Far-infrared spectrum of N2 and N2-noble gas mixtures near 80 K

The far-infrared absorption spectra of gaseous nitrogen, and mixtures of nitrogen with the foreign gases argon and neon, have been measured at pressures near 1 atm and at temperatures near 78 and 89 K. Spectra were obtained over the wave number range 20–100 cm−1 using a Fourier transform spectrometer and a multiple reflection absorption cell of 52 m path length. They show a broad continuum associated with the pure rotational collision-induced S branch of the N2 molecule plus structure attributed to transitions in dimers, not previously observed in this spectral region. In the case of N2–Ar, there are strong similarities with the fundamental vibrational band under similar conditions of pressure and temperature as reported by McKellar [J. Chem. Phys. 88, 4190 (1988)]. The integrated absorption coefficient has been evaluated for the N2–N2 rotational band; at 78 K it equals 3.1×10−31 cm5 sec−1, a factor of 2 greater than typical values at temperatures above 100 K.