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^, 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 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. 51 meV in TlBr and 112 meV in TlCl. The exciton-optical transition is discussed and the importance of lattice and polaron effects emphasized.