Knowledge Bank

Infrared absorption spectra of single crystals of sodium and potassium nitrate have been obtained under moderately high resolution and at temperatures down to $5\circ K$ . Obsevations have also been made using polarized light with oriented single crystals. A number of absorption bands due to isotopic molecular species in their natural abundance have been identified. The splitting of the normally degenerate vibrational modes ($\nu +a$ and $\nu 4$) in $NO18O216-$ and $NO17O216-$ is rather large and provides additional data for the calculation of intramolecular force constants. A number of combination and overtone bands have also been observed for the various isotopic species in natural abundance but it is doubtful that a complete set of anharmonicity constants can be obtained. The influence of crystal structure on certain aspects of the spectra will be discussed. Of particular interest is the totally symmetric mode of the nitrate ion which is forbidden in $NaNO3$ but allowed in $KNO3$. Frequency shifts with temperature have been accurately measured and follow a pattern very similar to that observed in other ionic solids.