Absorption Coefficients for Exciton Absorption Lines

The absorption coefficient for excitons is calculated in the effective-mass approximation. The coefficient is shown to be decreased over what one would expect for a collection of free atoms by the factor ${\epsilon }^{-1\mathrm{}}{\left(\frac{a_0}{a_B}\right)}^3$, where $\epsilon$ is the dielectric constant and $\frac{a_0}{a_B}$ is the ratio of the lattice constant to the exciton radius. This result seems not inconsistent with the identification of the observed absorption lines in ${\mathrm{Cu}}_2$O, CdS, Hg${\mathrm{I}}_2$, Pb${\mathrm{I}}_2$, and Cd${\mathrm{I}}_2$ as exciton lines.