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High-pressure scanning tunneling microscopy was used to study the room-temperature adsorption of CO on a Pt(111) single-crystal surface in equilibrium with the gas phase. The coverage was found to vary continuously, and over the entire range from 10-6−760 Torr pressure-dependent moiré patterns were observed, characteristic of a hexagonal or nearly hexagonal CO overlayer. Two different pressure ranges can be distinguished: below 10-2 Torr, the moiré lattice vector is oriented along a 30° high-symmetry direction of the substrate, corresponding to a pressure-dependent rotation of the CO overlayer with respect to the (1 × 1) Pt surface lattice, while above 10-2 Torr, the CO layer angle is independent of the pressure. This behavior is analyzed in terms of the interplay of the repulsive CO−CO interaction potential and the substrate potential.
Longwitz et al. (2004) studied this question.