Improved mode assignment for molecular crystals through anisotropic terahertz spectroscopy.

Rohit Singh, Deepu Koshy George, Jason B Benedict, Timothy M Korter, Andrea G Markelz

Department of Physics, University at Buffalo, Buffalo, New York, USA.

The journal of physical chemistry. A 2012 Oct 25

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We report the anisotropic terahertz response of oxalic acid and sucrose crystals in the 0.2-3.0 THz range using terahertz time domain spectroscopy on large, single crystals. We compare the observed anisotropic response with the response calculated using solid-state density functional theory and find good agreement in the orientation dependence and relative intensities of the crystal phonons. It was found that oxalic dihydrate can be reversibly converted to anhydrous by controlled relative humidity. In addition, oxalic acid was found to have a large birefringence with Δn = 0.3, suggesting the material may be useful for terahertz polarization manipulation. Sucrose has a smaller birefringence of Δn = 0.05, similar to that of x-cut quartz. The anisotropic measurements provide both mode separation and symmetry determination to more readily achieve mode assignment for the more complex sucrose spectrum.

Citation

Rohit Singh, Deepu Koshy George, Jason B Benedict, Timothy M Korter, Andrea G Markelz. Improved mode assignment for molecular crystals through anisotropic terahertz spectroscopy. The journal of physical chemistry. A. 2012 Oct 25;116(42):10359-64