The infra-red absorption spectra of sodium nitrate and calcite

Abstract

The infra-red absorption spectra of sodium nitrate and calcite crystals have been studied in the range 1 μ to 22 μ with a Beckman infra-red spectrophotometer, using three different thicknesses of a cleavage section in each case. The thinner plates exhibit the structure of the more intense absorption bands while the thicker ones show their harmonics as well as the weaker bands very clearly. The remarkable sharpness of the harmonics is a noteworthy feature. Much new detail has been observed with sodium nitrate in the region of wavelength greater than 8 μ . A strong absorption in the region beyond 17μ has been found in both crystals. This is ascribable to harmonics of lattice oscillations. The entire body of data is satisfactorily explained in terms of the normal modes of vibration jointly of the two units (NaNO₃ or CaCO₃) contained in the unit cell of the crystal lattice.