THz (terahertz) properties of MgF2 crystal (sample 1) and MgF2: Co crystal (sample 2) is measured using transmission THz time domain spectroscopy in the frequency range between 0.5 and 2.5 THz. The absorption coefficients α(ν) of sample 1 increases and has a maximum 24cm-1 with increasing frequency ν from 0.5 THz to 2.5 THz. The absorption coefficient of sample 2 is much larger than that of the sample 1, and its absorption band edge of the crystal lattice shifts to lower frequency side because of the dopant Co. Moreover, there is an absorption peak at 1.9THz and the absorption coefficient is greater than 70 cm-1, the force constant K of F-—Co2+ ionic chemical bond stretch vibration calculated from this peak is 3.40×10-2 N/cm. The THz spectroscopy can be an important technique for research of chemical bond in crystal. Real dielectric functions between 0.5 and 2.5 THz of the two samples are calculated by relations between optical constants. The ε1(ν) of sample 1 varies from 4.67 to 4.73 and ε1(ν) of sample 2 varies from 4.62 to 5.01.
Terahertz time-domain spectroscopy has been used to measure the spectral properties of polycrystalline asparagines at room temperature. The absorption spectra and the indexes of refraction are obtained. This result shows the sample has the spectral response in terahertz spectral range and the fact demonstrates the terahertz spectra of asparagine could be used to study the molecular structure and vibration of sample. Two absorption peaks, the broad band ranged from 1.642 to 1.758THz and the narrow one located at 2.266THz, are observed in the frequency range 0.5—2.4THz. The assignments of vibrational modes in this spectral region are made using density functional theory at the level of B3LYP/6-31+G(d,p). The theoretical results agree well with the experimental data.
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