Faraday Rotation in the Absorption Edge of Amorphous Selenium

The Faraday rotation in amorphous Se has been measured with an ac technique between 1.1 and 0.64 μ (1.127 - 1.94eV). The results are compared with a theoretical dispersion formula based on the Tauc model of optical absorption in an amorphous semiconductor. A key feature of the model is a description of the violation of the $k$-conservation rule, induced by the disorder of the amorphous state, with a single parameter $z$. For $z=0\mathrm{}$ the dispersion formula reduces to the standard one for direct transitions, and for $z>\mathrm{}1\mathrm{}$ it reduces to the form for indirect transitions. A satisfactory fit is obtained with $z=0.2\mathrm{}$, which indicates a range of local order about half that in amorphous Ge. The gap energy obtained in the fit is $E_g=2.13\mathrm{}$ eV. This value is interpreted as further evidence for a two mechanism absorption in the absorption edge in amorphous Se. One is due to localized-molecular states and the other to delocalized bandlike states. The present experiments have allowed an optical resolution of these two absorption mechanisms, because of the different magneto-optical properties of the associated states.