ATR Infrared Spectra of Uniaxial Nitrate Crystals

The experimental ATR bands for the ${\nu }_3$ nitrate ion mode, in particular as measured for trigonal alkalimetal nitrate crystals with their trigonal axes perpendicular to the ATR element–sample interface, display interesting shapes which are radically different for TM and TE polarizations. Further, there is only a marginal amount of overlap of the intense bands which appear for the two polarizations. It is shown that the band structures are in complete accord with expectations based on a damped oriented oscillator model of the nitrates combined with the complex form of Snell's law and Fresnel's reflection equations as developed for a uniaxial crystal. The theoretical development indicates that the TM band structure closely reflects both the transverse and longitudinal modal frequencies since the band peaks sharply near ${\nu }_3\mathrm{(t)}$ and has an inflection point corresponding to ${\nu }_3\mathrm{(l)}$ . This insight is used to deduce ${\nu }_3\mathrm{(t)}$ and ${\nu }_3\mathrm{(l)}$ from ATR data for the high‐temperature trigonal forms of sodium and potassium nitrate.