Thermoluminescent excitation measurements of photoionization in doped insulators
Fleniken, Julian david
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This study is focused on the utilization of Thermally Stimulated Luminescence (TSL) as an excitation spectroscopic technique capable of measuring photoionization and charge transfer in optically active materials. These electron transfer processes play an important role in phosphors, scintillators, photovoltaic and photorefractive materials, spectral (chemical) hole burning and can influence the efficiency of solid state lasers to name a few examples. The primary materials investigated in this study are rare earth and transition metal doped insulators where optical absorption and emission is explained in terms of electronic transitions of the impurity. The optical behaviors of many such systems have been extensively studied and are well explained based on theoretical models describing the effect of the host lattice environment as a perturbation of the impurity free ion potential. However, a full description of the optical properties of these materials must include the effect of electron transfer, which is comprised of transitions involving the transfer of electronic charge from the localized states of the impurity to the delocalized states of the host conduction and valence bands. TSL is utilized to this purpose in two manners; firstly, the direct measurement of the relative strength of the electron transfer process can be measured and secondly, the energy of the impurity ground state relative to the host conduction and valence bands can be determined. These type measurements are often performed using photoconductivity and photoelectron emission, but this dissertation illustrates that TSL offers many advantages such as increased sensitivity in many materials, discrimination between different types of electron transfer processes and impurity sites, and versatility of measurable sample types. Measurements of CaS:Ce3+ and SrS:Ce3+ show clear evidence of the observation of multiple photionization processes. Measurements using a two-color excitation scheme performed on ruby (Al2O3:Cr3+) allow for the determination of photoionization as distinguished from charge transfer.