The Normal Mode Frequencies of Chemisorbed Atoms and Molecules

The normal modes of vibration of a molecule chemisorbed on a metal are modified because the metal atoms to which the molecule is bonded themselves form part of a giant molecule (the crystal lattice) with a quasi-continuous band of normal mode frequencies. This problem has the structure of a system of particles (the molecular modes) interacting with a field (the lattice modes), and the effect of the interaction is to raise the molecular frequencies. If a molecular frequency already lies above the top of the lattice band, it still remains as a discrete frequency when the interaction is included. On the other hand, a molecular frequency which falls within the lattice band can only persist as a discrete frequency if it is forced clear of the band by the interaction. If this does not happen, the molecular frequency disappears, and an extra frequency is added to the lattice band. The chemisorbed atom and the chemisorbed diatomic molecule are treated in detail. Two molecules chemisorbed on the same crystal are in interaction with each other via the normal modes of the lattice. This interaction, which is formally analogous to the retarded van der Waals interaction between neutral molecules, is small. The important interaction between chemisorbed molecules is connected with changes in the electronic structure as the two molecules approach each other.