Two highly ordered, epitaxially rotated phases of bilayer ice are obse
rved on Pt(111) in high resolution helium atom diffraction. Analysis o
f helium diffraction patterns shows that the two phases differ slightl
y in their density and alignment with respect to the surface, The lack
of any isotope effect for both phases indicates that the ice bilayers
have structures and hydrogen bond lengths very similar to bulk ice. (
C) 1997 American Institute of Physics.