DYNAMICS OF GENERATION AND DECAY OF LUMINESCENT CLUSTERS IN PURE KBR CRYSTALS

V. Kochubey , Yu. Sedova

Saratov State University, Department of Physics, Astrakhanskaya 83, Saratov, 410026, Russia, kochubey@mailexcite.com

Keywords: alkali halide crystals, defects, luminescence, XEOL-spectra

It is known that "pure" alkali halide crystals are able to emit light in near ultraviolet region after heating at near-melting temperature or growing from the melt.

Up to date, the nature of such luminescent centres has not been elucidated. It is connected with the fact that impurity concentrations are much less than detection limit. It does not allow to associate these defects with any impurity. In our opinion, assumptions about the influence of gas environment, in which the crystal has been grown, on efficiency of luminescent centre formation seem to be unsound, because such centres are formed when crystals grow both under various gas atmosphere and in vacuum. Moreover some authors studied the luminescence in the same spectral region using various excitation methods (optical, thermal and X-ray luminescence). But identity of emission centres in that crystals was not unambiguously proved. Therefore, the study of luminescence properties of the same sample under ultraviolet and X-ray excitation is actual.

We have investigated KBr crystals which were grown from the melt of the purified salt. That crystals emit light in the region of 392 nm under UV excitation. After X-ray irradiation of the sample we observed the decrease of luminescence intensity.

To study the dynamics of X-ray luminescence the crystals under investigation were exposed to synchrotron radiation with quantum energy near the K edge of Br absorption (13474 eV). Succession of XEOL (X-ray Excited Optical Luminescence ) spectra was obtained by non-stop irradiation of the same crystal section. It was shown, that in long-wavelength region of luminescence spectrum we observed reversion of a jump sign when crossing the Br absorption edge. The jump size, as a rule, increases in this case. In low-wavelength region, we observed no reversion of a jump sign. The jump size decreases. Reversion of a jump sign, as well as the increase of it in the first case, may be explained by destruction of defects of one type and production of centres of other type under X-ray irradiation.

So, experimental data obtained suggest that in "pure" KBr crystals there are at least two types of luminescent centres, which have emission bands overlapping in ultraviolet region and different mechanisms of interaction with X-ray radiation. And, at least, one type of mentioned defects has complex structure and they may be an analogue of a cluster.