A39

Surface Science 277 (1992) 184-192 North-Holland

A semiempirical quantum approach to possible structures of copper electrodeposits at submonolayer and monolayer levels on Pt(100) and Pt(111) clusters. M.B. L6pez, G.L. Estifi, E.A. Castro and A.J. Arvia lnstituto de Investigaciones Fisicoqu[rnicas Tedricas y Aplicadas (INIFTA), Facultad de Oencias Exactas, Universidad Nacional de La Plata, Sucursal 4, Casilla de Correo 16, (1900) La Plata, Argentina Received 21 January 1992; accepted for publication 2 June 1992

The development and reconstruction of surface structures formed by underpotential deposition (UPD) of Cu atoms on Pt(100) and P t ( l l l ) clusters are approached through extended Hiickel molecular orbital (EHMO) calculations applied to a statistical Cu atom deposition. These results allow us to obtain a qualitative description of the different stable configurations of Cu atoms on the Pt cluster surfaces in terms of the corresponding binding energies.

Surface Science 277 (1992) 193-206 North-Holland

The formation of misfit dislocations by climb in pseudomorphic monolayers D.L. T6nsing, P.M. Stoop and Jan H. van der Mel'we Department of Mathematics, Applied Mathematics and Astronomy, University of South Africa, P.O. Box 392, Pretoria 0001, South Africa Received 5 May 1992; accepted for publication 24 June 1992

We have used a hybrid Lennard-Jones/Frank-van der Merwe (LJ-FM) model and an embedded-atom-method (EAM) to analyse the energetics of an isolated adatom A, which is adsorbed at a second-layer lattice point above a pseudomorphic (ps) monolayer (ML) of Ni or Co on a Mo{ll0} substrate, and is allowed to climb down quasi-statically into the ps ML where it constitutes a critical nucleus for the formation of a misfit dislocation (MD). In the LJ-FM approach misfit could be introduced in a somewhat artificial, but adequate way to predict tendencies. The LI -FM analysis showed (i) that a Ni adatom has a well defined stable second-layer position when Mo is replaced by bcc Ni, (ii) that the equilibrium position is less well defined with Mo substrate even with imposed zero misfit and (iii) that as the mistfit with Mo becomes more negative, approaching the Ni and Co misfits of about - 20%, a flat energy trough develops which allows spontaneous access to the interior of the ps ML. In the EAM approach only Co reaches, more or less, the interior of the ps ML; in the case of Ni, and Cu which was also considered, strong core repulsion of substrate adatoms limits the downward climb to only 1 .~,. Clearly, negative (positive) misfit of large magnitude, is a prerequisite to the nucleation of heavy (light) MDs by climb in a process which is facilitated by strong (weak) absorbate-substrate bonding.

Surface Science 277 (1992) 207-219 North-Holland

Photoemission and scanning tunneling microscopy on K/Si(100) U.A. Effner, D. Badt, J. Binder, T. Bertrams, A. Brodde, Ch. Lunau, H. Neddermeyer Institut fiir Experimentalphysik, Ruhr-Universitiit Bochum, D-4630 Bochum, Germany

and M. Hanbiicken CRMC2-CNRS *, Campus Luminy, F-13288 Marseille, France Received 4 March 1992; accepted for publication 7 July 1992

We have measured valence band photoemission, the Si2p and K3p transitions and the work function from K/Si(100)2 × 1 in the entire coverage range with K deposited at room temperature up to saturation coverage. In addition, we have studied this system by scanning tunneling microscopy (STM). In STM at about ~ ~ of saturation coverage a surface structure with local order has ~ -~