X-ray reflectivity study of the surface of liquid gallium

X-ray reflectivity from the surface of liquid gallium was measured under ultrahigh vacuum conditions using a novel technique for curved surfaces. The small deviations between the measured and theoretical Fresnel reflectivity for an ideally sharp flat interface for wave-vector transfer ≲0.5 A${\mathrm{̊}}^{\mathrm{-}1}$ imply an interfacial width for the electron density profile of ≲1.3±0.2 Å. This is consistent with a model of atomic close packing which lacks structure along the surface normal at length scales >10 Å.