Abstract
The far-infrared absorption spectra due to the two-phonon difference process have been observed. Peaks are found at about 27 and 35 cm -1 in TlCl and at 35.2 cm -1 in TlBr, which are due to saddle points in the difference energy surface of two phonons. The second order Raman spectra of TlCl and TlBr at low temperature have been measured and the assignments of the spectral peaks to particular combinations of two phonons are made. In the near resonant region, the scattering efficiency of the overtone of the LO phonon shows a more remarkable resonant behavior than that of other phonons. From the analysis of the experimental result by the theory of Ganguly and Birman, it is concluded that the two LO phonons at the \(\varGamma \) point are successively scattered by the exciton, while two phonons of other modes with finite wave vectors are, presumably, scattered simultaneously.
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