Determination of Rayleigh scattering cross sections and indices of refraction for Ar, CO2, SF6, and CH4 using BBCES in the ultraviolet

Accurate Rayleigh scattering cross sections are important for understanding the propagation of electromagnetic radiation in planetary atmospheres and for calibrating mirror reflectivity in high finesse optical cavities. In this study, we used Broadband Cavity Enhanced Spectroscopy (BBCES) to measure Rayleigh scattering cross sections for argon, carbon dioxide, sulfur hexafluoride, and methane between 333 and 363 nm, extending the region of available UV measurements for all four gases. Comparison of our results with refractive index based (n-based) calculations demonstrates excellent agreement for Ar and CO2, within 0.2% and 1.0% on average, respectively. For SF6, our mean Rayleigh scattering cross sections are lower by 2.2% on average relative to the n-based calculation and lie outside the 1-σ measurement uncertainty; however, the results still fall within our 2-σ uncertainty. The measured Rayleigh scattering cross sections for CH4 are in substantial disagreement (22%) with those calculated from the most recent n-based values in the literature and lie far outside our mean 1-σ uncertainty of 1.6%. Extrapolation of several older index of refraction measurements from visible wavelengths to the UV yields better agreement with our results for CH4, but the agreement is still generally outside our 1-σ measurement uncertainty. Use of the dispersion relation derived in this work provides significantly improved Rayleigh scattering cross sections for CH4in the UV-A spectral region.