Angular behaviour of electrons inelastically scattered from the shape resonance for CO chemisorbed on Ni(110)

Recent vibrational high resolution electron energy loss experiments (HREELS) have shown evidence for molecular shape resonances in the inelastic scattering of electrons from chemisorbed molecules. Such resonances arise from the capture of the incident electron in a quasibound state of the molecule, leading to the formation of a temporary negative ion. They are manifest as an enhancement in the intensity of a specific vibrational mode at a characteristic incident electron energy. In contrast to gaseous species, the alignment which the surface provides for the chemisorbed species, can be exploited to determine the angular characteristics of the resonant state. In this work, we show evidence for a shape resonance, centred at an incident energy of approx. 18 eV, for CO/Ni(110). The angular dependence of the scattered electron intensity has been measured for the CO stretching vibration. The results are discussed in terms of the spherical harmonic components of the resonant state, modified by vibrational broadening caused by low frequency bending modes associated with the bonding of the CO molecule to the surface.