Field Desorption of Barium from Tungsten

Abstract

The field desorption of Ba has been studied over a range of fields and coverages in an attempt to determine the mechanism of desorption and the charge of the desorbing species. It was found that adsorption seems to be polar but not ionic and that desorption of Ba++ occurs under our conditions. If this interpretation is correct α≅60 Å3 for the polarizability of adsorbed Ba results, close to the value for the free atom. The accessible field-temperature range is restricted by the onset of field-dependent surface diffusion, which was also investigated. It was found that positive fields have more effect on the activation energy Ed and pre-exponential term of the diffusion coefficient than negative ones. While a ½ αF2 dependence of Ed is compatible with the limited data obtained for negative fields, no simple behavior was found for positive ones, Ed going through a minimum of 0, accompanied by a drastic reduction in pre-exponential term. A tentative explanation in terms of a compensation effect is advanced. Values for the zero field heat of adsorption were obtained over a wide coverage interval and agree well with those of Moore and Allison where overlap occurs.

Surface electronic structures of Ba overlayers on W(100), W(110), and W(111)

“…The electronic structures of metal-adsorbate-vacuum interfaces play an important role in such fields as epitaxy and heterogeneous catalysis, while electron emission from overlayers of alkaline-earth metals and their oxides is widely used for cathodes in discharge lamps and photoelectron emission devices because of its well-known effect in reducing the work function. The work function [1][2][3][4][5][6] and the diffusion [7][8][9] of Ba atoms adsorbed on tungsten have received a lot of attention, but relatively little work, either experimental or theoretical, has been published on electron emission at W-Ba-vacuum interfaces.…”

Field- and photofield-emission spectroscopy of sodium overlayers on the (100) and (110) facets of tungsten

“…Field-induced diffusion in adsorbate overlayers has been observed by other workers at field strengths comparable to those in the present experiments. [17][18][19][20][21][22][23] The present observations can be accounted for qualitatively in terms of field-induced changes in the adsorption energies of sodium on the various crystal planes of the tungsten field emitter.…”

Stationary stochastic adparticle flip-flop as a precursor to surface diffusion

“…A related expression for the change in surface diffusion energy including a term with a permanent dipole moment has been discussed by Utsugi and Gomer [30]. If the microfield a t the adsorbed particle is assumed to be proportional to the field F a t t h e tip, F , = B F = @'/I,F, Fa = /lap, the field dependence can be expressed by…”

“…An additional small contribution by a changing field (and a permanent dipole influence which could easily be added to equations ( 2 ) to (4)) cannot be excluded at present. The effect of a changing a has been considered already for an explanation of the field dependence of the activation energy of barium diffusion on tungsten [30] and should be even more pronounced with the highly polarizable flavanthrene molecules. More data on the field dependence are required, however, for further analysis.…”