Raman spectrum of ammonium chloride and its variation with temperature

Abstract

Using the λ 2536·5 mercury resonance radiation as exciter, the Raman spectrum of ammonium chloride has been investigated in detail over the range of temperature from 85° T. to 395° T. The recorded spectra at all temperatures exhibit many more frequency shifts than those obtained by the previous workers. The present study reveals the following facts: (1) The Raman speetrum in the region of frequency shifts 90-200 cm.^-1 exhibits seven lattice lines, the frequency shifts and intensities of which show marked variations with temperature. These lines have been identified as the fundamental modes of vibration of the cæsium chloride structure, in which the NH₄ ions osciUate as units, as demanded by the new crystal dynamics. (2) The lattice spectrum persists far above the transition temperature. This fact indicates that Pauling's hypothesis that there is onset of free rotation of the NH₄ ions immediately above the transitiort temperature is not corrcct. (3) The internal oscillations of the NH₄ ions exhibit nine distinct Raman shifts, showing thereby that these osciUafions are not those of a freely vibrating ting tetrahedrally symmetric group. (4) The spectra recorded at all temperatures reveal the presence of Raman shifts characteristic of HCl and NH₃ molecules.