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Structure of NH/sub 3/ on Ni(111)
In a recent study of the adsorption of NH/sub 3/ on NI(111) at T approx. 190 K using angle resolved UPS, it was concluded that NH/sub 3/ is molecularly adsorbed, and is bonded to the surface via the N atom with the H atoms oriented away from the surface. To study the bonding configuration using a direct and independent technique, we have examined NH/sub 3/ on Ni(111) using the electron stimulated desorption ion angular distribution (ESDIAD) method, coupled with temperature programmed desorption (TPD) and low energy electron diffraction (LEED). For NH/sub 3/ coverages achievable at T > 150K, (THETA approx. < 0.75 monolayers) the ESDIAD patterns are dominated by a halo of ion emission with little ion yield normal to the surface; the halo pattern is consistent with molecular NH/sub 3/ bonded to Ni via the N atom. Whereas angle-resolved UPS data indicate a specific azimutal registry of NH/sub 3/ with Ni(111) a well-defined azimuthal orientation is not evident from the ESDIAD results. Possible reasons for the differences between these results are examined, including final state effects in ESDIAD, and difference in sensitivity of the two methods to more than one possible configuration of the adsorbed NH/sub 3/