SYMMETRY AND THE SURFACE INFRARED SELECTION RULE FOR THE DETERMINATION OF THE STRUCTURE OF MOLECULES ON METAL-SURFACES

Reflection absorption infrared spectra for several adsorbates on Pt(111) are presented that illustrate the use of symmetry in applying the surface IR selection rule. The selection rule states that only vibrations that belong to the representation of the surface normal will be surface IR active. The selection rule does not forbid vibrations that are dominated by the stretching of bonds that are parallel to the surface, and we show two such examples: the O-O stretch of adsorbed Oz and the C-C stretch of di-sigma bonded ethylene. Using multilayer CH3I as an example, we show how IR bands can be split due to formation of structures with symmetry different from the isolated molecule. Spectra of submonolayer coverages of CH3, ICH3, CCH3, and CCD3 demonstrate that, even for adsorbates having the same symmetry and same functional group, the relative intensities of the allowed vibrations can be quite different.