Structure and energetics of potassium overlayers on ruthenium (101̄0)

Abstract

The chemisorption and desorption of K at the (101̄0) surface of Ru has been investigated by LEED, Auger spectroscopy, Δφ, neutral atom and ion desorption measurements. The results reveal that substantial K-enrichment of the immediate subsurface region occurs for small potassium coverages at 430 K. Desorption of neutral K atoms is characterised by a monotonic fall in activation energy from 280 kJ mol −1 at zero potassium coverage to 180 kJ mol −1 near the monolayer coverage point. Analysis of the K + desorption data yields a value for the activation energy to atomic K desorption which agrees well with that determined directly from the neutral atom desorption spectra. Work function measurements indicate a constant adatom dipole moment of ~ 3.9 D over the coverage regime of 0 ⩽ θ(K) ⩽ 0.18. At higher coverages, the alkali overlayer depolarises (α = 30 Å 3), a near metallic overlayer being formed at completion of the first monolayer (θ(K) = 0.67). LEED shows the existence of a series of ordered phases for 0.36 ⩽ θ(K.) ⩽ 0.67. An examination of the splitting of the fractional-order beams indicates that the Ru(101̄0) surface can accommodate increasing K coverage by two distinct mechanisms: growth of c(2 × 2) domains when θ(K) ⩽ 0.5 and uni-axial compression along [1̄21̄0] when θ(K) ⩾ 0.5.