  Raman spectra of few-layer WS2 have been measured with up to seven excitation
energies, and peculiar resonance effects are observed. The two-phonon acoustic
phonon scattering signal close to the main E2g1 peak is stronger than the main
peaks for excitations near the A or B exciton states. The low-frequency Raman
spectra show a series of shear and layer-breathing modes that are useful for
determining the number of layers. In addition, hitherto unidentified peaks (X1
and X2), which do not seem to depend on the layer thickness, are observed near
resonances with exciton states. The polarization dependences of the two peaks
are different: X1 vanishes in cross polarization, but X2 does not. At the
resonance with the A exciton state, the Raman-forbidden, lowest-frequency shear
mode for odd number of layers appears as strong as that for the allowed case of
even number of layers. This mode also exhibits a strong Breit-Wigner-Fano line
shape and an anomalous polarization behavior at this resonance.
