THE REFLECTION AND ABSORPTION SPECTRUM OF LIQUID AMMONIA IN THE INFRARED

Abstract

In view of the importance of $NH_{3}$ in the atmospheres of the major planets, there is a need for knowledge of the optical constants of $NH_{3}$ in condensed states. We have attempted to obtain these constants for liquid $NH_{3}$ by quantitative measurements of spectral reflectance and spectral absorptance. Spectral reflectance at near-normal incidence was measured from 5100 to $350 cm^{-1}$ by comparing the reflection at a free liquid surface with reflection from a plane reference mirror, the mirror’s reflectance having been measured in an auxiliary reflectometer. The Lambert absorption coefficients for liquid $NH_{3}$ from 7000 to $980 cm^{-1}$ were determined by the methods used in our earlier study of $water.^{1}$ The imaginary part of $n_{i}$ of the refractive index was determined directly from the Lambert coefficients. From the spectral reflectance data and $n_{i}$, the real part $n_{r}$ of the refractive index can be determined from the Fresnel equation. Values of $n_{i}$ and $n_{r}$ can also be determined from an analysis of the spectral reflectance data using the Kramers-Kroning relations. There was close agreement between the $n_{r}$ values determined by the two methods.