NITROGEN ADSORPTION ON W(110) VICINALS - ROLE OF STEP SITES AS DISSOCIATION CENTERS

Nitrogen adsorption has been investigated on a multi-faceted W sample exposing the (110) plane and four vicinal surfaces. The vicinals are stepped surfaces exhibiting two different step densities (inclination angle 5° and 10°) and two edge structures (steps parallel to the [001] and [1̄10] directions). The [001] steps contain {100} sites and the [1̄10] steps not. Nitrogen adsorbs dissociatively on the stepped surfaces but not the (110) plane. It is concluded that vacant step sites act as N2 dissociation centres from which atoms may migrate onto the terraces. In contrast to previous assumptions not only {100} sites are found to accomplish N2 dissociation but also the structural arrangement present on [1̄10] steps. However, [001] steps give rise to larger N sticking coefficients than [110] steps. The saturation coverage does not depend on edge structure. Coadsorbed oxygen acts as poison for N adsorption. [1̄10] steps are more affected than [001] steps. The termination of N adsorption as function of N and/or O coverage is explained by the complete occupation of step sites by the adsorbed species. © 1979.