Resonant Raman scattering in GaSe

The resonant Raman spectrum of GaSe has been investigated using a cw tunable dye laser. A detailed investigation of the wavelength dependence of the scattering efficiency has been carried out in the region just below the energy gap, which for GaSe falls midway in the spectral range of the dye laser. The scattering efficiency was observed to undergo a maximum when the laser energy was equal to the energy of the free exciton. The general wavelength dependence was found to be in good agreement with that predicted by Martin for $q$-dependent scattering. The observed angular independence of the scattering is, however, in discord with this theory, and for one mode, ${A_2}^{\prime \prime }\left(\mathrm{LO}\right)$, momentum conservation is observed to be broken. The nonpolar modes have been observed to exhibit a strong antiresonant behavior similar to that observed in CdS but the present results cannot be interpreted in terms of the previous empirical models.