The Interaction of Simple Gases with Hot Metal Filaments

Abstract

The interaction of iodine, methyl bromide, methyl cyanide and methyl iodide with tantalum, tungsten, molybdenum and platinum filaments has been studied in a magnetron system. The interaction of two of these compounds, iodine and methyl iodide, with tantalum filaments has also been studied in an ultra high vacuum system. The study of the surface ionisation of iodine and the methyl compounds in the magnetron revealed the mechanisms of ion formation for the bromide, cyanide and iodide negative ions from these compounds. Adsorption and dissociation on the filament surface were found to be important in the ion formation reaction. The study in the ultra high vacuum system consisted of observing the thermal desorption of neutral species from the surface and the effects of gas adsorption on the electron work function of this surface. The study of the thermal desorption of iodine and methyl iodide revealed the presence of adsorbed molecular iodine and from a quantitative analysis of this reaction the surface coverage of the desorbing species, the order of reaction and the enthalpy and entropy of activation for the desorption reaction were ascertained. The effect of the vapours of all the compounds mentioned above on the various surfaces employed was observed using both thermionic emission and contact potential measurements and the most important factor determining the emission properties was found to be the formation of carbides by reaction of the metal with the methyl group. The physical and thermionic emission properties of tantalum carbide have been studied in detail.