Resonant enhancements of multiple-phonon Raman scattering in CuGaS2 excited by a wide range of laser energy are studied. At 8.6 K, the intensities of the multiple-phonon modes show a sharp resonance maximum at the scattered-photon energy just below that of the free exciton. Polarization characteristics of these modes have the same selection rule as the free exciton. The behavior is attributed to the intraband Fröhlich interaction for longitudinal E modes of polar phonons. The resonant scattering by this mechanism exhibits a temperature-dependent enhancement correlated with the thermal shift and broadening of the free exciton energy. On the other hand, the temperature dependence of the resonance is greatly decreased for the nonpolar A1 mode which has a large deformation potential.
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Susaki et al. (1996) studied this question.