Optical Absorption of Cuprous Oxide

The relative optical absorption coefficient $\alpha$ of polycrystalline slabs of cuprous oxide was measured at 295°K, 77°K, and 4.2°K. At 4.2°K, $\alpha$ is proportional to ${[\sigma -E_0]}^{\frac{1}{2}}$ for $16510 {\mathrm{cm}}^{-1\mathrm{}}<~\sigma <~16900 {\mathrm{cm}}^{-1\mathrm{}}$, where $\sigma$ is the wave number. At 77°K an additional component appears, so that $\alpha ={\alpha }_1+{\alpha }_2$, with ${\alpha }_1\propto {[\sigma -E_1]}^{\frac{1}{2}}$ and ${\alpha }_2\propto {[\sigma -E_2]}^{\frac{1}{2}}$. This is attributed to indirect transitions to exciton levels, in agreement with a theory by Elliott. The ratio of the integrated absorption coefficient of the first two exciton lines, after corrections for the background were applied, is also in satisfactory agreement with the Elliott theory.