Coupling of vibrational modes of adsorbates: Application to field-induced shifts for CO and CN on Cu(100)

We have studied the coupling of the metal-ligand and the intramolecular-ligand stretching vibrations with ab initio cluster model wave functions. When there is no applied electric field, uncoupled internal coordinate modes and the fully coupled normal modes give essentially the same results. In the presence of an applied field, the coupling is important for CN. The coupling is shown to arise from the large field-induced changes in the metal-ligand distance for the ionic bond between the metal and CN. The mechanism of the field-induced changes in the bond lengths and vibrational frequencies as a Stark effect is confirmed; chemical changes are shown to be small. © 1987 The American Physical Society.