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 (S8)
rings and short chains, with a small concentration of hexameric rings (S6). 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 (~106 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.
A tunable terahertz source (TTS) based on a Smith-Purcell emitter will be described. The tunable THz source is analogous to low frequency electron beam devices such as magnetrons, backward wave oscillators and traveling wave tubes. The device offers continuous tunability, compactness, and robust operation. Examples of THz spectroscopy will be given.
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