Absorption Spectra at 4°K of Single Crystals of Normal and Isotopically Substituted Sodium and Potassium Nitrates

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Abstract
Sharp line structure is observed in the low energy shoulder of the 3000‐A absorption band of crystalline nitrates that have been cooled to 4°K. Single crystals of normal and of nitrogen‐15 isotopically substituted sodium and potassium nitrates were examined spectrographically. Correlation between the spacings, polarization and orientation properties, isotope effect, and crystal field splitting of the spectra permitted their analysis. In terms of the D3h point symmetry of a free nitrate ion the excited electronic state is E′. This electronic configuration is one of three considered likely upon the basis of a LCAO molecular orbital treatment of the ion. The two asymmetric E′ vibrations of the ion predominantly contribute to the band structure. A large Franck‐Condon shift, an extensive weakening of force constants, and the character of the excited vibrations indicate that the excited ion is planar but expanded with perhaps some, but not a great amount of asymmetric distortion. Predissociation appears at a level corresponding to a dissociation energy of 3.76 ev for the nitrate ion.

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