Resonant coupling in the van der Waals interaction between an excited alkali atom and a dielectric surface: an experimental study via stepwise selective reflection spectroscopy

We present a detailed experimental study of the evaluation of the van der Waals (vW) atom-surface interaction for high-lying excited states of alkali-metal atoms (Cs and Rb), notably when they couple resonantly with a surface-polariton mode of the neighbouring dielectric surface. This report extends our initial observation [Phys. Rev. Lett. 83, 5467 (1999)] of a vW repulsion between Cs(6D(3/2)) and a sapphire surface. The experiment is based upon FM selective reflection spectroscopy, on a transition reaching a high-lying state from a resonance level, that has been thermally pumped by an initial one-photon step. Along with a strong vW repulsion fitted with a blue lineshift, - 160 +/- 25 kHz mum(3) for Cs(6D3/2) in front of a sapphire surface (with a perpendicular c-axis), we demonstrate a weaker vW repulsion (-32 +/- 5 kHz mum(3)) for Cs(6D(3/2)) in front of a YAG surface, as due to a similar resonant coupling at 12 pm between a virtual atomic emission (6D(3/2)-7P(1/2)) and the surface polariton modes. A resonant behaviour of Rb(6D(5/2)) in front of a sapphire surface exists also because of analogous decay channels in the 12 mum range. Finally, one demonstrates that fused silica, nonresonant for a virtual transition in the 12 mum range and hence weakly attracting for Cs(6D(3/2)), exhibits a resonant behaviour for CS(9S(1/2)) as due to its surface polariton resonance in the 8-9 mum range. The limiting factors that affect both the accuracy of the theoretical prediction, and that of the fitting method applied to the experimental data, are discussed in the conclusion.