Anisotropy of the spatial distribution of CO molecules desorbing from tungsten

The desorption of CO from an anisotropic surface of polycrystalline tungsten after different periods of annealing has been studied. Anisotropy in the spatial distribution of CO molecules in the desorbing flux was detected at early stages of annealing. Smoothing out of the surface texture during annealing recorded by means of STM resulted in the disappearance of the spatial distribution anisotropy. The results are quantitatively explained by the model of a rough surface.⁷ Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 999–1002, June, 1994.The authors express their profound gratitude to Prof. V. I. Panov and his collaborators for making it possible to carry out measurements with a tunneling microscope and for their help.     

Two-dimensional model of a rough surface and spatial distribution of the desorbed species

We consider the angular distribution (AD) of species desorbed from a rough surface described by various distribution functions (DF) of the surface topography. It is shown that the width of the AD of the desorbed flow depends mainly on the roughness coefficient and is almost independent of the DF, except for the indirect dependence through . It is established that the secondary reflection of desorbed species from an uneven surface (with 1.2) has no appreciable effect on the form of the AD. Taking two models as examples, it is shown that anisotropy in the surface topology leads to anisotropy in the AD of the desorbed species.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2699–2704, December, 1990.     

Reactions of Adsorbed CH3 Radicals and the Products of Their Decomposition at Mo and Cu Surfaces According to Spatially Resolved TPR Data

Reactions in the layer of CH₃ radicals adsorbed on the surfaces of polycrystalline molybdenum and copper were studied using the method of temperature-programmed reaction (TPR). After N₂ and CH₃ ^· adsorption (the products of azomethane pyrolysis) on molybdenum, N₂, H₂, and CH₄ were observed in comparable amounts in the TPR spectrum. At the same time, only methane was detected in the TPR products on the copper surface. The spatial distributions of CH₄ desorption flows were measured, which were indicative of translational excitation of these molecules. The direct measurements of the rates of the CH₄ molecules desorbed from the copper surface showed that their translational energy was 10–15 times greater than the thermal one. The mechanisms of reactions on the Mo and Cu surfaces are proposed. The rate constants were calculated for some of the elementary steps.     

Focusing effect of coadsorbed CO on the spatial distribution of H2 flow desorbed from iridium

Angular distributions of H₂ desorbing from the surface of polycrystalline iridium are studied by temperature-programmed desorption with spatial resolution. The presence of coadsorbed CO strongly affects the spatial distribution of the desorption flow (SDDF) of H₂. In the absence of CO, SDDF of H₂ is described by the Knudsen law. If H₂ desorbs from the layer of coadsorbed CO and H₂, SDDF of hydrogen concentrates along the normal to the sample surface. A model is proposed to explain this phenomenon.     

Spatial distributions of desorption flow as an indicator for a precursor state in several systems

Spatial distributions of the desorption flow (SDDF) of CH₄, N₂ and azomethane (CH₃N=NCH₃) on molybdenum, O₂ and N₂ on indium, and N₂ on iron were measured by TPD/TPR with angular resolution and described by the Comsa equation. The activation energy of chemisorption and the effect of the precursor state on the total desorption rate were estimated from the parameters of the Comsa equation. The pathway via a precursor state is the main one (60–90%) for oxygen on indium and for nitrogen and azomethane on molybdenum. For other systems, this pathway cannot be completely excluded either. According to our upper estimates, its contribution can be essential (∼40%). Deceased.     

Catalytic decomposition of azomethane and reactions of adsorbed methyl radicals on the molybdenum surface

The methods of temperature-programmed reaction/desorption (TPR/TPD) are used to study azomethane (CH³N=NCH³) decomposition and the reactions of the products of its pyrolysis (CH ₃ ^* radicals and N₂) on the polycrystalline molybdenum surface. A TPR spectrum of adsorbed azomethane decomposition shows mainly N₂, H₂, and unreacted azomethane. Upon preliminary adsorption of azomethane pyrolysis products on a catalyst sample, a TPR spectrum shows N₂, H₂, and CH₄ in comparable amounts. The difference in the composition of desorption products found for these two types of experiments shows that, in the decomposition of adsorbed azomethane, surface methyl moieties are not formed. The rate constants were calculated for the dissociation of adsorbed CH₃, CH₂, and CH, recombination of hydrogen atoms with each other and with CH₃ and CH₂, and the recombinative desorption of nitrogen atoms. Deceased.