Selective reflection from sodium vapour at low densities

We report an extensive study of the reflectivity of a glass-sodium vapour interface near the sodium D1 and D2 resonance lines for low sodium vapour densities (1014-1016 atoms cm -3). The observed reflection spectra are narrower than predicted by conventional dispersion theory. The experimental results are compared with recent calculations based on a theory in which not only atom-atom collisions are taken into account, as in the conventional theory, but also atom-wall collisions.