Infrared and Raman Studies of Long-Wavelength Optical Phonons in Hexagonal MoS2

Infrared and Raman measurements of lattice vibrations in hexagonal Mo${\mathrm{S}}_2$ have been made over the combined range 20-4000 ${\mathrm{cm}}^{-1\mathrm{}}$. Two infrared-active modes at 384 and 470 ${\mathrm{cm}}^{-1\mathrm{}}$ and three Raman-active modes at 287, 383, and 409 ${\mathrm{cm}}^{-1\mathrm{}}$ have been observed. Classical dielectric oscillators are fitted to the reflectivity data, which are also analyzed by means of the Kramers-Kronig relations. The degeneracy of the $E_{1u}$ and $E_{2g}^1$ modes is interpreted as indicating that the layer-layer interaction is weak. All 15 optical modes are assigned to irreducible representations and their frequencies tabulated or predicted.