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. 
Oleg G. Shpyrko, Alexei Yu. Grigoriev, Peter S. Pershan, Binhua Lin,   Mati Meron, Tim Graber, Jeff Gebhardt, Ben Ocko and M...