A466

164 SurfaceScrencc 13.5 (19X3) 164-1X3

North-Holland Puhlishtng Company

ADSORPTION AND ORIENTATION OF NH, ON Ru(OO1) Carsten BENNDORF * and Theodore E. MADEY

Sur$~e Scrence Diomon, Nu~wnul Bureuu of Stondard.s. U’ushrngrm, DC‘ 302.14. LjSA

Received 18 April 1983: accepted for publication 24 June 19X3

The interaction of NH, wtth clean Ru(OOI) surfaces has been studted using LEED (IOU energy

electron diffraction). ESDIAD (electron stimulated desorption ion angular dtstrthution). TDS

(thermal desorption spectroscopy). and work function changes (A+). Four different bindmg states

(denoted as a,, al. ,B and y ) were detected with TDS. At low coverages. NH 7 deaorha from the cr,

state with a TDS peak maximum at - 310 K. The broadening of the TDS peaks and their shift to

lower temperature with increasing NH, coverage are related to repulstve lateral interactions

between neighboring NH, molecules. At higher NH, coverages (0 NH, 2 0.15). a second desorption

peak ( a2) develops at T = 180 K. accompanied by a (2 x 2) LEED structure. With further increase

of NH, exposure a sharp desorption peak (p state) is found at T = 140 K. and is interpreted as due

to NH, species desorbing from a second adsorption layer. Finally a desotptton peak due to

multilayer adsorption (y state) is found at 115 K. At low NH, coverages (a, state). a “halo”-lihe

H ’ ESDIAD pattern gives evidence of randomly oriented or freely rotatmg NH, monomers.

bounded via the N atoms to the surface with the H atoms pointing away from the surface. This

orientation of NH, is supported by work function measurements showing a linear decrease of &$

in the a, state. Structural information concerning the adsorption geometry of NH, in the /3 state

has been obtained from LEED and ESDIAD. During the formation of the second NH, layer (p) a (26 x 2fi)R30” LEED pattern is observed and is accompanied by an ESDIAD pattern with a

hexagonal outline. A structural model of the p-state bonding. in which second layer NH,

molecules are bonded via threefold hydrogen bonds to the first layer NH,, IS proposed.

I84 Surface Science 135 (19X3) 1X4-19X

North-Holland Publishing Company

SURFACE SEGREGATION IN Au,,,Cuo,, CRYSTALS M.J. SPARNAAY * and G.E. THOMAS

Phdrps Research Lrrhorrrrorres, P. 0. Bos X0.000, 5600 JA Emdhorwt. The lirrhubml.\

Received 3 May 1983: accepted for publication 30 August 1983

A study was made of the surface segregation of the (111). (110) and (IOU) surfaces of single

crystals (fee) of the alloy Au ,Cu, ,, where .r = 0.1 (and .x = 0.9). Also polycrystalline samples

were used. The experimental technique applied was that of LEIS. The results are that in all cases

there is a gold enrichment of the surface accompanied by a depletion layer beneath the surface.

There is only a weak plane specificity. The kinetics of the segregation process is dtffuston

determined and the diffusion process is believed to be responsible for the occurrence of the

depleted region. A simple thermodynamic analysis of the alloy A, B, , results in the difference

~aoa - yAuA (~,,a are the surface tensions. aA,a are the atomic areas) as the factor determining the

segregation. For Au .Cu, _ ); alloys this difference is only 8% of the separate yu values. Since the

current value and sign of this difference pomts to a surface copper enrichment, the role of possible

correction is discussed.