Adsorption of carbon monoxide on tungsten (210)

Thermal desorption spectra taken after adsorption of carbon monoxide at room temperature on W(210) show sequential formation with increasing coverage of strongly bound β₂ and β₁ binding states, correlated to the sequential formation of P(2 × 1) and (1 × 1) adsorbate structures as observed by LEED. Adsorption at room temperature gives a poorly ordered arrangement of adsorbed CO molecules, but well-ordered structures are produced by subsequent anneal. For adsorption without anneal the work function increases monotonically with coverage to a maximum of Δφ = + 0.70 eV at saturation coverage of 1 monolayer. For adsorption followed by anneal the work function dependence upon coverage is less simple, with even a decrease of work function at coverages less than a quarter monolayer. LEED intensity-voltage measurements from P(2 × 1)CO and P(2 × 1)N structures suggest that CO molecules occupy the sites of 4-fold symmetry upon which nitrogen is believed to be adsorbed. The distinction between the β₂ and β₁ states of adsorbed CO is attributed to heterogeneity induced by the reduction in binding energy of a CO molecule when its nearest-neighbor sites are occupied.