Ammonia Ices Revisited: New IR Intensities and Optical Constants for Solid NH3

Solid ammonia (NH3) is the only nitrogen-containing polyatomic molecule reported in both interstellar and solar system ices. However, an examination of the literature reveals significant omissions and difficulties in earlier work that can hinder quantitative measurements of solid NH3 by infrared (IR) methods by both astronomical observers and laboratory spectroscopists. Here we reinvestigate the IR spectra of NH3 ices in amorphous and crystalline forms to determine mid- and near-IR intensities. The IR absorption coefficients, band strengths, and optical constants are presented for both amorphous and crystalline NH3, along with new density and refractive index (lambda = 670 nm) measurements needed to quantify our IR results. We find that two widely used approximate IR band strengths for amorphous NH3 are nearly 30% higher than measured values after corrections for the compound's density. We have also used our new results to rescale two NH3 near-IR band strengths in the literature, finding that they increase by about 60%. Some applications of our new results are described along with suggestions for future studies. Optical constants are available in electronic form.