Influence of resonances on the phase and the group refractive indices of air

The Barrell and Sears and Edlen formulae use the Sellmeier form away from resonances, which gives a ''smooth'' dependence within the visible and near-infrared spectra. All known atlases indicate numerous resonances in these regions, which cause local disturbance of the dispersion curve. Anomalous dispersion may be estimated from an absorption by the Kramer-Kronig ratios. It gives a change of phase refractive index of air in a range from several ppt to 1 ppb within narrow optical regions, causing large local dispersions. Computation of the group refractive index with the use of Rayleigh's ratio shows that the influence of resonance effects may be more than several ppm. For example, the error of the group refractive index for high-precision EDM using HeNe lasers (633 nm) varies between 0.2 and 0.7 ppm, depending on the laser stability.