Effects of the off-center position of theLi+ion on the optical properties of theFAce

Abstract

The off-axis symmetry of the F& center in KCl:Li has been investigated by optical experiments. We have studied static and dynamic reorientational properties of the F& center by measurements of absorption and emission. The off-axis model of the center has been confirmed. A theoretical treatment, which takes into account the small deviation of the center with respect to the crystal axes, is in good agreement with the experimental results. Best fittings of the theory to the experimental data yield a value of the off-axis angle 8= 8'.

The contribution to lattice defects physics by Umberto Maria Grassano

“…Baldacchini and Grassano extended the analysis to other F A centers. The new approach was based on a theoretical model of 532 R. CAPELLETTI the F A optical reorientation, which takes into account the off-axis angle, on the measurements of 1) the angular dependence of the stationary emission under polarized excitation, 2) the F A reorientation time for different pumping powers, and 3) the F A realignment kinetics [23]. In this way the off-axis symmetry was confirmed for F A (Li) in different matrices, as KF, KCl, and RbCl and evaluated as 14, 8, and 7 degrees, respectively.…”

Self-consistent analysis of absorption and emission data forF A centres in alkali halides

“…Since the first works on the dichroic absorption and subsequent emission of the Fa centres in alkali halides [1][2][3][4] the problem of taking into account the peculiarities of their orientational relaxation was a point of particular interest for different research groups [5][6][7][8][9][10][11][12][13]. The FA (Li) centre is formed by an F centre with a nearest-neighbour Li + substitutional impurity.…”

“…2 the following steady-state solutions for the populations ~t: For excitation with FA1 light Z=a2(oJ)/al(o))<<l and ~1--1/2tg 20<<1, being 0 << 1 [9]. After averaging eq.…”

“…(9) give the following set of transcendental equations: Due to the centre reorientation ~t ~ pit, and therefore absorption indices as well as phase velocities for the components Ex, Ey are different.…”

“…These curves were calculated for the ratio ~)/yd = 0.2 and ~) --0.015 in the case of RbC1, and ~)/7"d = 0.3 and ~ ~ 0.03 in the case of KF. If this parameter reduces only to ~1 [11,9], then we obtain the tilt angles reported in the fifth column of table I. In the case of KC1 the best fitting was obtained without taking into account the exciting light depolarization.…”