High-frequency (94.9 GHz) EPR spectroscopy of paramagnetic centers in a neutron-irradiated sapphire single-crystal fiber

Initial results are reported of an EPR study, conducted at 94.9 GHz, of a thermally annealed, neutron-irradiated white sapphire (α-Al₂O₃) single-crystal fiber. The optical centers produced in sapphire by neutron irradiation and thermal annealing are of interest for optical technologies involving the phenomenon of spectral hole-burning. While these centers have been modeled as consisting of electrons localized at anion vacancies, experimental tests of this model have been very limited. EPR spectroscopy — a choice technique for elucidating structural details of such color centers — reveals signals from numerous paramagnetic centers in this material. The predominant signals, with amplitudes a hundredfold greater than any other signals, derive from three closely related, highspin-multiplicity centers. These centers do not, however, derive from radiation-induced lattice defects: they are readily identified as Cr³⁺ (S = 3/2) and a pair of crystallographically equivalent Fe³⁺ (S=5/2) impurity ions. The advantages of high-frequency-EPR instrumentation in facilitating this identification are presented and discussed in detail. These advantages include enhanced sensitivity for this volume-limited, fiber sample. Moreover, the analysis of the spectra — entailing spectral assignments, evaluation of the spin-Hamiltonian parameters, and spin-counting — is greatly facilitated when the Zeeman interaction is dominant.