The octopole and hexadecapole moments of N2O from collision-induced absorption measurements

The absorption spectrum of gaseous N2O was measured in the spectral region from approximately 15–120 cm−1 at 11 different densities from 3.3 to 64.5 amagat. (An amagat is defined as the density of a gas at N.T.P.) The data were obtained with a Michelson Fourier transform IR spectrometer and a far IR laser using a stainless steel absorption cell 9.26 cm in length. From the density dependence of the absorption, the collision-induced spectrum above approximately 60 cm−1 was clearly separated from the relatively strong allowed transitions peaking at the lower frequency near 22 cm−1. From theoretical expressions for the line shape, derived using information theory, and a reliable value of the quadrupole moment (obtained from the birefringent measurements), we estimate the octopole moment to be 7.6 × 10−34 esu and the hexadecapole moment to be 16.9 × 10−42 esu. Alternatively, if the hexadecapolar contribution is considered to be negligible, then we estimate the octopole moment to be 9 × 10−34 esu. These results are consistent with measurements of the total integrated absorption of the IR band assuming a theoretical expression for the contribution due to the allowed dipole transitions.