COVERAGE-DEPENDENT AZIMUTHAL ORIENTATIONS OF PF3 ON RU(0001) - ADSORBATE-SUBSTRATE VERSUS INTERADSORBATE INTERACTIONS IN ROTATIONAL-DYNAMICS

We present evidence for a coverage dependent rotational dynamics and a zimuthal orientation of PF3/Ru(0001). ESDIAD data are interpreted to i ndicate that at low coverages at temperatures above 75 K PF3 rotates n early freely about an axis normal to the surface, while the rotation i s hindered at 20 K; the barrier height is 1 +/- 0.5 meV. At saturation coverage (0.33 ML) the rotation is suppressed at all temperatures bel ow 300 K. Surprisingly, the azimuthal orientation at saturation covera ge differs by 30 degrees from that for the low coverage at 20 K, which we explain on the basis of geometrical packing arguments and competin g interaction potentials.