Attenuated Total Reflection of the Rubidium D₂ Line in Optically Dense Vapor

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Absorption spectrum of the evanescent light at a glass/Rb vapor interface has been observed with atom density from 9×10¹⁹ m⁻³ to 5×10²⁰ m⁻³. Absorption is as high as 60% at the peak of a hyperfine structure split line. The absorption spectrum of the experiment is compared with that of the calculation based on Fresnel’s relation on the assumption of a homogeneous electric susceptibility of the vapor. Near the critical angle the calculation which incorporates the Lorentz–Lorenz local-field correction reproduces the experimental red shift only partly.