  Terahertz time domain spectroscopy (THz TDS) is used to measure the melting
kinetics of fructose molecular crystals. Combining single crystal anisotropy
measurements with density functional calculations we assign the phonon
frequencies and interrogate how specific phonons behave with melting. While
nearly all the low frequency phonons continuously red shift with heating and
melting, the lowest energy phonon polarized along the c-axis blue shifts at the
melting temperature, suggesting an initial structural change immediately before
melting. We find that the kinetics follow a 3D growth model with large
activation energies consistent with previous differential scanning calorimetry
(DSC) measurements. The large activation energies indicate multiple H-bonds
must break collectively for the transition. The results suggest the generality
of the kinetics for molecular crystals and that THz TDS with picosecond
resolution could be used to measure ultra-fast kinetics.
