Probing the Surface Brillouin Zone by Infrared Absorption Spectroscopy: Asymmetric Line Shape of Vibrational Combination Band

The combination band of the C-O and the Ru-CO stretch vibrational modes of CO adsorbed on Ru(001) has been used to study vibrational dispersion effects by infrared absorption spectroscopy. For an ordered CO layer both vibrational modes can be excited far from the $\gamma$ point while fulfilling the $\mathbf{q}\approx 0$ condition through $\mathbf{q}$ vector matching. It is found that diluted layers lacking intermolecular interactions display a symmetrical and narrow line shape which broadens substantially for the $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°\mathrm{CO}$ layer and displays a pronounced asymmetry, in good agreement with theory.