Diffusion and evaporation of nitrogen on tungsten (100)

Surface diffusion and evaporation from a layer of nitrogen chemisorbed on the (100) plane of tungsten was examined using Auger spectroscopy. The initial localized deposit required for diffusion measurements in a concentration gradient is created by a highly collimated molecular beam. The sample, under computer control, is moved across an electron beam, and the Auger spectra are recorded automatically at different points on the surface. Nitrogen diffusion over the (100) tungsten surface was found to be concentration independent, taking place over a barrier of 28 kcal/mole with a prefactor D0 of 3.0 x 10-2cm-2/sec. At temperatures 900 to 1050 K necessary to observe spreading on the order of 50-100 μm in the concentration profile, evaporation was found to be a competing process. Therefore, the desorption kinetics of β chemisorbed nitrogen on W(100) was studied by a combination of Auger spectroscopy and flash desorption. The activation energy for desorption was found to be 82 kcal/mole with a prefactor of 0.01 molecules/(cm2sec). A comparison with the diffusion of nitrogen on the W(110) surface is made, showing for the first time the effect of surface structure on the diffusion of a chemisorbed gas on a solid surface