MEAN SURFACE PHONON-DISPERSION CURVES FOR LOW-INDEX FCC (A(C)B(1-C))(1) B(KLM) SURFACE ALLOYS/

A theoretical formalism for calculating the mean surface phonon disper sion curves for surface alloy systems (A(c)B(1-c))(1)/B(klm) is develo ped, using the matching method and an effective field approach. Numeri cal results which are presented for the (100), (110), and (111) surfac es of the (PtcCu1-c)(1)/Cu(klm) system are, however, carried out withi n the virtual crystal approximation which is described by random distr ibutions of Pt atoms. In this approximation short range disorder effec ts are not predicted. Significant shifts of surface dispersion branche s are found, as functions of the defect concentration c (0 less than o r equal to c less than or equal to 1), that would be measurable within instrumental resolution of EELS and HAS. Thus, the fit of the calcula ted branches to the experimental data, in particular for small wavevec tors, would lead to information concerning the values of the defect cr ystal interaction parameters.