Determination of Adsorbate Molecular Orientation from Resonance Electron-Scattering Angular Distributions

We have measured the angular profile of electrons inelastically scattered from oriented molecular ${\mathrm{O}}_2$, physisorbed on graphite at 20 K. Electrons ejected after vibrational energy loss from a temporary negative-ion resonance observed near 9 eV have an angular profile which peaks at 15°±5° from the normal to the crystal surface, independent of the incident beam angle. From an analysis of these angular distributions we are able both to identify the ${\mathrm{O}}_2$ negative-ion state (as ${}_{}{}^2{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}$) and to determine the orientation of the ${\mathrm{O}}_2$ molecular axis (as 25°±5° from the vertical) in what we believe to be the monolayer $\zeta 2$ phase.