Choice of Compounds with Fast Core-Valence Transitions
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INTRODUCTION It is now well established that in a number of wide gap ionic crystals a specific intrinsic luminescence can be observed due to radiative hole transitions between the upper core band and valence band. The progress in knowledge about core-valence (CV) transitions is considered in papers [1-3]. The CV transitions were first observed [4] and interpreted [5] in BaF 2 crystals. The intrinsic luminescence due to CV transitions has a short decay time ( - Ins), a high thermal stability in all its parameters, and a relatively high yield. Because of these luminescence properties, crystals with radiative CV transitions are promising scintillators. Such scintillators are particularly important in devices with a high counting rate, for instance in emission tomography. The presence of a filled valence band above the core band in which the hole is created presents the possibility of population inversion at high excitation densities. This aspect creates new possibilities for producing optical amplification and generation [6,7]. This work examines different approaches to the problem of CV transitions. A class of ionic crystals, in which the radiative transitions are most efficient, is identified. It is shown how to obtain the necessary spectral width of emission, and the possibility of controlling the decay time of the emission is discussed. PHYSICAL PROCESSES AND MODELS OF CORE-VALENCE TRANSITIONS Let us consider the physical processes which are responsible for CV transitions. Fig. 1 shows schematically the energy bands of a BaF 2 crystal, constructed on the basis of optical [8,9] and photoemission data [10] and confirmed by calculations [11]. Usually luminescence transitions (excitonic, impurity ones) occur within the upper gap of the crystal, Egj wide. Under a high energy excitation, holes are formed in the valence band (2p, F ) as well as in the upper core band (5p, Ba) of the crystal. After going up to the top of the core band the hole can recombine with the electron of the valence band. Fig. 1. shows appropriate electron transitions. These transitions occur in the region of the second energy gap of the crystal E8 2 or the gap between the core and valence bands. In recent times the luminescence due to the transitions considered here has been called core-valence luminescence [12].There are other names which, in our opinion, are less suitable, e.g., Auger-free luminescence [13], cross-lulminescence [1]. Another approach used for consideration of CV-transitions is a cluster one. As a result of relaxation of the core hole, a Ba 3 + ion surrounded by F ions is formed (Insert, Fig. 1). The transition with the charge transfer from an F ion to a Ba 3 + ion corresponds to a core-valence transition in the [BaF8 ] cluster. The following facts, which were obtained experimentally, prove that the detected 77 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
luminescence of a crystal is attributed to CV transitions: 1. In the luminescence excitation spectrum (curve 2, Fig. 1) a threshold is detected, at whi
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