On the trap depth of the IR-sensitive trap in Na- and K-feldspar

Feldspars are natural radiation dosimeters used in geological dating. The processes that underlie the measured natural- and laboratory induced infrared stimulated luminescence (IRSL) signals are, however, not fully understood. In this study we aim to determine the optical depth of the electron trap that is used in luminescence dating. A series of optically stimulated luminescence (OSL) experiments were performed on two feldspar minerals (single-crystal museum specimens of Na- and K-feldspar) at a low temperature (10 K). The low temperature suppresses thermal processes that may be active during excitation and facilitates the monitoring of mainly optical transitions. Additional experiments at room temperature were performed for comparison. The continuous wave OSL and the post-stimulation phosphorescence were recorded in the ultra violet (3.2e4.8 eV) for stimulation energies ranging between 1.7 and 2.8 eV. From the dependence of the OSL intensity on the excitation energy is deduced that band-tail states play a significant role during charge transport, thus complicating the precise determination of the optical trap depth. Both minerals display a small resonant transition when stimulated at∼2.05 eV. From the OSL data, the trap depth is tentatively estimated to be ∼2.1 and ∼2.5 eV for Na- and K-feldspar, respectively. The fast phosphorescence following IRSL in both crystals is likely the result of tunnelling from bandtail states in the vicinity of the excited state of the IR-trap. The post-OSL phosphorescence decay is an order of magnitude slower, owing to the contribution from tunnelling of charge from a range of occupied band-tail states. The lack of dependence of the phosphorescence decay rate on the excitation energy (in the range of 1.7e2.8 eV), unambiguously indicates that a single mechanism causes the post-OSL phosphorescence signal.
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