Adsorption of Activated Nitrogen on Tungsten

Abstract

Adsorption and desorption of activated nitrogen on a polycrystalline tungsten filament were studied by the flash filament desorption technique incorporated with mass spectroscopy and field emission microscopy. Nitrogen in excess of the β‐nitrogen structure on tungsten is conveniently prepared by the adsorption of atomic nitrogen resulting from electron impact on nitrogen gas, or, less effectively, from thermal activation. The structure thus prepared is shown to be equivalent in all major properties to that prepared by decomposition of ammonia on tungsten at 800°K. Specifically, the (β + x)‐nitrogen structure, prepared by activated nitrogen adsorption, can contain nearly twice the nitrogen of the β‐nitrogen structure. x‐Nitrogen flash desorption can be resolved from β‐nitrogen desorption and is found to be second order in x‐nitrogen with activation energy about 47.6 kcal/mole. When a $\beta$ ¹⁵N predose is followed by an activated ¹⁴N dose extensive isotopic mixing occurs in both x‐ and β‐nitrogen flash desorption peaks. All of these properties are, within experimental error, properties of the $\mathrm{(\beta \hspace{0.17em}+\hspace{0.17em}x)}$ structure obtained by ammonia decomposition on tungsten. Work‐function–flash‐temperature curves and field emission pattern photomicrographs obtained from a tungsten tip dosed with ammonia at 600°K can be very nearly duplicated by dosing with activated nitrogen instead. A model involving electron impact dissociation of nitrogen, adsorption and desorption of atomic nitrogen on glass, and formation of x‐nitrogen from adsorption of atomic nitrogen on β‐nitrogen predosed tungsten surface provides a correct dependence of the amount of x‐nitrogen on dosing pressure, time, and current with reasonable values of parameters.