Adsorbate Vibrations on Transition Metal Surfaces: Applications and Theory

Adsorbate vibrational excitations are an important descriptor of molecule/surface interactions, affecting temperature contributions to the free energy and impacting reaction rate and equilibrium constants. Furthermore, vibrational spectra aid in identifying species and adsorption sites present in experimental studies. Despite their importance, rigorous theory for surface dependent adsorbate frequencies is lacking. By combining previously reported experimental data and our own density functional theory calculations, we reveal surface induced frequency shifts and develop theory to explain these shifts. As coupling of the motion in Cartesian coordinates generates non-zero off-diagonals of the Hessian matrix, we developed algorithmic criteria for assigning normal modes to translations of the adsorbate, frustrated rotations, and internal motions. Through replication of low coverage spectra computationally, we validate these results with high resolution electron energy loss spectroscopy (HREELS) experiments and identify fingerprints for describing surface vibrations.