Surface structure of liquid Bi and Sn: An x-ray reflectivity study

X-ray reflectivity measurements of the liquid Bi surface are presented and analyzed together with previous liquid Sn results. Published measurements on liquid Ga, In, and K all exhibit a single strong maximum at a wave-vector transfer of the order of the reciprocal of an atomic-diameter, due to surface-induced layering. In contrast, both Sn and Bi exhibit\char22{}in addition\char22{}a weak broad peak at much smaller wave-vector transfers. This feature is an unambiguous signature of an enhanced electron density in the near-surface region. Possible ways of modeling this enhancement are presented. Once the different surface-roughening effects of thermal capillary waves are accounted for, the surface structure factors of Sn and Bi are remarkably similar. The principal difference between the two is that the depth of the layering below the surface is more than $\ensuremath{\sim}40%$ larger for Bi than for Sn. This is considerably larger than the ratio of their covalent radii which is only $\ensuremath{\sim}10%$. No theoretical explanation can be offered at this time for the surface structure difference between Sn and Bi and other elemental liquid metals studied to date: Ga, In, and K.