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

Abstract

The incoherent inelastic neutron spectrum of a monolayer of n-hexane adsorbed on graphite has been analyzed. The two-dimensional lattice structure that was used in the analysis of a monolayer of n-butane on graphite is adopted. The intramolecular valence force fields of the n-butane system require only a minor modification of the CCC/CCC bending interaction and HCCH torsion force constants, while the intermolecular pairwise potentials transfer with no modification. The frequency dispersions and hydrogen amplitudes calculated from normal mode analysis are used to construct the neutron scattering cross sections. These are then 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 agreement between the calculated and observed spectra is even better than that obtained in our earlier analysis of n-butane. The intermolecular potential constants for the hydrogen–substrate and hydrogen–hydrogen pairs are 0.04 and 0.003 mdyn/A, respectively.