An experimental study and classical treatment of the far infra-red induced dipolar absorption in gaseous ethylene

The far infra-red absorption spectrum of the compressed gas ethylene has been measured in the region 10–200 cm⁻¹ at pressures from 14.7 to 55.7 bar at 296 K. The origin of the broad bands observed is a bimolecular collision induced absorption as indicated by the dependence of the total integrated intensity on the square of the molecular number density. The quantum theory [6] of resonant, multipole induction, giving rise to sets of rotational transitions is an inadequate description of the observed broad continua, the peaks of which shift from 125 cm⁻¹ to 150 cm⁻¹ over the pressure range investigated. However, they are matched satisfactorily by a spectral function c(ω), generated from an approximation to its related classical autocorrelation function c(t), containing two equilibrium averages K_o(o) and K_l(l) (which are related to the coefficients in the Maclaurin expansion of c(t)); and a width parameter γ. All three phenomenological parameters vary with pressure, contrary to the quantum prediction of their constancy, and go through a smooth minimum at $\text{ca}$. 30 bar, where the correlation time τ is longest.