We measured ratios of van der Waals potential coefficients ($C_3$) for different atoms (Li, Na, K, and Rb) interacting with the same surface by studying atom diffraction from a nanograting. These measurements are a sensitive test of atomic structure calculations because $C_3$ ratios are strongly influenced by core electrons and only weakly influenced by the permittivity and geometry of the surface. Our measurement uncertainty of 2% in the ratio $C_3^{\mathrm{K}}/C_3^{\mathrm{Na}}$ is close to the uncertainty of the best theoretical predictions, and some of these predictions are inconsistent with our measurement.

• Received 2 August 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.233202