Crowding and tilting in dense adsorbate layers; An ESDIAD investigation of CO on Ru(001) and Ni(110)

The orientation of CO on Ru(001) and Ni(110) at high coverages, where deviations from the perpendicular orientation of the molecular axis are expected from existing geometric models, has been investigated by recording the angular directions of electronically stimulated ion desorption (ESDIAD). Optimum ESD parameters for minimal influence of effects other than initial state orientation (orientation of final state potential, angle-dependent recapture, image charge interaction), which have been derived earlier, have been utilized (excitation energies above adsorbate core thresholds; low surface temperature; use of O⁺ fragment ions). In saturated layers on Ru(001), CO is inclined on the average by about 5° to the normal, independent of azimuth. On Ni(110) at saturation, an inclination by 19° from the normal, strictly in the [100] azimuth alternating to both sides going along [110] is found. Occupation of bridge sites appears likely for the tilted species.