Type A‐centres are found in KJ:Na single crystals. The conversion F → A is followed in the visible region and near the fundamental absorption edge. The intensity of the β‐band decreases with the formation of A‐centres and at the same time a new band (βA) is formed at longer wavelengths. The disappearance of A‐centres results in a broad Ac‐band in the visible region which is accompanied by the formation of an αA‐band in the ultra violet region, at wavelengths shorter than those corresponding to the α‐band. As the intensities of the A1‐and βA‐bands are proportional the occurrence of the βA‐ and αA‐bands can be explained qualitatively by exciton formation near A‐centres or vacancies which are due to negative ions with a Na ion as nearest neighbour.
Um ein Modell für das A‐Zentrum aufzustellen, wurde die optische Absorption in natürlichem und polarisiertem Licht an Li‐dotierten KCl‐Kristallen gemessen. Die Ergebnisse sind in guter Übereinstimmung mit denen anderer Autoren, die Na‐dotierte KCl‐Kristalle untersucht haben, und können mit Hilfe des A‐Zentrenmodells, das aus einem Na‐ oder Li‐Ion in unmittelbarer Nachbarschaft einer Anionenleerstelle besteht, erklärt werden.
It is well known that the larger the concentration of F-centers, the smaller the efficiency of the F-F' conversion (1). This phenomenon arises also if a colored crystal is illuminated in its F-band at room temperature. A less efficient F -F1 conversion is connected with a change in equilibrium accompanied by a reverse conversion from FI-to F-centers owing to a tunnelling effect (2 to 6), or to the preferentially stronger Coulomb attraction of the 6 -centers for the electrons coming from FI-centers (8) and (5). Another process with the same effect at larger concentration of F-centers is the resonant energy transfer between F-centers (7) and (4) ' The efficiency of the F-F' conversion is maximum in freshly quenched crystals and is reduced in crystals kept in darlaess at room temperature.The difference between the increase in the 6 -band as a function of time (Fig. l), immediately after the thermal treatment (curve la) and 48 h later (curve lb) may be -3 observed in additively colored KI crystals containing about 1. 9x10l6 F-centers cm .The curves were obtained with the help of a SP 700 Unicam spectrophotometer, illuminating in the F-band at LNT, 0.5 mm slit width. After keeping the crystal for 48 h in the dark, only 88% from the initial number of 6 -centers is formed. A further illumination for 10 min in the F-band at room temperature, 0.5 mm slit width, results in an increase in the M-band and the total number of&-centers is reduced to 27% from that initially produced (curve lc).At the same time the excited luminescence spectra of the& -center were measured by means of a VSU-1 Zeiss monochromator. The spectra a, b, c on Fig. 2b correspond to curves a, b, c on Fig. 1. A narrowing as well as a slight shift towards higher energies are observed in the excitation band after keeping crystals in the dark, this effect becoming even marked when M-centers are formed, The perturbation of the anionic vacancy is quite difficult to be explained from the present data.
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