An electron energy loss spectroscopy study of resonance population in ethylene chemisorbed on Pd(110)

We investigated energy and angle dependencies of vibrational losses of ethy lene chemisorbed on Pd(110) by means of high-resolution electron energy los s spectroscopy (HREELS). A resonant enhancement of the C-H stretching modes is observed at the electron primary energy of similar to 1.3 eV. It is sho wn that the resonance cross section is significantly dependent on the elect ron incidence direction. The resonance is forbidden when the electron incid ence direction is aligned along the C-C axis, which is qualitatively explai ned in terms of the overlap integral between the incident plane wave and th e unoccupied orbital. Based on the previous finding that ethylene is anisot ropically adsorbed with the C-C axis preferentially aligned along the [<1(1 )over bar>0] direction, we assign the resonance state as 2b(2u). The result s are compared to those obtained by near-edge x-ray absorption fine structu re (NEXAFS). (C) 2000 American Institute of Physics. [S0021-9606(00)70431-4 ].