An Angle-Resolved Photoelectron Spectroscopy and Near-Edge X-ray Absorption Fine Structure Study

The nature of the surface formed by a saturated layer (8(NO) = 1.0) of NO on a Mo(100) surface at 150 K has been determined by using both angle-resolved ultraviolet photoelectron spectroscopy and the angular variation of near-edge X-ray absorption fine structure (NEXAFS). Photoelectron spectroscopy indicates that NO can adsorb molecularly at this temperature but that a portion of the NO is also dissociated, even at 150 K. The orientation of the molecular species is measured by using both photoelectron spectroscopy and NEXAFS. Both techniques yield identical geometries, indicating that chemisorbed NO has a molecular axis that is tilted at 35' f 5' with respect to the surface normal. In addition, measurement of the u-a spacing in NEXAFS suggests that the NO bond in chemisorbed NO is up to 0.03 A longer than in the gas phase. Heating this surface to 720 K causes the intensity of photoelectron peaks due to chemisorbed molecular NO to decrease by 50%. This temperature range corresponds to the desorption of molecular NO. Further heating of the surface to 1090 K causes the majority of the remaining NO to dissociate into chemisorbed nitrogen and oxygen. However, small NO photoemission peaks remain in the photoelectron spectrum after annealing the surface to 1090 K, suggesting the presence of a small amount of chemisorbed molecular NO, even after annealing to this temperature.