Selective reflection by Rb

Perpendicular reflection of low-power, near-resonant radiation at a sapphire–Rb-vapor interface (selective reflection) has been measured for Rb densities of 3×10¹⁴–3×10¹⁷/cm³. At the lowest density the eight hyperfine components are essentially isolated, and at the highest pressure they are highly blended. The data have been fitted to a coherent superposition of the reflected field amplitudes of the eight components, including the well-known starting transient and wall interaction. At low to moderate densities this yields a good fit to the data, and a result for the Rb-Rb collisional shift. At the higher densities the rapid attenuation of the incident field has a major effect on the reflection spectrum, and an approximation to this is included in the calculation, assuming exponential attenuation at the equilibrium-vapor rate. This provides improved fits to the data and allows the apparent Lorentz-Lorenz plus collisional shift to be evaluated. However, the data do not fully fit this calculation, and a complete self-consistent-field calculation appears necessary to fully understand the high-pressure results.