THEORETICAL APPROACH TO THE LYOLUMINESCENCE OF ALKALI-HALIDES

When previously irradiated halide crystals are dissolved into a solven t like water, the radiative recombination of hydrated electrons with t he holes on the surface of the crystallites, gives rise to the light e mission. The intensity of lyoluminescence (LL) emission depends on dif ferent parameters and it can be expressed as I=eta beta gamma alpha nF (N)0/(beta - alpha) {exp(-alpha t) - exp(-beta t)}, where eta is the p robability of radiative recombination, beta the rate constant for the recombination of hydrated electrons with holes, gamma the factor corre lating the number of hydrated electrons and the number of dissolved F- centres, oc the rate of dissolution of solute in the solvent, n(F) the density of F-centres, No the initial number of the molecules of solut e, and t the time of dissolution. The equation shows that the LL inten sity should initially increase with time, attain an optimum value and then it should decrease exponentially with time. As cc increases with temperature, an increase in the value of I with temperature is expecte d. However, at higher temperature thermal bleaching takes place, and c onsequently the LL intensity should be maximum at a particular tempera ture. The LL intensity initially increases and then tends to attain a saturation value for higher mass of the solute added into the solvent. A good qualitative correlation is found between the experimental and theoretical results.