X-ray studies of atomic layering at liquid metal surfaces

Surface-induced layering has been observed in liquid gallium and mercury using X-ray reflectivity. The specular reflectivity R(q,) has been measured to wavevector transfers as large as q, = 3.0 A-‘. For Ga, the only major deviations from pesnel theory R, are near q, = 2.4 A-‘, where there is a peak in the ratio R/R,. For Hg, there is a broad peak near 2.15 A-‘. The data have been collected on a sputtered clean, ultra-high vacuum Ga surface and on a Hg surface in a reducing atmosphere of hydrogen. The data can be explained with a layered liquid/vapor interface that is roughened by thermally excited capillary waves, The ,‘ayer spacing is $nilar to the Ga or Hg atomic dimensions, extending into the bulk with an exponential decay length of 6 A for Ga and 3 A for Hg.