Exciton-Optical Properties of TlBr and TlCl

The effect of strain on thallous-halide films has been investigated, and a technique developed for producing essentially strain-free films. The optical absorption of strain-reduced TlBr and TlCl films has been measured down to 4.2°K, and the results analyzed for the optical constants. Relatively narrow (2-meV) firstexciton lines were found in the near ultraviolet accompanied by longitudinal-optical-phonon sidebands. The Faraday rotation pattern was measured through the first peak in TlBr and found to be linear in $H$. An analysis of the rotation pattern together with the known $n$ and $\kappa$ yields an exciton effective $g$ value of 0.7 for TlBr. Oscillatory magnetoabsorption has been observed in both TlBr and TlCl originating from the edge of the sideband. The exciton binding energies are 6.5±1 meV in TlBr and 11±2 meV in TlCl. The exciton-optical transition is discussed and the importance of lattice and polaron effects emphasized.