Coupling of Vibrational Modes of Adsorbates: Application to Field Induced Shifts for CO and CN on Cu(100)

摘要

We have studied the coupling of the metal-ligand and the intra-molecular ligand stretching vibrations for CO and CN chemisorbed on Cu(100) using potential energy surfaces obtained with ab initio cluster model wave functions. When there is no applied electric field, approximate internal coordinate modes for these vibrations and the fully coupled normal modes give essentially the same results, showing that their coupling is small. In the presence of an applied field, the coupling is important for CN. The origin of the coupling is shown to arise from the large field induced changes in the metal-ligand distance for the ionic bond between metal and CN. The present calculations also confirm the mechanism of the field induced changes in the bond lengths and vibrational frequencies for both chemisorbed CO and CN as a Stark effect. Chemical changes are shown to be small.