Abstract
Monte Carlo (MC) simulations of D-2 molecules on the MgO(001) surface are reported and show that a series of interesting structures form with increasing coverage, viz. p (2 X 2)-> p (4 X 2) -> p (6 X 2), with coverages Theta = 0.5, 0.75, and 0.83 respectively, and are stable up to 13 K. The p (2 X 2) structures contain two D-2 molecules per unit cell, with each molecule lying parallel to the plane of the surface (theta = 90 degrees) directly above every other Mg2+ site. The molecules adopt a "T" configuration with respect to their nearest neighbors. The p (4 X 2) and p (6 X 2) structures, have two kinds of adsorption sites: a parallel site, as in the case of p (2 X 2), and a tilted site, where the D-2 molecules sit between cationic and anionic sites with the molecular axis directed towards the anionic site, with theta approximate to 60 degrees. These structures are consistent with recent neutron scattering results in terms of coverage and stability, but disagree in terms of symmetry; the neutron scattering work found "c" type structures whereas the MC simulations (without quantum considerations) yield a "p" type structures. To reconcile the results of the simulations and experiments, the quantum mechanical rotational motion of the adsorbed D-2 molecules was studied using perturbation theory. These calculations show that the adsorbed D-2 molecules are azimuthally delocalized and hence the structures are indeed "c" type rather than "p" type.