First measurement of the Rayleigh cross section

Summary form only given. A century ago Lord Rayleigh formulated a theory of light scattering by ideal gases that not only explained the molecular origin of atmospheric scattering and the blue colour of the sky but also provided a quantitative expression for the amount of light scattered. The cross section (in cm/sup 2/) is given by the well known equation: /spl sigma/(/spl nu/)=(24/spl pi//sup 3//spl nu//sup 4/(n/sup 2/-1)/sup 2//(N/sup 2/(n/sup 2/+2)/sup 2/))F/sub k/(/spl nu/) with the so-called King correction factor F/sub k/(/spl nu/) accounting for the anisotropy of nonspherical molecules. Accurate tabulations of frequency dependent Rayleigh scattering coefficients have been produced based on accurate measurements of refractive indices; predicted cross sections are considered accurate at the 1% level. These calculations have never been subjected to a laboratory test. We have performed a measurement of the Rayleigh cross section for N/sub 2/, Ar and SF/sub 6/ with the technique of cavity ring-down spectroscopy (CRDS) and based on the assumption that in the absence of absorption the scattering cross section is equal to the extinction.