Structure and energetics of potassium overlayers on ruthenium (1010 )

Structure and energetics of potassium overlayers on ruthenium (1010 )

A560 Surface Science 176 (1986) 505-529 North-Holland, A m s t e r d a m CO ADSORPTION Cu(lll) STUDIES 505 ON PURE AND Ni-COVERED SURFACES W...

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A560 Surface Science 176 (1986) 505-529 North-Holland, A m s t e r d a m

CO

ADSORPTION

Cu(lll)

STUDIES

505 ON

PURE

AND

Ni-COVERED

SURFACES

W. KIRSTEIN,

B. K R I S G E R

and F, THIEME

lnstitut fur Physikalische Chemie, Universitiit Hamburg, Laufgraben 24, D 2000 Hamburg 13. Fed. Rep. of German), Received 17 March 1986; accepted for publication 16 June 1986 The adsorption of CO on pure and Ni-covered Cu(111) surfaces has been studied by means of LEED, TDS, UPS and work function measurements during adsorption and desorption. Different Ni-coverages between 0.1 and 2 monolayers were obtained by Ni-evaporation controlled by a quartz micro balance and by AES. Near room temperature Ni grows in a layer-by-layer mode on Cu(111). The island structure of the surfaces with submonolayer Ni-coverages is clearly demonstrated by TDS und LEED results obtained after CO adsorption. As with surfaces of bulk C u - N i alloys CO adsorption on Cu(111) with submonolayer Ni-coverage is dominated by a site effect. Cu-, Ni-, and mixed adsorption sites can be distinguished. The CO induced work function changes for Ni- and Cu-site adsorption show the same sign as observed with the pure metals. Mixed site adsorption has only a minor influence on the work function. A "ligand effect" observed only for the Ni-site adsorption, and only at small Ni-coverages is discussed in detail. Studies on the adsorption kinetics reveal that the Cu-sites may serve as precursor sites for Ni-site adsorption. Detailed UPS studies demonstrate that the CO-induced emission maxima observed on Cu surfaces with submonolayer Ni-coverages can be interpreted as a superposition of the respective adsorption features observed with the pure metals, roughly separated by their work function difference.

530 STRUCTURE

Surface Science 176 (1986) 530-546 North-Holland, A m s t e r d a m AND

ENERGETICS

OF POTASSIUM

OVERLAYERS

ON RUTHENIUM (1010) K. HARRISON,

R.M.

LAMBERT

* and R.H. PRINCE

**

Department of Physical Chemistry, University of Cambridge, Cambridge CB2 1EP, UK Received 20 March 1986; accepted for publication 10 June 1986 The chemisorption and desorption of K at the (1010) surface of Ru has been investigated by LEED, Auger spectroscopy, Aq~, 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 tool -1 at zero potassium coverage to 180 kJ mol -~ 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 m o m e n t of - 3.9 D over the coverage regime of 0 ~< 0(K) ~< 0.18. At higher coverages, the alkali overlayer depolarises ( a = 3 0 ~3), a near metallic .overlayer being formed at completion of the first monolayer (0(K) = 0.67). LEED shows the existence of a series of ordered phases for 0.36 ~ 0(K) ~< 0.67. An examination of the splitting of the fractional-order beams indicates that the Ru(1010) surface can accommodate increasing K coverage by two distinct mechanisms: growth of c(2 × 2) domains when 0(K) ~< 0.5 and uni-axial compression along [1210] when 0(K) >~ 0.5.