Abstract
Resonant Raman scattering has been studied in trigonal Se at low temperatures in the region of its indirect and direct excitonic absorption edges. Whereas no enhancement in Raman cross sections was observed at the indirect absorption edge, strong dispersion was found in the region of the direct excitons. In the vicinity of the direct absorption edge Raman cross sections of one-phonon modes decreased monotonically with increase in photon energy in all three scattering configurations studied. This monotonic decrease in Raman cross sections was explained qualitatively by resonant cancellation between a constant background and a dispersive term due to the 2.20-eV peak in the reflectivity spectrum of Se. Resonant enhancements were observed at the direct excitons only in the scattering configuration where the incident and scattered photons are both polarized perpendicular to the $c$ axis. The enhancements are explained quantitatively by a simple model involving two direct excitons of Se as resonant intermediate states. We have also determined directly the longitudinal and transverse mode splitting of the low-energy $E$ mode in trigonal Se as 7 \ifmmode\pm\else\textpm\fi{} 2 ${\mathrm{cm}}^{\ensuremath{-}1}$.
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