Resonant electron scattering by N2 molecules on Ag(110): study of the orientational ordering of the N2 physisorbed layers

Inelastic electron scattering data are used to study the geometry of molecular adsorbates on metal surfaces. The analysis of the vibrational excitation of N2 molecules physisorbed at 15 K on Ag(110) by electron impact provides information on the orientational ordering of the molecules on the surface. The incident electron energy (19 eV) is in the range where vibrational excitation is dominated by the N−2(2Σu) resonance. The angular distributions (polar angle) of the inelastically scattered electrons, determined at monolayer, bilayer and multilayer N2 coverages, reflect the orientational distribution of the molecules on the surface. Using the inelastic angular distributions of the free molecule, it is shown that the experimentally observed distributions for physisorbed N2 molecules on Ag(110) are consistent with the theoretical ordering predicted in a previous paper [Ramseyer et al., Phys. Rev. B 58 (1998) 4111]: herringbone for the monolayer and a dense hexagonal pinwheel structure for the bilayer.