ADSORPTION OF CO ON THE UNRECONSTRUCTED AND RECONSTRUCTED IR(100) SURFACE

The adsorption of CO on both the (1 x 1) and the (5 x 1) reconstructed Ir(100) surface at room temperature has been investigated using electron energy loss spectroscopy (EELS) and LEED. CO is adsorbed on both surfaces at all coverages in on-top sites. All four vibrational modes of the adsorbate have been detected. Upon adsorption of CO on the (5 x 1) surface the reconstruction is locally lifted giving rise to a (1 x 1) LEED pattern. Adsorption of CO on the (1 x 1) surface leads to a c(2 x 2) structure. The vibrational frequencies of the CO-molecules on both surfaces differ only slightly. At saturation the iridium-CO and the C-O stretching frequencies are 485 and 2075 cm-1 on the (5 x 1) and 497 and 2068 cm-1 on the (1 x 1) surface, respectively. The frequency of the rotational mode of the CO molecule is found to be 425 cm-1 and the frustrated translation at 53 cm-1, both showing no dispersion along GAMMA-BARMBAR direction. The C-O stretching vibration shows dispersion due to dipole-dipole interaction, also when the overlayer is not ordered.