Low-Temperature Chemisorption. II. Flash Desorption of Carbon Monoxide

Flash desorption studies of the interaction of carbon monoxide with a tungsten surface indicate that this gas forms at an initial rate considerably higher than that for nitrogen (CO sticking coefficient: s∼0.5; N2 sticking coefficient: s∼0.25). This rate also remains independent of the amount adsorbed for higher coverages, up to n∼300×1012 molecules cm—2. Unlike nitrogen, CO does not dissociate on the surface. It retains its molecular identity in both the primary chemisorbed state β, and the weaker state of 20 kcal mole—1 binding energy α, which forms as the rate of β growth diminishes. At T=298°K, the α state reaches a concentration of ∼200×1012 molecules cm—2. No additional weak binding is found even at low temperatures, T∼115°K. The β state is itself made up of three subpeaks, arising from different surface structures, and designated β1, β2, and β3 in order of increasing binding energy. Only β2 and β3 form initially, with desorption energies of 75 and 100 kcal mole—1, respectively; β1 appears at higher coverages, at which the desorption energy of β2 has also diminished.