Resonant Raman effect in cinnabar

In α-HgS, cinnabar, grown by chemical vapor transport, a sharp photoluminescence feature is found at an energy E(X₁)=1.873 eV. Absorption measurements show that this peak is due to resonance fluorescence associated with an electronic transition. When the Raman spectrum of such samples is excited with laser energy ωL in the range 1.865-1.885 eV provided by a dye laser, new Raman lines appear at 21, 33.5, 67, and 101 cm^-1. These lines exhibit a pronounced resonance in the spectral dependence of their scattering intensity. The 33.5- and 67-cm^-1 lines, for which we have a complete spectral dependence, show a resonance peak at ωL=E(X₁) as well as for ωL=E(X₁)+the phonon energy. The shape of their resonance curves agrees well with that predicted by the theory of the resonance Raman effect. These resonance curves also exhibit a peak at ωL=E(Z₁)+the phonon energy, where E(Z₁)=1.870 eV. The 33.5-, 67-, and 101-cm^-1 Raman lines are ascribed to the first, second, and third harmonics of a Raman inactive A₂ (TO) mode, activated under resonance conditions as a result of a breakdown of the usual selection rules. The 21-cm^-1 line can be interpreted as an "in-band resonant mode." The zone-center Raman-active phonons at 43 and 48 cm^-1, of A₁ and E symmetry, respectively, also show a resonance enhancement for ωL=E(X₁).