Abstract

Detailed information on structural, chemical, and physical properties of natural cleaved (10.4) calcitesurfaces was obtained by a combined atomic force microscopy (AFM) and infrared (IR) study using COas a probe molecule under ultrahigh vacuum (UHV) conditions. The structural quality of the surfaces wasdetermined using non-contact AFM (NC-AFM), which also allowed assigning the adsorption site of COmolecules. Vibrational frequencies of adsorbed CO species were determined by polarization-resolvedinfrared reflection absorption spectroscopy (IRRAS). At low exposures, adsorption of CO on the freshlycleaved (10.4) calcite surface at a temperature of 62 K led to the occurrence of a single C–O vibrationalband located at 2175.8 cm1, blue-shifted with respect to the gas phase value. For larger exposures, aslight, coverage-induced redshift was observed, leading to a frequency of 2173.4 cm1for a fullmonolayer. The width of the vibrational bands is extremely small, providing strong evidence that thecleaved calcite surface is well-defined with only one CO adsorption site. A quantitative analysis of theIRRA spectra recorded at different surface temperatures revealed a CO binding energy of0.31 eV. NC-AFM data acquired at 5 K for sub-monolayer CO coverage reveal single molecules imaged asdepressions at the position of the protruding surface features, in agreement with the IRRAS results. Sincethere are no previous experimental data of this type, the interpretation of the results was aided byemploying density functional theory calculations to determine adsorption geometries, binding energies,and vibrational frequencies of carbon monoxide on the (10.4) calcite surface. It was found that thepreferred geometry of CO on this surface is adsorption on top of calcium in a slightly tilted orientation.With increased coverage, the binding energy shows a small decrease, revealing the presence ofrepulsive adsorbate–adsorbate interactions.

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