Librational Modes of Molecules on Metals
The hindered rotational motion of a molecule bound to a metal surface by its image potential is studied. The potential is anharmonic in the tilt angle and leads to low frequency modes with frequencies less than 20 cm−1.
Molecules with permanent dipole moments as well as induced dipole moments are considered and the modification of the image potential due to the finite plasma frequency of the metal included. The distance of the center of mass (charge) of the molecule from the surface, which strongly affects the strength of the image dipole well, is chosen for the case of pyridine and CN- on silver.
The possible connection of the calculated librational modes to recently observed Raman-active modes of pyridine on a silver surface showing strong surface enhancement will be discussed.