RESONANCE ELECTRON-SCATTERING BY O2 MONOLAYERS ON GRAPHITE

We present a high resolution electron energy loss spectroscopy study of δ and ζ2 phase O2 monolayers physisorbed on graphite. Various molecular negative ion resonances are observed in the cross sections for vibrational and electronic excitation of the O2 molecules for electron beam energies in the range 4 < Ei < 10 eV. In particular, vibrational excitation is found to proceed via the 4∑u transient negative ion state in the δ phase, but via the 2Πu negative ion in the ζ2 phase. This "selective resonance population" is shown to arise because the resonance scattering cross sections are functions of the adsorbate molecular orientation in each phase, as well as the resonance symmetry and scattering geometry. Electronic excitation of the O2 molecule (X3σ-g → a1Δg) is proceed via the X2Πg negative ion resonance in both δ and ζ2 phase, though the resonance is shifted in energy between the two phases due to multiple electron scattering in the surface. The angular distributions of scattered electrons are used to study the structure of the O2 monolayer phases. The postulated molecular orientations in the δ phase (molecular axes approximately parallel to the surface) and in the ζ2 phase (approximately upright) are directly confirmed. Further, multiple electron scattering calculations, performed to allow a quantitative analysis of the measured angular distributions, are consistent with substantial dynamical tilting of the O2 molecules away from their equilibrium orientations in both phases, with an estimated mean tilt angle of 20° in the δ phase and 10° in the ζ2 phase. © 1990.