CASE OF UNDERLAYER CHEMISORPTION - BETA-NITROGEN ON W[110]

The dissociatively bound β state of nitrogen on tungsten is known to adsorb with a low sticking probability (∼10-3) on the {110} plane, and to relatively low saturation coverages. We report here on a combined LEED, angle-resolved photoemission, ion-scattering (IS) spectroscopy, AES and thermal desorption study of this system. Adsorption of β-nitrogen at 300 K was found to be accompanied by the formation of a good (2 × 2) LEED pattern. The coverage can be increased from 2 × 1014 atom cm-2 to 3 × 1014 atom cm-2 by thermally cycling to 900 K. IS spectra were obtained using 1000 eV He+ ions over a wide range of incidence, scattering and azimuthal angles; the W ISS peak is attenuated by nitrogen adsorption but no N peak could be observed, indicating that the N adatoms lie beneath the surface layer of W atoms. Auger spectra were obtained over a range of incidence angles which confirm this underlayer model: the ratio of peak-to-peak heights for N and W passes through a maximum at an incidence angle θ of 50°, in contrast to the overlayer system O/W{110}, where the O/W ratio was found to increase continuously with θ. The underlayer model also provides an explanation for angle-resolved photoemission spectra obtained from the N/W{110} system. © 1979.