Infrared-Active Combination Band in a Surface Formate Species

Infrared reflection-absorption spectroscopy of a formate species on Cu(110) shows not only ${\nu }_s\left(\mathrm{COO}\right)$ and $\nu \left(\mathrm{CH}\right)$ stretching modes characteristic of a bridge species but also a combination band ${\nu }_a\left(\mathrm{COO}\right)+\delta \left(\mathrm{CH}\right)$. 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 $\nu \left(\mathrm{CH}\right)$.