Using Infra-red stimulated luminescence and phototransferred thermoluminescence to investigate electron trapping and charge transport in feldspars

Abstract

Previous studies have observed mineral specific variations in the ratios of IRSL signals stimulated at the different discrete wavelengths of lines corresponding to Xe lamp emission lines, which fall within the broad excitation spectrum of IRSL. Meanwhile recent accounts of IRSL have referred to single trapping systems within the excitation band, and modelled excitation spectra as continuous gaussian distributions. A simple single trap model should not result in variations in the ratio of IRSL intensities produced at different wavelengths. In the study reported here 22 different feldspar samples, which have all been characterised by SEM and x-ray analysis, were measured using a portable OSL instrument equipped with 890 nm and 940 nm diode arrays to confirm and extend the previous observations of variations in IRSL intensity ratios. Of the samples investigated the sensitivity varied over 3–4 orders of magnitude. Albite samples in this selection showed consistently high responses. The plagioclase samples show significant variations, and the microcline and K-feldspar samples produced a large range of responses. Excitation ratios of 940 nm–890 nm show mineralogically linked variations which together with the earlier Xe lamp data point to distributions of trapping systems within the variable environments of these minerals. These results do not readily fit simple single trap models for luminescence from feldspars; instead, they suggest multiple traps or a series of similar traps which are significantly perturbed by their local lattice structural environments. Post Stimulation Phosphorescence (PSP) was observed for all samples, and phototransferred thermoluminescence (PTTL) has also been confirmed following IR stimulation suggesting that recombination mechanisms involve charge mobility and interactions with shallow traps in band-tail states rather than primarily being associated with direct quantum tunnelling recombination. The relative proportions of PSP to prior IRSL also varied between the two stimulation wavelengths which further emphasises the physical differentiation between processes influencing different subpopulations of the trapping states associated with IRSL and their local lattice environments.