The optical properties of liquid chalcogens

The temperature and pressure variations of the optical reflectivity of liquid Te, Se, Se₈₀Te₂₀ and Se₈₀S₂₀ have been measured in the energy range of 0.5-5.4 eV. The optical conductivity, σ(ω), has been derived by using an oscillator fitting analysis. The σ(ω) curve of liquid Te, which has a stacked planar zigzag geometry, has a broad peak around 2.2 eV and a Drude term on the low-energy side. The peak can be mainly ascribed to the transition from the bonding σ band to the antibonding σ^* band and from the filled σ^* band to the unfilled σ^* band. On the other hand, the σ(ω) curve of liquid Se at 500°C has a broad peak around 3.5 eV, maintaining a semiconducting character. The peak is assigned to the transition from the lone-pair band to the σ^* band. With increasing temperature, the peak shifts to the lower energy side. The optical gap closes between 1100 and 1300°C and a Drude term appears. This implies that the structure of liquid Se changes from the spiral chain geometry to the geometry of the metallic liquid Te. The temperature of the semiconductor-metal transition is lowered by replacing Se with Te.