Far IR transmission measurements on triglycine sulphate (TGS), at 5K

Abstract

New measurements with a special cryostat, and a high resolution grid interferometer show that TGS exhibits Far Infrared (FIR) oscillations at θ = 5K with much smaller damping coefficients (δ = δv/v), than previously described (Gerbaux, X. and Hadni, A. (1991). Int. J. Infared and Millim. Waves, 12, 981). The values of δ are smaller than 10⁻⁴ for several lines. The lowest frequency and strongest line, located at 37.3 cm⁻¹ can be used to predict a low temperature peak in pyroelectric response at 15.4 K, which has been observed at 16K (Mangin, J. and Hadni, A. (1983). Phys. Rev. B, 27, 7730). The low damping coefficient of the line seems to rule out an explanation in terms of anharmonicity, and strongly favors one with a second-order dipole moment which has been shown to give the same temperature behaviour (Szigeti, B. (1975). Phys. Rev. Let., 35, 1532). With Lorentz oscillators of such low damping coefficients, it is possible to fit the spectra of crystal plates with thicknesses extending from a few micrometers to several millimeters. This shows that they are homogeneous, and any evidence of a surface layer different from the bulk has not been found at low temperatures.

Key Words:

• Far infrared spectra
• triglycine sulphate
• amino acids
• surface layers
• pyroelectricity
• harmonicity