IR quenching of luminescence of ZnS=Cu phosphors and structure of their luminescence centers

Abstract

The effect of IR irradiation on the shape of the individual luminescence band of ZnS=Cu phosphors was investigated in [1][2][3][4]. In this paper we use this effect to determine the structure of the luminescence centers and the mechanism of IR quenching.There are two different explanations of the change in the shape of the individual luminescence band due to IR light. According to the first, some of the energy of the iR quanta is transferred directly [2], or by means of optical electrons [3], to the luminescence centers (optical electrons are electrons which are released by the IR light and have excess energy in comparison with the equilibrium electrons [3]). This excess energy, which brings some of the centers into a nonequilibrium state, is the reason for the indicated effect. According to the second viewpoint [4], the effect is due to the redistribution of holes between different energetically close activator levels by the IR light, i.e., to the nonelementary nature of the luminescence band itself.We think that these two essentially different modes of action of iR emission can be distinguished by the rate of restoration of the initial shape of the luminescence band after the IR light is turned off. In the first case the shape of the luminescence band will be restored in the time required for dissipation of the excess energy of the luminescence center or the optically released carrier. This time will not exceed i0 -9 sec [5, 6]. In the second case the restoration of the initial distribution of holes among the different activator levels after alteration by IR light will be brought about by recombination transitions to these levels. The time for restoration of the shape of the luminescence band will be close to the time required to restore the total intensity of the band itself and, hence, will be very large.We investigated ZnS=Cu and ZnS CdS=Cu phosphors with an activator (copper) content of 10 -4 and 2.10 -4 g per g of the host substance.The phosphors were prepared by the method described in [7] and were kindly presented to us by K. Piir. Some of these specimens were investigated earlier [8].The phosphors were excited by a SVD-120A mercury lamp. The ZnS = Cu phosphors were excited by the 365 n_m line and the mixed ZnS (60 mole %) CdS = Cu phosphors were excited by the 436 nm line.For measurements of the IR quenching and its relaxation in relation to the wavelength of the phosphor luminescence we used a UM-2 monochromator and anFEU-17A or FEU-27 photomultiplier.The quenching long-wave light was isolated by another UM-2 monochromator from the light flux of a 300 W projection lamp.The iR luminescence quenching factor #(~, t) as a function of the energy ~ of the luminescence quanta and the time t after removal of the long-wave radiation was calculated from the formula ~ (~, t) -A/(~, t)Io (~)The intensity of the luminescence due to the short-wave exciting light I0(~) and its reduction AI(e, t) due to the long-wave light were determined separately for each spectral region.The investigated phosphors were contained in...