Theoretical study of the monolayer structures of CO adsorbed on NaCl(100)

Abstract Interaction potential calculations are performed to interpret recent experimental results on the geometry of CO monolayers adsorbed on clean NaCl(100) substrate deduced from polarized Fourier transform infrared spectroscopy and helium beam scattering. Two isoenergetical structures are calculated: the (1 × 1) geometry with the molecular axes standing upright or inclined with respect to the surface normal and the (2 × 1) geometry with the molecular axes inclined and tilted anti-parallel. A statistical analysis of the energy minima for the CO monolayer is performed and it is shown that the transition from the (2 × 1) to the (1 × 1) structure can proceed in a very small energy range ( ~ a few meV) leading to a rearrangement of the molecular centers of mass above the Na sites with a concomitant redistribution of the molecular axis orientations. These results are in close agreement with experiments indicating a phase transition at 35 K from the (2 × 1) to the (1 × 1) phase.