α‐Band in KI Crystals Containing F‐Aggregate Centers

Abstract

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.