Angular-resolved photoemission of formate and methoxy on Cu(110): Orientation and orbital assignment using dipole selection rules

Using synchrotron radiation the decomposition intermediates of formic acid, formate, and of methanol, methoxy, on a Cu(110) single crystal surface have been investigated. By applying dipole selection rules — which are particularly powerful for molecules of low symmetry on two-fold surfaces — the orbitals due to formate on Cu(110) can be assigned and compared to theory assuming perpendicular and symmetric bidentate formate orientation parallel to [110]. Other geometries are shown to be inconsistent with our data. For methoxy, however, the situation is more complex. While at first glance the reduction of the number of photoemission peaks when going from methanol to methoxy indicates high symmetry for methoxy on Cu(110) (C_3v), more detailed investigations show little influence upon changing photoemission parameters. This indicates the presence of tilted, bent, or more than one single species on Cu(110) and prevents a detailed analysis of adsorbate geometry using dipole selection rules in this case.