Knowledge Bank

Studies of Raman and infrared spectra of several molecules adsorbed to metal surface indicated special selection has govern these spectra. In the case of infrared reflection adsorption spectroscopy (IRS) as in the related techniques of electron energy loss spectroscopy (ELS) and inelastic electron tunneling spectroscopy (IETS), only modes which develop dipole moment perpendicular to the metal surface can be excited. We have shown that an image field model which has been proposed to account for these observations is equivalent to the presence of a new symmetry operation which consists of \emph{simultaneous} reflection in the plane of the metal surface plus charge conjugation. As result, instead of the point group of the molecule, the symmetry group which should be used to analyze the motions of a molecule adsorbed to a metal surface consists of the direct product group of the molecular point group and another which is isomorphous with $C2$. Furthermore, the activity representations of the system, molecule + image, for both infrared and Raman spectra, are those which are compatible with the totally symmetric representation of $C2$. Both infrared and Raman spectra of oriented metal adsorbates can therefore be used to probe molecular orientation. Depolarization ratios for specific orientation and scattering geometries have been derived and used to analyze the results of several experimental examples.