STRUCTURE OF W(100) FOR NO ADSORPTION STUDIED BY ESDIAD

The adsorption state of nitric oxide on a W(100) surface at room tempe rature has been studied by means of electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LE ED). Only O+ ions are detected by ESD from the NO adsorbed W(100) surf ace. The ESDIAD pattern from the NO-adsorbed surface has a bright cent er spot and 4-symmetry [100] and [110] lobes with very weak intensity. At low NO exposure (below 0.4 L), the center spot is a hazy square in shape, and becomes circular in shape with increasing exposure. The [1 00] and [110] lobes appear around the center spot above 0.4 L. With in creasing temperature, the intensities of the center spot and the [100] lobes exhibit similar variation, and the [110] lobe intensities decre ase monotonically. The surface after NO adsorption has a diffuse (4 x 1) structure, which changes to the sharp (4 x 1) structure around the temperature at which the center spot intensity decreases (similar to 9 00-1100 K), and a (2 x 1) structure appears above 1100 K. There are 4 adsorption sites for O atoms on the NO-adsorbed surface.