A292

analyzer. The data were compared with differential photoionization cross sections calculated for

free and oriented molecules. The results demonstrate the upright orientation of CO on Ni( 100) and

Pt(l ll), while CO on Pt(ll0) shows a marked difference which can be explained by assuming that

the CO molecules are tilted in the [OOl] directions of Pt(l10). yielding a (2 X l)p2mg superstructure

observed in LEED. The tilt angle is estimated to about 20”. The structure model is supported by

the shape resonances of the 40 (5~) orbitals of CO/Pt( 110) as compared to CO/Pt( 111).

Surface Science 143 (1984) 177-187

North-Holland, Amsterdam

177

SURFACE SEGREGATION AND MIXED VALENCY IN DILUTE Yb-Al INTERDIFFUSION COMPOUNDS Ralf NYHOLM *, Ib CHORKENDORFF ** and Jenss SCHMIDT- MAY ***

HAS YLAB, Hamburg, Fed. Rep. of Germany

Received 16 December 1983; accepted for publication 20 March 1984

Photoemission experiments on mixed valent Yb-Al compounds prepared by diffusing Yb into

an Al(110) single crystal are reported. At high Yb concentrations surface core level peaks reveal a

purely divalent surface in agreement with recent results for single crystal YbAl,. In the dilute

limit, when the intensity from bulk Yb is reduced below the limit of detection, a surface layer of

divalent Yb still persists and a surface segregation of Yb in Al is made evident. When the bulk

concentration of Yb is reduced the bulk mean valence increases. For diluted samples, when the

formation of YbAl, in a surrounding of elemental Al is expected, Yb is still in a mixed valent

state. The highly resolved structures from trivalent Yb are compared with a recent calculation of

the 4f13 --+ 4f” multiplet pattern.

188 Surface Science 143 (1984) 188-203

North-Holland, Amsterdam

MODEL STUDIES OF THE INTERACTION OF H ATOMS WITH bee IRON Stephen P. WALCH *

Po(patomics Research Institute, 1101 San Antonio Road, Suite 420, Mountain Vieus, Cdiforniu

94043, USA

Received 13 October 1983; accepted for publication 13 March 1984

Ab initio/effective core potential cluster studies are reported for the interaction of H atoms

with bee iron. The calculations use a one-electron ECP based on the 4s’3d7 state of the Fe atom.

Two-fold and four-fold sites on the (100) surface as well as octahedral, tetrahedral, and trigonal

interior sites were studied. Four-fold surface sites are found to be bound by - 1.5 eV with the H

atom - 0.5a, above the surface. Penetration of the surface at a four-fold site involves movement

toward a second layer atom and is expected to be unfavorable. Two-fold surface sites have small

binding energies - 0.25 eV. Penetration of the surface at this site involves movement toward a

tetrahedral interior site and is downhill in energy. Tetrahedral interior sites are found to be bound

bY - 1.3 eV and are a minimum on the potential energy surface. Octahedral sites are a maximum on the potential energy surface and are estimated to be - 0.2 eV higher (including lattice relaxation effects). Trigonal sites are found to be a saddle point connecting adjacent tetrahedral

sites and this pathway leads to an estimated barrier to diffusion of - 0.1 eV (including lattice relaxation effects). The volume expansion for a H atom in a tetrahedral site is calculated to be 21%.