Terahertz spectroscopy of molten sulfur using a tunable THz source

We have studied the liquid-liquid allotropic transitions in molten sulfur using terahertz (THz) spectroscopy. Liquid sulfur is selected as an initial choice of materials because its structure and properties are well established from previous in-situ studies by one of the current investigators (and by other researchers) using a variety of physical and chemical methodologies. It is known that sulfur melts to an equilibrium mixture of octameric (S 8 ) rings and short chains, with a small concentration of hexameric rings (S 6 ). As temperature is increased, thermal energy initiates ring scission and the resulting diradically-terminated short chains undergo covalent bonding to induce polymerization at 159-166°C. Further increase in temperature causes an increase in chain length and an increase in chain species concentration until a temperature of 188°C is reached at which the long chains (~10 6 atoms in length) undergo chain scission, and although the chains start to break up, the polymer concentration of the mixed phases still increases. We have experimentally mapped THz absorption, transmission, and reflection/scattering effects with these known transitions in liquid sulfur, as a function of temperature and wavelength.