We explore a situation where the van der Waals long-range atom-surface interaction is repulsive. This repulsion originates in a resonant coupling between a virtual emission at $12.15\mu \mathrm{m}$ of a ${\mathrm{Cs}}^{*}\left({6D}_{3/2}\right)$ atom and a virtual excitation of a surface polariton in sapphire. The experimental evidence is based upon the analysis of the spectroscopic response of Cs* in the near-infrared range with a technique that probes a distance range $\sim 100\mathrm{nm}$ away from the sapphire surface. We also demonstrate the critical dependence of atom-surface forces on the sapphire crystal orientation.

• Received 17 May 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.5467