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Phonon Kinetics at the Solid to Liquid Phase Transition

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.