Atoms on tungsten

Todd C J and Rhodin T N 1974 Adsorption of single alkali atoms on tungsten using field emission and field desorption Surf. Sc/. 42 109-21... 

The Heats of Adsorption of Cesium Atoms on Tungsten at High Degrees of Occupation... 

Rates and k2 will both be of the form ve El/kT and ve E kT. We are going to assume that the desorption energy depends on surface coverage in the manner E2 = E2o — acesium atoms on tungsten. Rewriting Equation 4 so that the dependence on surface coverage is plain, we get... 

The ability of the ion microscope to resolve the details of the atomic arrangement of a surface is well established this by itself is exceedingly important for the study of adsorption phenomena. However, the full power of the microscope can be brought to bear upon surface studies only if adsorbed gas atoms can be resolved as well. This question has been examined in detail for nitrogen atoms on tungsten (76). Because of its importance, the evidence showing that direct observation of adatoms is indeed possible will be outlined in some detail. 

Elkowitz A B, McCreery J H and Wolken G 1976 Dynamics of atom-adsorbed atom collisions Hydrogen on tungsten Chem. Phys. 17 423... 

Fig. 47. Arrhenius plot of diffusion coefficient for (a) H and (b) D atoms on the (110) face of a tungsten crystal at coverage degree 0.1-0.9 as indicated. The cusps on the curves correspond to the phase transition.

Difoggio and Corner [1982] and Wang and Comer [1985] have discovered tunneling diffusion of H and D atoms on the (110) face of tungsten. They saw that the Arrhenius dependence of the diffusion coefficient D sharpy levels-off to the low-temperature limit (D = D ) at 130-140 K (fig. 47) the values of depend but slightly on the mass of the tunneling particle for the D and... 

Equation (89) shows that the allowance for the variation of the charge of the adsorbed atom in the activation-deactivation process in the Anderson model leads to the appearance of a new parameter 2EJ U in the theory. If U — 2Er, the dependence of amn on AFnm becomes very weak as compared to that for the basic model [see Eq. (79)]. In the first papers on chemisorption theory, a U value of 13eV was usually accepted for the process of hydrogen adsorption on tungsten. However, a more refined theory gave values of 6 eV.57 For the adsorption of hydrogen from solution we may expect even smaller values for this quantity due to screening by the dielectric medium. 

Field electron emission coupled with flash-filament studies have been employed by Condon and Hansen to study benzene chemisorption on tungsten (21). Evidence was obtained for the chemisorption of benzene by a single bond (probably of -character) to the surface. This form of asso-ciatively adsorbed benzene [(I), Scheme 1] appeared to exist in equilibrium with cr-adsorbed-CeHs (II) and adsorbed atomic hydrogen. 

Hydrogenolysis of 2-methylpentane, hexane, and methylcyclopentane has been also studied on tungsten carbide, WC, a highly absorptive catalyst, at 150-350 °C in a flow reactor [80], These reforming reactions were mainly cracking reactions leading to lower molar mass hydrocarbons. At the highest temperature (350 °C) all the carbon-carbon bonds were broken, and only methane was formed. At lower temperatures (150-200 °C) product molecules contained several carbon atoms. 

G. Ehrlich and F.G. Hudda, Atomic view of surface self-diffusion tungsten on tungsten, J. Chem. 

Fig. 6. Total electron yield versus ion kinetic energy for singly charged ions of the noble gases incident on atomically clean tungsten. Ions are all in the ground state. (From Ref. )...

In an applied field of the order of a few V/A, gas atoms and molecules, which normally will not adsorb on a surface at a given temperature, may adsorb on the surface by an effect of the applied field. Field adsorption can occur at a temperature much higher than that in ordinary adsorption. For example, He and Ne can be field adsorbed on tungsten surfaces at a temperature as high as 100 K with a probability of near one, whereas in ordinary physisorption the probability is nearly zero at such a tempera-... 

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