Structure of helicoptering H2 molecules on MgO(001)

Monte Carlo (MC) simulations of helicoptering (J = 1; mj = ± 1) H2 molecules on the MgO(001) surface using rotationally averaged interaction potentials are reported and show that at the coverage Ө = 0.5 the film forms a structure with c(2 × 2) symmetry in agreement with experiment. This structure is stable up to 6 K. The unit cell is arranged in a quincunx pattern and has a density of two H2 molecules per unit cell. Each molecule sits above the cation sites with its axis of quantization pointing perpendicular to the surface on average. The unit cell symmetry is “c” type in agreement with helium atom scattering experiment and in contrast with the “p” type structures observed in previous simulations which did not include quantum mechanically averaged potentials from the start. The occurrence of defects was also observed with increasing frequency as the temperature rose. An increase in the number of defects was accompanied by greater aggregation at the expense of the c(2 × 2) structure.