X-ray determination of the substrate modulation potential for a two-dimensional Rb liquid in graphite

Using a nonlinear response theory that accounts for correlation in the liquid, we have determined the modulation potential for a 2D Rb liquid in graphite from a measurement of the Rb contribution to the graphite Bragg peaks. The resulting potential is almost perfectly represented by a set of six simple sine waves; its value at a hexagon center is V₀=-0.0735 eV. At a bond midpoint the potential is Vb=+0.0185 eV. This result is checked with numerical simulations, and the implications for other 2D systems, including rare gases on graphite, are discussed.