Oscillatory Magneto-Optical Absorption and Exciton-Phonon Interaction in Thallous Halides

Oscillatory magneto-absorptions due to excitons have been studied in TlCl and TlBr. In zero magnetic field, absorptions due to exciton-phonon quasi-bound state and one and two free-phonon states of exciton quasicontinuum are observed. In magnetic field, members of s exciton series with a subsidiary peak on the high energy side appear. The exciton binding energy is 11.7 meV in TlCl and 9.8 meV in TlBr and the reduced mass is 0.24 4 m 0 in TlCl and 0.23 4 m 0 in TlBr. A direct evidence of exciton polaron effect is obtained by observing the clamping of zero-phonon exciton lines of high quantum number in the vicinity of one free-phonon threshold of 2 s exciton. Above the threshold, absorptions of another series grow as the field increases. They are one-phonon replicas of zero-phonon excitons in magnetic field. The existence of two minima in the conduction band of each halide is suggested.