Field Desorption of Barium from Tungsten

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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.

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