  We present measurements of the growth rates of the principal facet surfaces
of ice from water vapor as a function of supersaturation over the temperature
range -2 C > T > -40 C. Our data are well described by a dislocation-free
layer-nucleation model, parameterized by the attachment coefficient as a
function of supersaturation \alpha(\sigma) = Aexp(-\sigma_0/\sigma). The
measured parameters A(T) and \sigma_0(T) for the basal and prism facets exhibit
a complex behavior that likely originates from structural changes in the ice
surface with temperature, in particular the onset and development of surface
melting for T > -15 C. From \sigma_0(T) we extract the terrace step energy
\beta(T) as a function of temperature for both facet surfaces. As a basic
property of the equilibrium ice surface, the step energy \beta(T) may be
amenable to calculation using molecular dynamics simulations, potentially
yielding new insights into the enigmatic surface structure of ice near the
triple point.

Materials Science