Vibrational Anharmonicity of Carbon Monoxide on Metal Surfaces: Backdonation, Electric Field, and Structure Effects

We investigate the origins of vibrational anharmonicity for CO adsorbed on bare and oxidized Cu(110) and adsorbed to Ruthenium atoms in a Ruthenium tetraphenyl porphyrin (RuTPP) layer on Cu(110) using hot band transitions excited by intense IR–visible sum frequency generation. Observation of both υ = 1 → 2 and υ = 2 → 3 transitions of CO on bare and oxidized Cu(110) shows that the Morse anharmonic oscillator potential does not fully describe the vibrational energy levels. A compilation of our new data and published data on other surfaces shows the expected trend of weaker C–O bonds with lower C–O stretching frequency, being described by more anharmonic potentials. Deviations from this trend reveal changes in the C–O bond that are not captured by its frequency. Coadsorbed oxygen increases the anharmonicity of the C–O bond, likely due to a local electric field (vibrational Stark effect). Anharmonicity of the C–O stretch of CO bound to RuTPP can be tuned by porphyrin coverage, which changes the conformation of the phenyl ligands and influences the adsorbed CO in turn.