Abstract
The vibrational features of carbon monoxide (CO) adsorbed on Ir(111) were studied by means of high-resolution core-level spectroscopy. By monitoring the Ir 4f7/2 core level as a function of exposure, we proved that the CO adsorbs on the surface always in on-top sites, in agreement with the results of vibrational spectroscopy techniques and density functional theory studies. The C 1s vibrational splittings measured for the p(√3 × √3)R30° (233.4 ± 0.5 meV) and c(4 × 2√3)rect (231.4 ± 0.4 meV) structures were in good agreement with the Z + 1 model. Despite the very small error bar of the measurements, it was not possibile to reveal any anharmonic contribution to the spectral lineshape. We speculate that the contribution of the unresolved vibrational mode of the frustrated translation or the effect of phonon-mediated interaction with the substrate can account for the observation of this outcome.
Highlights
The investigation of vibrational properties of molecules and solids typically relies on infrared-radiation spectroscopies or monochromatized electron energy loss spectroscopy experiments
Since the pioneering experiments of Siegbahn and co-workers in the 1970s,1−3 it has been understood that the vibrational fine structure in simple molecules in the gas phase can be detected by means of high-resolution X-ray photoelectron spectroscopy (HR-XPS)
It has been observed for the specific case of carbon monoxide (CO) that the vibrational fine structure is strongly dependent on the photon energy of the X-radiation used for the experiment
Summary
The investigation of vibrational properties of molecules and solids typically relies on infrared-radiation spectroscopies or monochromatized electron energy loss spectroscopy experiments. The photoionization process is accompanied by vibrational excitation caused by the presence of a localized hole that induces a strong rearrangement of the electron density distribution in the excited molecule It has been observed for the specific case of carbon monoxide (CO) that the vibrational fine structure is strongly dependent on the photon energy of the X-radiation used for the experiment. In which the C−H bond leads to final-state vibrational frequencies on the order of 350−400 meV, other chemisorbed molecules, such as CO, have been investigated using HR-XPS The outcomes of these experiments showed smaller core-level vibrational splitting. To decrease the contribution of spectral broadening caused by inhomogeneities (molecules in slightly different chemical environment), it was necessary to prepare long-range ordered surface layers with a low density of defects and with excellent degree of cleanliness, where preferably only a single adsorption site was occupied. We measured the vibrational fine structure associated with the C 1s core level for both of the ordered structures that we prepared, extracting the values of the adiabatic transitions observed
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