Coupled-harmonic-oscillator model for the vibrational modes of an adsorbed diatomic molecule

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Abstract
The normal vibrational modes for a diatomic molecule adsorbed on a surface are obtained in terms of the free-space intramolecular and bound-rigid-molecule surface modes. The consequences on both eigenfrequencies and damping rates of surface-induced coupling of these modes are pointed out. A set of self-consistency conditions relating the line positions and widths are derived. With these relations, it is demonstrated how shifts in intramolecular vibrational energies due to chemisorption bond formation can be extracted from total observed shifts resulting from both bond formation and coupling with the rigid-molecule modes. Electron-energy-loss data due to Andersson for CO adsorbed on Ni are analyzed in the light of the ideas put forth here.