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

Abstract

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\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} 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 $^{2}\ensuremath{\Pi}_{\mathrm{u}}$) and to determine the orientation of the ${\mathrm{O}}_{2}$ molecular axis (as 25\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} from the vertical) in what we believe to be the monolayer $\ensuremath{\zeta}2$ phase.