Nitrogenadsorption and desorption at iron pyrite FeS2{100} surfaces

Abstract

We have investigated the interaction of nitrogen with single-crystal iron pyrite FeS₂{100} surfaces in ultra-high vacuum. N₂ adsorbs molecularly at low temperatures, desorbing at 130 K, but does not adsorb dissociatively even at pressures up to 1 bar. Atomic surface N can, however, be obtained with nitrogen ions and/or excited neutral species, generated by passing N₂ through an ion gun. Substantial nitrogen-induced disorder is seen with both ions and neutrals, and no ordered N overlayers form; a decrease in the S/Fe ratio is seen when exposing to nitrogen ions. Recombinative desorption leads to temperature-programmed desorption peaks at 410 and 520–560 K which we associate with interstitial atomic N and substitutional ionic N, respectively, in the surface regions. Thermal repair of sputter damage necessitates segregation of bulk S to the surface, which, over repeated experiments, leads to gross cumulative damage to the bulk crystal. The desorption temperatures associated with recombinative desorption of atomic N from FeS₂{100} are significantly lower than those measured for Fe surfaces. This is linked to the inability of FeS₂{100} to dissociate N₂, but suggests that N_ads will be significantly more able to react with other species than it is on Fe surfaces.