Physisorption of CO andN2on Al(111): Observation of surface-molecule vibrations in electron resonance scattering

Monolayers of physisorbed CO and ${\mathrm{N}}_2$ on Al(111) were investigated by means of high-resolution electron-energy-loss spectroscopy (HREELS). The line shapes of the ν=0→1 molecular stretching modes differ largely. In the high-energy-loss tail of this mode, surface-molecule stretching losses are detected with an energy of 10.5 meV in the case of ${\mathrm{N}}_2$. It is demonstrated that this large inelastic intensity is due to the negative-ion resonance scattering mode operating for ${\mathrm{N}}_2$ but not for CO. The results are discussed in the framework of Gadzuk’s wave-packet method for time-dependent quantum behavior of molecules on surfaces.