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Anomalous layering at the liquid Sn surface
X-ray reflectivity measurements on the free surface of liquid Sn are
presented. They exhibit the high-angle peak, indicative of surface-induced
layering, also found for other pure liquid metals (Hg, Ga and In). However, a
low-angle peak, not hitherto observed for any pure liquid metal, is also found,
indicating the presence of a high-density surface layer. Fluorescence and
resonant reflectivity measurements rule out the assignment of this layer to
surface-segregation of impurities. The reflectivity is modelled well by a 10%
contraction of the spacing between the first and second atomic surface layers,
relative to that of subsequent layers. Possible reasons for this are discussed.Comment: 8 pages, 9 figures; to be submitted to Phys. Rev. B; updated
references, expanded discussio