A Kinetic Study of the Electron Stimulated Adsorption of Ammonia on Tungsten Single Crystal Surfaces

Abstract

<p>This thesis reports a comprehensive investigation of the effects of ammonia gas pressure, crystal temperature, electron beam current and angle of incidence on electron stimulated adsorption (ESA) of ammonia on W(100) and W(1lO) surfaces. An understanding of the kinetics of the ammonia dissociative adsorption has tremendous technological importance. AES studies on the interaction of NH3 with W(100) and W(110) have been conducted. The cross-section for ESA on W(110) is calculated to be 3.4x10-16 cm2. The ESA rate increases with ammonia pressure at very low pressures but becomes independent of it at higher pressures. The ESA rate is first order in electron beam current but decreases with increasing crystal temperature. Except at low pressures, the ESA rate is found coverage-independent over a large coverage range. The desorption energies of two precursor states found are calculated to be 120 kJ mol-I (N-W bonded) and 7 kJ mol-1 (H-bonded). All the evidence suggests a precursor state mechanism in which the activation sites and chemisorption sites are different. Yet it is inappropriate for a well-ordered single crystal surface and a novel alternative mechanism has been discovered. The electron beam study leads to an understanding of how different adsorbates (NH3, NH2, NH and N) make different contributions to the measured overall N AES signal, and the signal attenuation is related to the shielding of the N atom by the bonded H atoms. An adsorption mechanism involving concepts about vacant site exclusion and stimulated dissociation of adsorbates was used to simulate the coverage evolution of individual adsorbates. The Auger sensitivity factors describing the signal attenuation from hydrogen shielding and from the electron beam's incident angle were deduced. The factors and the simulated coverages combined to produce simulated AES data that match the experimental results and reproduce the apparent coverage-independence.</p>