Laboratory measurements of the 7.5–9.38-mm absorption of gaseous ammonia (NH 3) under simulated jovian conditions

Abstract

Previous laboratory results have verified that to within experimental limits, the modified Ben-Reuven lineshape correctly describes the opacity of gaseous ammonia (NH 3) in an H 2/He atmosphere from 1.38 to 18.5 cm (1.62–21.7 GHz) (P. G. Steffes and J. M. Jenkins 1987, Icarus 72, 35–47). However, at wavelengths shortward of 1 cm, significant uncertainties exist as to the actual absorption spectrum of gaseous ammonia under simulated planetary conditions. We have made laboratory measurements of the millimeter-wave opacity from gaseous ammonia under simulated conditions for the Jovian atmosphere so that the abundance and distribution of ammonia can be more accurately inferred from radio emission measurements. The measurements were made at several frequencies between 32 and 40 GHz (7.5–9.38 mm), at a temperature of 203°K, and at a total pressure of 2 atm in a mixture consisting of 88.34% hydrogen, 9.81% helium, and 1.85% ammonia. The results of these measurements are significant in that the experimental data can be modeled quite well by using the lineshape factor given by E. P. Gross (1955, Phys. Rev. 97, 395–403) in place of that given by J. H. Van Vleck and V. F. Weisskopf (1945, Rev. Mod. Phys. 17, 433–443) as applied to the given conditions. Various parameters in the Ben-Reuven lineshape can also be modified to give a good fit to the laboratory results. More laboratory results at shorter wavelengths will be needed before it is clear which theory will be able to best describe the opacity of ammonia over the entire inversion spectrum.