Exciton Absorption and Photoconductivity in Mercurous Halides

The optical absorption spectra of thin films of ${\mathrm{Hg}}_2$ ${\mathrm{F}}_2$, ${\mathrm{Hg}}_2$ ${\mathrm{Cl}}_2$, ${\mathrm{Hg}}_2$ ${\mathrm{Br}}_2$, and ${\mathrm{Hg}}_2$ ${\mathrm{I}}_2$ were measured in the wavelength range 190-750 mμ at 77°K. and 300°K. With the exception of ${\mathrm{Hg}}_2$ ${\mathrm{F}}_2$, sharp exciton peaks were observed in the spectra of all these halides, both near the fundamental absorption edge and at higher energies. The multiplicity and positions of the exciton peaks are characteristic of the Mott-Wannier (delocalized) type of excitons. Absorption peaks due to the splitting of the halogen doublet were also observed. The photocurrent spectrum of ${\mathrm{Hg}}_2$ ${\mathrm{I}}_2$ shows close resemblance to the corresponding absorption spectrum.