Site symmetry of surface adsorbed molecules

Abstract

A procedure for determining the allowed local site symmetry groups S for surface adsorbed molecules is presented. An analogy can be drawn between the perturbing field experienced by a matrix isolated molecule in a three-dimensional crystal and that experienced by a molecule adsorbed on a surface. In the former case the field has the symmetry of the host lattice site whereas in the latter case the field has the symmetry of the adsite. If the surface is viewed as a homogeneous two-dimensional plane, then the symmetry of the adsite is described by the group G = C∞v. If the surface cannot be viewed as an unstructured homogeneous plane so that the perturbing field experienced by the adsorbed molecule must be considered on a microscopic level, then the adsite symmetry will be described by the group appropriate for the substrate atoms. This can only be one of the groups G = Cn or G = Cnv. The symmetry operations of the local site symmetry group S consist of an operation in the molecular point group M applied to the molecule combined with an operation in G applied to the substrate. A mapping of the operations in M and G to operations in their isomorphous Longuet-Higgins groups allows the operations to be identified as permutation P or permutation–inversion P operations. The operations in S are restricted to be combinations of P operations in M with P operations in G and P operations in M with P operations in G. An adsorbed molecule having M = D3h symmetry is used as an example to demonstrate the procedure for determining S. The local site symmetry group of the adsorbed molecule is, in general, different for the homogeneous surface approximation as opposed to the microscopic surface approximation. An attempt is made to apply predicted spectroscopic selection rules to adsorbed pyridine and ethylene. Several factors which complicate the interpretation of Raman spectra of molecules adsorbed on metal surfaces are discussed, one of these being depolarization effects due to rough surfaces.