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 ethylene chemisorbed on Pd(110) by means of high-resolution electron energy loss 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 shown that the resonance cross section is significantly dependent on the electron incidence direction. The resonance is forbidden when the electron incidence direction is aligned along the C-C axis, which is qualitatively explained in terms of the overlap integral between the incident plane wave and the unoccupied orbital. Based on the previous finding that ethylene is anisotropically 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 results are compared to those obtained by near-edge x-ray absorption fine structure (NEXAFS). (C) 2000 American Institute of Physics. [S0021-9606(00)70431-4].