A VIBRATIONAL CHARACTERIZATION OF THE O A1(111) SYSTEM - A REASSIGNMENT OF HREELS DATA/

In light of recent STM measurements of the O/Al(111) system, we reassi gn the dipole active modes observed at low coverage to resolve discrep ancies between the interpretation of Strong, Firey, deWette and Erskin e [Phys. Rev. B 26 (1982) 3483], invoking subsurface oxygen, and a var iety of other studies which find no evidence for subsurface oxygen. Th e STM results, which show that very small island sizes are stabilized over an exposure range up to similar to 200 L with a total coverage le ss than or equal to 0.2 ML, are incompatible with the assumption of lo ng range periodicity required for lattice dynamical modeling. The cons equence is that vibrational modes polarized parallel to the surface ma y become dipole active. Within an Al3O cluster model appropriate to ex posures less than or equal to 3 L where most oxygen atoms are isolated species in three-fold hollow sites, the strong feature at 584 cm(-1) (72 meV) is still attributed to top-layer oxygen motion perpendicular to the surface (the symmetric Al3O stretch) but the second intense fea ture at 480 cm(-1) (60 meV) is assigned to the umbrella mode involving predominantly Al motion parallel to the surface rather than the motio n of two Al-O layers moving perpendicular to the surface out of phase with each other. The lowest frequency mode near 224 cm(-1) (28 meV) de rives from the frustrated translation of the cluster perpendicular to the surface. At higher exposures (> 10 L) where multiple oxygen island s begin to appear, totally symmetric combinations of the E-derived asy mmetric Al3O stretching motion polarised nominally parallel to the sur face become dipole allowed and can be assigned to the loss at 850 cm(- 1) (105 meV), which was previously attributed to subsurface oxygen.