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We present an analytic model applied to quasi-elastic scattering from an adsorbed surface species undergoing jump diffusion between adsorption sites described by a Bravais lattice combined with a basis of multiple points. The model allows for hops between adsorption sites which are both symmetrically and energetically inequivalent. We give results for 1-D hopping, which are applicable to a species jumping between the top and bridge sites along the [110] direction on an fcc-(110) surface or for jumps along a step edge. In 2-D, results for hopping between fcc and hcp hollow sites and between the bridge sites of an fcc-(111) surface are presented. These examples give characteristic signatures which will allow these forms of motion to be recognized in experimental data and will enable the underlying physical parameters to be extracted by comparison with the analytical forms derived here.

A classical molecular dynamics study of the diffusive motion of benzene molecules on graphite basal planes has been performed in the sub-monolayer coverage regime. Atomistic calculations were performed using the second generation forcefield COMPASS as well as the general purpose forcefields Dreiding and Universal. The COMPASS based calculations show evidence for a Brownian nature of the diffusion with a very small diffusion activation barrier of 11 ± 2 meV in agreement with recent helium and neutron spin-echo spectroscopy data [Fouquet P, Hedgeland H, Jardine AP, Alexandrowicz G, Allison W, Ellis J. Measurements of molecule diffusion on surfaces using neutron and helium spin echo. Physica B 2006;385–386:269-71]. Reasonable agreement is also found for the general purpose forcefields if screened charges are used in the description of the Coulombic non-bond interaction. The less computationally intensive Dreiding forcefield was shown to give a good qualitative description of the diffusion validating its applicability for future large scale calculation, i.e., for long times or enlarged systems. A potential energy surface (PES) has been established for the translational and rotational diffusive motion. The PES shows a peculiar dependence of the lateral diffusion barriers on the rotation angle of the benzene molecule which leads to a preferential selection of certain rotational states in the MD trajectories.

We present a new approach to gain detailed surface-molecule potential information by combining neutron and helium atom scattering. First results are presented for a prototype study of benzene molecule diffusion on graphite surfaces. The measurements show good agreement and confirm the transferability of the results between the two methods. Future instrumental improvements are described that will improve the sensitivity of NSE by a factor of 50 for studies of molecules adsorbed on surfaces and mesoporous substrates.