Atomic linewidths and line shifts near a dielectric layer, and the limit of a semi-infinite medium

The linewidth and line shift of an atom near a dielectric medium have been studied. The effect of the presence of the material is either a line narrowing or a line broadening, as compared to the natural linewidth, and it appears that the general tendency is a line narrowing. The corresponding shift is predominantly towards the blue. We illustrate graphically that the theoretical limit of a perfect conducting metal is unrealistic for any existing material, in particular when the atomic dipole is oriented perpendicular to the surface. We show that the limit of a semi-infinite medium follows from a delicate average over fast oscillations, and that the result is different from what one would expect. Explicit expressions for the linewidth were obtained for the case where the atom is close to the surface.