This thesis presents a technique to locate the ground state of impurity ions relative to the host conduction band, using thermoluminescence. The technique makes use of the concept that thermoluminescence probes the occupation of electron traps, and the fact that these traps are filled via promotion of impurity electrons into the conduction band. Using tunable radiation we determine the threshold for trap filling, which is given by the ionization threshold of the impurity. This technique is a form of excitation spectroscopy, sensitive to electron (or hole) transport processes in solids. Advantages of thermoluminescence excitation spectroscopy over complementary techniques, such as photoconductivity, include impurity specific signals, applicability to both bulk and power samples, and high sensitivity.