| Popis: |
We present an analytical potential expansion for a molecule interacting with a two-dimensional lattice of atoms which is convenient in quantum-mechanical scattering and lattice-dynamics calculations. It combines the following three ingredients: a spherical expansion in symmetry-adapted free-rotor functions of the molecule which represents the anisotropy of the potential explicitly, a translational-displacement expansion that goes beyond the harmonic approximation, and a two-dimensional Fourier expansion, which reflects the translational symmetry parallel to the substrate. As an example, we show some numerical results for N/sub 2/ on graphite which illustrate the convergence of the various expansions. The potential anisotropy is visualized explicitly. At an adsorption site, the in-plane anisotropy appears to be negligible with respect to the out-of-plane anisotropy. The anharmonic terms in the molecular-displacement expansion are so important that they will influence the out-of-plane translational vibrations. |
