OPTICAL AND THERMAL STABILITY HALF-WIDTHS AND OSCILLATOR STRENGTHS OF F-AGGREGATE BANDS IN HIGHLY PURE KBR CRYSTALS

The decay and growth of F, F-aggregate and colloidal centres in additively coloured crystals have been studied (i) by bleaching the additively coloured crystal with white or F light, (ii) by annealing the optically bleached crystal at room temperature in the dark and (iii) by heating the coloured and 'coloured and optically bleached' crystal up to 400°c. The thermal stability of the centres can be described by the equations colloids, and colloids F. N centres are stable at room temperature and disappear completely on heating the crystal to 100°c. Correction in the F-band height due to the over-lapping absorption of the excited states of the M centres is evaluated. Peak positions and half-widths of the M, R2, N and colloid bands are determined in highly pure and 'pure' KBr crystals at room temperature. The variation of the half-width W of the M band in KBr with temperature is found to vary as The changes in the absorption coefficients at the peaks of the F and F-aggregate bands observed in the optical bleaching experiments, the room-temperature annealing experiments and the thermal bleaching experiments are used to obtain the oscillator strengths of the F-aggregate centres in KBr. By taking the oscillator strength of the F centre as 0.48, the values 0.11, 0.43 and 0.06 are obtained for the oscillator strengths of the M, R and N centres, respectively.