Anisotropic amplitudes of adsorbate bending vibrations from x-ray photoelectron diffraction: A multiple-scattering evaluation.

Abstract

The C 1s photoelectron angular emission from a disordered CO layer on Ni(110) at 300 K (coverage approximately 0.7 monolayer) was studied for polar angles up to 80\ifmmode^\circ\else\textdegree\fi{} and several azimuths. The experimental intensity distributions are compared to multiple-scattering cluster calculations which include the possibility of anisotropic bending vibrations of the adsorbed molecule based on a new formalism for thermal averaging in the diffraction theory. From the azimuth-dependent width of the forward scattering peak we have determined bending vibraional amplitudes of 12\ifmmode^\circ\else\textdegree\fi{} and 17\ifmmode^\circ\else\textdegree\fi{} for the [11\ifmmode\bar\else\textasciimacron\fi{}0] and [001] azimuths, respectively. The optimum agreement between experiment and theory over the whole range of polar angles is obtained for a CO layer with short-range order domains and equal occupation of on-top and short-bridge sites.