Lattice vibrations of a monolayer of n-butane adsorbed on graphite

Abstract

The incoherent inelastic neutron spectrum of a monolayer of n‐butane adsorbed on graphite has been analyzed. The system corresponds to a two‐dimensional lattice with a unit cell having one n‐butane molecule and four substrate atoms of infinite mass. The potential energy matrix for the vibration is set up in Cartesian coordinates with both intramolecular valence force fields and intermolecular pairwise potentials. The calculated frequency dispersions and hydrogen amplitudes are used to calculate the neutron scattering cross sections. They are summed for all normal modes and folded with both the instrumental resolution function and the incident beam profile to form a composite time‐of‐flight spectrum. The comparison of the calculated spectrum with observation provides a set of intermolecular force constants. The best agreement is obtained with the hydrogen–substrate and the hydrogen–hydrogen constants of 0.04 and 0.003 mdyn/Å, respectively.