Infrared-Active Combination Band in a Surface Formate Species

Infrared reflection-absorption spectroscopy of a formate species on Cu(110) shows not only νₛ(COO) and ν(CH) stretching modes characteristic of a bridge species but also a combination band νₐ(COO)+δ(CH). Both components of this band have dynamic dipoles parallel to the surface and are thus forbidden themselves. Adsorbate-adsorbate dynamic dipole interactions cause the frequency to shift downwards with increasing coverage. The high absorption cross section of the band is attributed to a Fermi resonance with ν(CH).