Structures and stability of CO layers on the MgO(001) surface

Elucidating the structure and dynamics of CO ad-layers on MgO surfaces

The combination of quantum-mechanical simulations and infrared absorption spectroscopy measurements provides a clear picture for a long standing puzzle in surface science: the actual structure and vibrational dynamics of the low-temperature ordered CO monolayer adsorbed on (001) MgO surfaces. The equilibrium structure of the commensurate (4 × 2) adsorbed phase consists of three CO molecules per primitive cell (surface coverage of 75%) located at two inequivalent sites: one molecule seats upright on top of a Mg site while two molecules, tilted off the normal to the surface, are symmetrically positioned relative to the upright one with anti-parallel projections on the surface. This configuration, long believed to be incompatible with measured polarization infrared spectra, is shown to reproduce all observed spectral features, including a new, unexpected one: the vanishing anharmonicity of CO stretching modes in the monolayer.

Structures of D2 layers on LiF(001)

Classical Monte Carlo (MC) simulations of D(2) molecules physisorbed on LiF(001) surfaces are reported and show a series of interesting commensurate structure forms, viz., p(2x2) -->p(8x2) -->p(4x2), with coverages Theta=0.5, 0.625, and 0.75, respectively, and are stable up to 8 K. These structures are consistent with recent helium atom scattering (HAS) results (the p(4x2) is not observed) in terms of coverage and stability, but disagree in terms of symmetry. The p(2x2) structure contains two D(2) molecules per unit cell, with each molecule lying parallel to the plane of the surface directly above every other cationic site. For the p(4x2) structure, there are two kinds of adsorption sites: a parallel site, as in the case of p(2x2), and a tilted site, where the D(2) molecules sit between cationic and anionic sites with the molecular axis directed toward the anionic site, with a tilt angle of theta approximately 63 degrees. Perturbation theory calculations show that the adsorbed D(2) molecules are azimuthally delocalized and hence the structures are indeed c-type. Our calculations also indicate that o-D(2) and helicoptering p-D(2) species prefer cationic sites, compared to cartwheeling p-D(2) species.

Monte Carlo simulations of the adsorption of CO2 on the MgO(100) surface

The adsorption of CO2 gas on the MgO (100) crystal surface is investigated using grand canonical Monte Carlo simulations. This allows us to obtain adsorption isotherms that can be compared with experiment, as well as to explore the possible formation of monolayers of different densities. Our model calculations agree reasonably well with the available experimental results. We find a "low-density" adsorbed monolayer where each CO2 molecule is bound to two Mg2+ ions on the MgO substrate. We also observe the formation of monolayers of higher density, where some of the CO2 molecules have rotated and tilted to expose additional binding sites. Low-temperature simulations of both the low- and high-density monolayers reveal that these states are very close in energy, with binding energies of approximately 7 kcal/mol at T=5 K. The high-density monolayer given by our model has a density that is significantly less than the reported experimental value. We discuss this discrepancy and offer suggestions for resolving it.