Mobility measurements of two kinds of two-dimensional fluids. Methane adsorbed on graphite

Abstract

The diffusion coefficient of a CH4 submonolayer adsorbed on (0001) graphite is measured as a function of coverage and temperature by quasi-elastic neutron scattering. The experimental data permit us to clearly distin- guish two kinds of adsorbed fluids, i.e. a 2D liquid coexisting with a 2D gas and a 2D hypercritical fluid whose density varies continuously with coverage. The mobility disappears at solidification which can be obtained either by decreasing temperature at constant coverage or by increasing coverage at constant temperature. The coexis- tence of 2D solid and 2D liquid is also observed. Finally a phase diagram of the first layer of methane adsorbed on graphite is proposed.