Institute for Data Analysis and Visualization

Neutron diffraction studies of monolayer and submonolayer films of CD4 on graphite have been carried out at two coverges (o=1.09 and o=0.92) to investigate the behavior of the structure factor in the two-dimensional solid and across the solid-liquid transition. Detailed lineshape and analyses have been carried out. It is found that in the solid phase a characteristic low-K ''tail'' is seen in the diffraction peak which can be fitted quite well with a power law divergence as expected from scattering by acoustic phonons in a two-dimensional crystal. In the liquid phase, a Lorentzian structure factor accounts reasonably well for the observed lineshapes and yields a correlation lenght that increases rapidly as the solidification temperature is approached. The results are qualitatively in agreement with the predictions of the Kosterlitz-Thouless theory of melting.