ANISOTROPIC AMPLITUDES OF ADSORBATE BENDING VIBRATIONS FROM X-RAY PHOTOELECTRON DIFFRACTION - A MULTIPLE-SCATTERING EVALUATION

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-degrees 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 rigorous formalism for thermal averaging in the diffraction theory. From the azimuth-dependent width of the forward-scattering peak we have determined bending vibrational amplitudes of 12-degrees and 17-degrees for the [110BAR] 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.