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^-1 imply an interfacial width for the electron density profile of {}1.3±{}0.2 A{}. This is consistent with a model of atomic close packing which lacks structure along the surface normal at length scales >10 A{}.