Davydov splitting of degenerate lattice modes in the layer compound GaS

The frequency separation between the essentially degenerate in-plane TO mode and its Davydov conjugate has been observed in the layer compound GaS in a measurement of the Raman and far-infrared spectra. The relative magnitude of the frequencies of the two modes is contrary to what is expected from interlayer Van der Waals forces. Wieting and Verble have observed a similar frequency reversal in the Davydov conjugates in the spectra of Mo${\mathrm{S}}_2$ and $\beta$-GaSe. They have argued that the long-range Coulomb interaction could be responsible for this optical anomaly. Our calculation shows, however, that the effect of such interaction in the deformation-dipole model approximation can not explain the anomaly.