Van der Waals forces between heavy alkali atoms and gold surfaces: Comparison of measured and predicted values

The Van der Waals potentials between heavy alkali-metal atoms (Cs, Rb, K) and gold surfaces have been investigated by the atomic-beam-deflection technique. The result is consistent with a potential of the form $V\left(R\right)=-\frac{k}{R^3}$. The observed interaction constant $k$ is 7.0 ± 0.29 ${\mathrm{D}}^2$, 6.04 ± 0.30 ${\mathrm{D}}^2$, and 5.10 ± 0.62 ${\mathrm{D}}^2$ for Cs, Rb, and K, respectively. These values are smaller than the predictions based on all available theories. At best there is agreement within 60% for estimates based on the macroscopic continuum theory of Lifshitz. Other models disagree with measurements by factors of three (200% difference) or more.