Tracking the evolution of interatomic potentials with high resolution inelastic neutron spectroscopy.

High resolution inelastic neutron scattering has been used to characterize the low temperature rotational dynamics of methane ( CH4) films on MgO(100) surfaces as a function of layer thickness. At low temperatures the films grow layer-by-layer similar to the (100) planes of bulk solid CH4. We observe a crossover in the CH4 tunneling spectrum from one characteristic of the 2D solid at monolayer coverages into one that is bulklike (i.e., 3D) when the film is six layers thick. These findings place significant constraints on the microscopic models used to describe CH4-CH4 and CH4-MgO interactions.