Time-resolved infrared stimulated luminescence of the blue and yellow-green emissions: Insights into charge recombination in chemically and structurally different alkali feldspars

Time-resolved luminescence measurements can be used to explore luminescence processes in minerals and the defects involved. It has also been applied to feldspars and knowledge has been gained regarding potential crystal defects associated with luminescence productions in these minerals, but also regarding processes governing electron-hole recombination leading to luminescence emission.

Here we present time-resolved infrared stimulated luminescence (IRSL) signals measured for a range of mineralogically well characterised single crystal alkali feldspars. We explore time-resolved luminescence for the blue (∼410 nm) and the yellow-green emission (∼550 nm) in response to different irradiation doses and by comparing different IRSL signals. Firstly, we explore whether the lifetimes measured represent excited state or recombination lifetimes. Secondly, we investigate sample-dependent changes in blue and yellow-green time-resolved signals and link those to physical properties of the samples.

Our results show that the timescales on which the blue and the yellow-green emission occur differ significantly, with the blue signal on the μs-scale, and the yellow-green emission on the ms-scale. We do not observe any dependence of the time-resolved signal on signal integration, dose given or IRSL signal measured. However, inter-sample variability is shown for both emissions. In the blue we only observe small differences in decay time scale between single-phase feldspars and perthites, however larger differences are measured between samples that were artificially disordered compared to ordered feldspars. Longer lifetimes observed for disordered feldspars are suggested to be linked to either changes in the recombination centre or to increased band-tail states transport due to an increase in the width or density of the sub-conduction band-tail states. The data indicates the potential of using time-resolved IRSL of the blue emission to get an indication of the state of order of a feldspar. For the yellow-green emission slow signal decays are observed for single-phase feldspars, likely indicating a spin-forbidden transition. Interestingly, similar lifetimes were observed for K- and Na-feldspar end members.