Observation of color centers inα- and Naβ- alumina by electron energy loss and optical spectroscopies

We have measured the Auger, electron-energy-loss, and optical-transmission spectra of $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ and Na $\beta -{\mathrm{Al}}_2{\mathrm{O}}_3$ from 0 to 45 eV and from 3 to 6 eV, respectively. Although Auger measurements were not able to distinguish between the two types of materials, energy-loss measurements showed additional absorption near 4.5 eV in Na $\beta -{\mathrm{Al}}_2{\mathrm{O}}_3$. The absence of strong impurity absorption in this spectral region in melt grown Na $\beta -{\mathrm{Al}}_2{\mathrm{O}}_3$ enabled us to resolve this structure into two peaks of approximately equal strength by optical transmission.