Dynamics of the deep red Fe³⁺ photoluminescence emission in feldspar

We present here new characteristics of the Fe³⁺ emission in feldspar using a combination of site/energy-selective spectroscopy and time-resolved photoluminescence at cryogenic temperatures. Although the variation in the peak energy of Fe³⁺ emission has been known to vary across feldspar samples depending on the composition and structure, it has never shown before that this emission can vary dynamically even within a single sample. We show that a strong excitation-energy dependent emission (EDE) occurs in Fe³⁺ for non-resonant excitation at cryogenic temperatures; this characteristic can be exploited for site discrimination within a complex feldspar sample. The radiative relaxation in Fe³⁺ (⁴T₁ → ⁶A₁) is shown to be a dynamic process depending on whether the ion is excited using resonant or non-resonant transition. We suggest that during resonant excitation, the excited state of Fe³⁺ undergoes a slow energy-transfer, possibly a spin-lattice interaction, a process that leads to a non-first order effect in the radiative relaxation process.