First principles calculations on nitrogen reactivity on tungsten surfaces.

Using the spin-polarized gradient-corrected density functional theory we investigate the adsorption and dissociation of nitrogen molecule and hydrogen nitride radicals on two model surfaces of tungsten, W{1 0 0} and W{1 1 0}. The goal of the investigations is to predict the number and nature of the nitrogenized moieties that could be found on a tungsten surface after nitrogen-seeded hydrogen (deuterium) plasma discharge. The surfaces are considered clean or saturated in hydrogen or nitrogen atoms as they are expected to be after nitrogen seeded hydrogen plasma irradiation. We find that the radicals NH, NH2 and NH3 are dissociated by the catalytic action of the metallic surfaces if they are animated with an initial kinetic energy of less than 2.5 eV. The products of the reaction are a nitrogen atom incorporated into the metal surface and an adsorbed proton. The general conclusion is that nitrogen reduces hydrogen retention on the surface and more generally that the nitrided surfaces should be less reactive than the clean tungsten even if the two surfaces we consider behave quite differently.