The adsorption of CO on Ir(111) investigated with FT-IRAS

The adsorption of CO on Ir(111) has been investigated with Fourier transform infrared reflection-absorption spectroscopy, temperature programmed desorption, and low-energy electron diffraction. At sample temperatures between 90 and 350 K, only a single absorption band, above 2000 cm⁻¹, has been observed at all CO coverages. For fractional coverages above approximately 0.2, the bandwidth becomes as narrow as 5.5 cm⁻¹. The linewidth is attributed mainly to inhomogeneous broadening at low CO coverages and to the creation of electron-hole pairs at higher CO coverages. The coverage-dependent frequency shift of the IR band can be described quantitatively using an improved dipolar coupling model. The contribution of the dipole shift and the chemical shift to the total frequency shift were separated using isotopic mixtures of CO. The chemical shift is positive with a constant value of approximately 12 cm⁻¹ for all coverages, whereas the dipole shift increases with coverage up to a value of 36 cm⁻¹ at a coverage of 0.5 ML.