Study of the C(4×2) Structure Resulting from the Adsorption of CO Molecules on the MgO (001) Surface

Abstract

This research presents a model simulating the c(4×2) surface superstructure formed by the physical adsorption of carbon monoxide molecules on the (001) MgO crystal surface, using the Metropolis Monte Carlo algorithm. Simulation results indicate that the stable structure at low temperatures adopts the c(4×2) pattern, where the primitive cell contains three molecules, while the centered unit cell contains six molecules. Adsorption sites are found to cluster near Mg²⁺ ions, with a clear dominance of vertical over tilted molecules at a ratio of approximately 2:1. The tilted molecules stabilize at a polar angle of about 31°, which aligns well with data extracted from PIRS experiments.

Previous findings have shown that vertical molecules centered directly above Mg²⁺ ions form regular rows occupying all surface sites, whereas tilted molecules deviate slightly from ion positions by distances on the order of tenths of an angstrom toward their inclination, forming alternating rows with vertical ones—every two tilted rows separated by one vertical row. The simulation also revealed that full monolayer coverage primarily consists of tilted rows, and that transformation and surface desorption processes lead to the formation of the c(4×2) arrangement, with one vertical row between tilted rows. The results of this work show strong consistency with data obtained from various experimental techniques.