Reaction of nitrogen with fcc- and bcc-iron films on copper(100)

Abstract

Up to ∼10 ML epitaxial iron films grow on Cu(100) with fcc and beyond ∼10 ML with a bcc structure. As revealed by UPS and work function change (Δ φ) measurements, even at T ad=75 K, nitrogen adsorption leads to spontaneous and complete N 2 dissociation on all of these iron films, followed by the formation of a merely chemisorbed nitrogen layer on iron films of <1.5 ML, but of both chemisorbed and incorporated nitrogen on iron films of >1.5 ML. This phase change is associated with the availability of octahedrally iron-coordinated interstitial sites. The critical exposure for nitrogen incorporation (1) increases with increasing iron-film thickness and (2) is no longer reached at T ad≥150 K due to a decreased N 2 sticking coefficient. Bcc films show a reduced tendency for nitrogen incorporation compared to fcc films.