The adsorption of CO on the (100) plane of tungsten; thermal and work function measurements

Abstract

Thermal desorption and work function measurements indicate that a largely molecular layer, with some dissociation, is formed at 80–100 K, with an increase in work function of 0.55 eV. The coverage in this layer is 11.5 × 10 14 molecules/cm 2, or CO/W = 1.15. On heating, equal amounts of a β precursor, possibly dissociated, and a molecular α species are formed at ≈300 K, with abundances of 5 × 10 14 molecules/cm 2 each. The α desorption is complete at 360 K. The β precursor evolves on heating without desorption in the range 400–700 K as indicated by work function decreases, to β-CO, which is almost certainly dissociated. This change occurs at lower temperatures for low coverages. Thermal desorption shows 3 peaks, which have been traditionally labelled β 1, β 2 , and β 3 at 930, 1070, and 1375 K. Of these only β 3 corresponds to a well defined state. Readsorption after heating to 950 or 1150 K results in a doubly peaked spectrum at 1070 and 1375 K. The β 1 and β 2 peaks obey complex desorption kinetics, probably corresponding to desorption and rearrangement. The coverage of β 3 is 2.5 × 10 14 molecules/cm 2, suggesting that the c(2 × 2) LEED pattern corresponds to occupany of every other unit cell by a C or an O atom. For coverages ⩽ 1.5 × 10 14 molecules/cm 2 β 3 desorption obeys second order kinetics with an activation energy of 83 ± 3 kcal/mole. For β 3 the work function decreases from the clean W value by 0.1 eV, suggesting adsorption of C and O in the center of the W unit mesh, below the surface layer of W atoms. Readsorption on β and β precursor layers leads to formation of electropositive α-CO, with a multiply peaked thermal desorption spectrum, indicating the existence of different binding sites. Adsorption-heatingreadsorption, -heating-readsorption sequences indicate that additional changes in the α desorption spectrum occur, suggesting reconstruction in the β layer.