Absorption Peak Shape of the Lowest Vibrational Mode of β-form D-mannitol Observed by High Frequency Accuracy Terahertz Spectroscopy

Absorption peak shape of the THz spectrum at low temperature was analyzed in detail using a CW-GaP THz spectrometer with high frequency accuracy and high frequency resolution. The absorption peak of the lowest vibrational mode of β-form D-mannitol has a very sharp FWHM below 1 GHz. The high-quality crystal shows a symmetrical peak shape that can be fitted by the Lorentz function, but becomes asymmetric and a tail is observed at the lower frequency side when defects are introduced. This may be the reason of the peak frequency shift with increasing crystal defects.