Photoconductivity Measurements in Ruby and Sapphire

Photoconductivity in ruby was discovered during an unsuccessful attempt to explore the Zeeman splitting of the fluorescent optical levels in ruby with a microwave spectrometer. Investigations showed that sapphire, the ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ host crystal of ruby, is responsible for this photoconductivity. At 350°K and a light intensity of about 5 W/${\mathrm{cm}}^2$ (high pressure $H_g$ arc spectrum between 3500 to 7000 Å) the order of magnitude of these photoconductivities for sapphire is ${4.10}^{-8\mathrm{}} {\left(\Omega m\right)}^{-1\mathrm{}}$ for 9390-Mc/sec microwave absorption with 100-cps modulated illumination; ${10}^{-11\mathrm{}} {\left(\Omega m\right)}^{-1\mathrm{}}$ for photoconductivity with 100-cps modulated illumination; and ${4.10}^{-13\mathrm{}} {\left(\Omega m\right)}^{-1\mathrm{}}$ for photoconductivity with steady illumination. Temperature and light frequency dependence of the conductivities were investigated and a Hall-type experiment pointed to electrons as charge carriers. For sapphire these results can be explained by assuming that electrons from an occupied impurity level are excited by light to the 7-eV conduction band. According to the measurements, this impurity level would have to be 0.85 to 1.1 eV below the 7-eV conduction band.