Relations between molecular gas absoptivities and emissivities

Theoretical expressions have been derived for the relations between gas absorptivities and emissivities for the limit of zero optical depth and for the following models of vibration-rotation bands: bands with constant average absorption coefficients and well-defined widths; just-overlapping spectral lines; non-overlapping dispersion lines with constant collision half-width and either regular line spacing or with lines of equal intensity; randomly distributed spectral lines with dispersion contom; non-overlapping Doppler lines with either regular line spacing or with lines of equal intensity; randomly distributed Doppler lines for a special {unrealistic} assumption relating to the temperature dependence of the effective mean line spacing; non-overlapping spectral lines with combined Doppler and collision broadening, constant collision half-width and either regular line spacing or with lines of equal intensity. The theoretical formulae have been shown to provide a good correlation for the available experimental data on CO_2, H_2O and CO.