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Abstract

We have studied the structure of the clean and Cs covered Ag(001) surface, using surface X-ray diffraction. For the clean unreconstructed Ag(001) surface the analysis of the integer-order crystal truncation rods gives evidence for a compression of the first interlayer spacing
relative to the bulk by ∆d₁₂/d_bulk=-0.8(8)%, whereas
for the second interlayer spacing we obtain an expansion of ∆d₂₃/d_bulk=1-0(8)%. For the first two Aglayers we observe enhanced thermal disorder as expressed by the isotropic mean-squared displacement amplitude,
U=0.011(1) Ų at 340 K relative to the bulk value
of 0.009 Ų at this temperature. Our X-ray results for
Ag(001) are in good agreement with previous experimental and theoretical results. The adsorption of 0.25 ML
(1 ML=1.25x10¹⁵ adatoms/cm²) Cs on the Ag(001) surface at 170 K leads to the formation of a c(2x4) superstructure. The Cs atoms are found to occupy fourfold
hollow sites at d_Cs= 2.49(20) Å above the Ag(001) surface thereby shifting the underlying Ag atoms laterally by 0.029(5) Å from their bulk positions. From the adsorption height we derive an effective Cs radius of 1.78(16) Å. Large anisotropic disorder is observed for the Cs adatoms.
Within the harmonic approximation we derive meansquared displacement amplitudes of U₁₁=0.08(1) Ų, U₂₂=0.10(1) Ų and U₃₃=0.27(3) Ų. An alternative
model, suggested by the very large value of U₃₃, was also tried, in which there are 15% of Ag vacancies in the top
layer and a lower Cs site. This 'unusual' model is discussed in the context of current theories of alkali metal induced reconstructions.