The lattice vibrations of nearly incommensurate overlayers Ag on Ni(100) and Cu(100)

A374 Surface Science 217 (1989) 529-562 North-Holland, Amsterdam

529

THE LATTICE VIBRATIONS OF NEARLY INCOMMENSURATE OVERLAYERS Ag on Ni(100) and Cu(100) J.E. BLACK Department

of Physics, Brock Unrversity, St. Cathannes,

Ontario, Canada

D.L. MILLS The Institute for Surface and Interface Irvine, CA 92717, USA

W. DAUM,

C. STUHLMANN

Instttut ftir Grenzjltichenforschung

Science and Department

of Physics,

University

of Californra,

and H. IBACH

und Vakuumphysik,

Kernforschunsanlage

Jiilich, Jiilich,

Fed. Rep. of German-v Received

20 September

1988; accepted

for publication

28 February

1989

A chemisorbed monolayer may form and N x M overlayer, where N or M (or both) may be large. We formulate an approximate description of the lattice vibrations in such structures that includes coupling between the overlayer vibrations, and the substrate phonons. The analysis is a lattice dynamical analogue of Brillouin-Wigner perturbation theory; the scheme is accurate, and readily employed to generate surface spectral densities. A key feature is the ability to analyze the response characteristics for wave vectors in the surface Brillouin zone of the substrate. This greatly simplifies the task of interpreting the output of the numerical computational schemes. We also discuss experimental data on the surface vibrations of a monolayer of Ag on the Ni(lOO) and the Cu(100) surface, which are closely approximated by a (2X 8) and (2X10) structure, respectively. We achieve a quantitative fit with a simple model, and discuss the physical origin of the interaction between adsorbate and substrate phonons, in regions of the substrate surface Brillouin zone where the adsorbate vibration frequencies lie well below those of the substrate phonons of the same wave vector.

563

Surface Science 217 (1989) 563-572 North-Holland, Amsterdam

SOME DYNAMICAL PROPERTIES OF THE ALLOY DISORDERED CHAIN ON A SEMI-INFINITE SOLID SQUARE LATTICE A. KHATER,

N. AUBY

Sernrce de Physique des Atomes et des Surfaces, 91 I91 GI/-sur- Yvette Cedex. France

Centre d’Etudes Nucl&aires de Saclay,

and R.F. WALLIS Department Received

of Physics, 19 January

University of California at Irvine, Irvine, CA 92717, USA 1989; accepted

for publication

3 March

1989