Surface Molecular-Orbital Excitations in Electron Energy-Loss Spectra

We have observed electronic transitions characteristic of molecules at a Ni(110) surface in the energy-loss spectra of 60- to 80-eV primary electrons. We present spectra of a clean Ni(110) surface, an unordered adlayer of nitrogen on Ni(110), and an ordered adlayer of sulfur in the $\mathrm{Ni}\mathrm{}\left(110\right)\mathrm{}-c(2\mathrm{}\times 2)\mathrm{S}$ surface structure. These spectra derive from a two-step electron backscattering process involving an inelastic loss and an elastic scattering event at the crystal surface. The loss spectra of an ordered surface differ from those of a disordered surface in their sensitivity to primary-beam energy.