was ' Agull6-L6pez et a1 (1982).…”

“…A feature of the Z-centres in alkali halides is that they can be produced by photoconversion of F-centres (Okhura 1964, Paus and Strohm 1979, Paus and Scheu 1980, Moharil 1980). In the present work we note that upon illuminating an irradiated TLD-100 sample with uv light of 245 nm, the F-band decreases sharply (figure 4(a)) as does all of the spectrum from around 225 nm to 450 nm.…”

“…Also, it does not include the several different lattice orientations that are available for Z3-structures. For example, Paus and Strohm (1979) and Okhura (1964) note at least two Z3-optical absorption bands per impurity in the alkali halide systems studied by them. To have just one Z3-centre optical absorption band in LiF:Mg would be unusual.…”

The excited state of Z2 and Z+2 centres

“…In both cases the ZJ(1) band is polarized parallel to the centre axis and possesses by a factor of ~ 2 higher an oscillator strength than the perpendicular transition Z 2 (2) length tail. The two other maxima are the ZZ(1) Nevertheless some points need to be mentioned : and the Zl(2) bands.…”

“…Introduction. -EPR experiments on the triplet state of Z 2 centres in KC1 doped with divalent impurities (Ca ++ , Sv ++ , Ba ++ , Yb ++ ) revealed their rather complicated atomistic structure : most probably, an impurity cation -cation vacancy complex is combined with two F centres in various relative positions [1][2][3][4]. The simplest atomistic configuration is found with Z 2 (Ca) and the most frequent type of Z 2 (Yb) centres : they show pure (100) axial symmetry and are orientable by polarized optical irradiation.…”