Orientation and symmetry of ethylene on Pd(110): A combined HREELS and NEXAFS study

The orientation and symmetry of low-coverage ethylene adsorbed on Pd(110) a t 150 K were studied by high-resolution electron energy-loss spectroscopy ( HREELS) and near-edge x-ray absorption fine structure (NEXAFS) analysis. HR EELS measurements unambiguously revealed that ethylene is pi-bonded with th e C-C axis preferentially aligned along the [<1(1)over bar>0] row; this fin ding was also confirmed by NEXAFS. The molecular symmetry of this species w as found to be C-1 and thus, we proposed that the C-C axis is tilted agains t the surface. The effect of coadsorption with hydrogen was examined, and a flat-lying species with C-2 symmetry was found to dominate on H-covered Pd (110). Furthermore, the coadsorbed hydrogen induced the C-C axis rotated ab out the surface normal, possibly reflecting the zigzag phase formation of a dsorbed hydrogen atoms. (C) 2000 American Institute of Physics. [S0021-9606 (00)70313-8].