Dynamics of interaction of H2 and D2 with Ni(110) and Ni(111) surfaces

A260 79

Surface Science 155 (1985) 79-100 North-Holland, Amsterdam ANISOTROPY

OF SURFACE

Daniel A. STEIGERWALD,

COMPOSITION

S a r a h J. M I L L E R

IN A Ni-Au

ALLOY

* and Paul WYNBLATT

Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon Uniuersity, Pittsburgh, Pennsylvania 15213, USA Received 13 August 1984; accepted for publication 26 December 1984 The anisotropy of the equilibrium surface composition in a Ni-0.1 at%Au alloy has been studied in a polycrystalline alloy. Surface compositions of 38 (randomly oriented) grains have been obtained in a scanning Auger microprobe after equilibration of the alloy at four temperatures in the range 810 to 900°C, and corresponding surface orientations have been determined by selected area channelling patterns in a scanning electron microscope. Experimental heats and entropies of segregation obtained from different grains have been found to vary by a factor of three, indicating a high degree of anisotropy. The experimental results have been compared with the results of calculations based on a nearest neighbor bond model which incorporates solute strain energy effects. The predictions of this model do not properly account for either the observed degree of anisotropy or the experimentally determined pattern of surface segregation anisotropy.

Surface Science 155 (1985) 101-120 North-Holland, Amsterdam DYNAMICS Ni(lll)

OF

INTERACTION

101 OF

H E AND

D z WITH

Ni(ll0)

AND

SURFACES

H.J. ROBOTA

*, W . V I E L H A B E R ,

M . C . L I N **, J. S E G N E R

and G. ERTL

lnstitut ff~r Physikalisehe Chemie, Unit~ersiti~tM~nchen, D - 8000 Mimchen 2, Fed Rep. of Germany Received 16 October 1984; accepted for publication 14 January 1985 Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H 2 and D 2 on Ni(ll0) and N i ( l l l ) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s o , irrespective of surface temi3erature T~ and mean kinetic energy normal to the surface ( E j_ ). The desorbing molecules exhibit a cos 0e angular distribution indicating full thermal accommodation of their translation energy. With N i ( l l l ) , on the other hand, s o is only about 0.05 if both the gas and the surface are at room temperature, s o is again independent of T~, but increases continuously with { E± ) up to a value of - 0 . 4 for ( E ± ) = 0.12 eV. The cosS0e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(ll0), it is about 0.1 eV for N i ( l l l ) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular "precursor'' state on the clean surfaces, but this effect may play a role at finite coverages.