Surface infrared reflection-absorption measurements for CO adsorbed on Cu(100) in the frequency range 200–500 ${\mathrm{cm}}^{\mathrm{-}1}$ reveal, in addition to the Cu-CO stretch mode at 345 ${\mathrm{cm}}^{\mathrm{-}1}$, a sharp derivativelike feature at 285 ${\mathrm{cm}}^{\mathrm{-}1}$, assigned to the CO frustrated rotation mode which is usually dipole forbidden. Its infrared activity and Fano line shape arise from coupling to substrate electronic transitions associated with CO-induced surface states.

• Received 13 March 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.480