SURFACE X-RAY-DIFFRACTION ON CLEAN AND CS-COVERED AG(001)

We have studied the structure of the clean and Cs covered Ag(001) surf ace, using surface X-ray diffraction. For the clean unreconstructed Ag (001) surface the analysis of the integer-order crystal truncation rod s gives evidence for a compression of the first interlayer spacing rel ative to the bulk by Delta d(12)/d(bulk) = -0.8(8)%, whereas for the s econd interlayer spacing we obtain an expansion of Delta d(23)/d(bulk) = 1.0(8)%. For the first two Ag layers we observe enhanced thermal di sorder as expressed by the isotropic mean-squared displacement amplitu de, U = 0.011(1) Angstrom(2) at 340 K relative to the bulk value of 0. 009 Angstrom 2 at this temperature. Our X-ray results for Ag(001) are in good agreement with previous experimental and theoretical results. The adsorption of 0.25ML (1 ML = 1.25 x 10(15) adatoms/cm(2)) Cs on th e Ag(001) surface at 170K leads to the formation of a c(2 x 4) superst ructure. The Cs atoms are found to occupy fourfold hollow sites at dc( s) = 2.49(20) Angstrom above the Ag(001) surface thereby shifting the underlying Ag atoms laterally by 0.029(5) Angstrom from their bulk pos itions. From the adsorption height we derive an effective Cs radius of 1.78(16) Angstrom. Large anisotropic disorder is observed for the Cs adatoms. Within the harmonic approximation we derive mean-squared disp lacement amplitudes of U-11 = 0.08(1) Angstrom(2), U-22 = 0.10(1) Angs trom(2) and U-33 = 0.27(3) Angstrom(2). An alternative model, suggeste d by the very large value of U-33, was also tried, in which there are 15% of Ag vacancies in the top layer and a lower Cs site. This 'unusua l' model is discussed in the context of current theories of alkali met al induced reconstructions.