Sukretnyi V. Adsorption of oxygen on the faces of single crystal tungsten at cryogenic temperatures

The thesis is devoted to study of oxygen adsorption on (100), (111), (112), (110) and vicinal [3(110x( 0 1)] і [5(110)x(0 1)] faces of a tungsten at low, down to 5K, temperatures. The UHV setup of the "black chamber" type has been designed to accomplish these researches. It has been established that low-temperature ТD spectra of molecular oxygen adsorption states posseses three peaks: at desorption temperatures 25K, 27K irrespective of crystallographic orientation of the tungsten substrate, and the peak at desorption temperature 45K, which is displaced to 60K only for W (111). The peaks correspond to multilayer condensation, physisorption and weak molecular chemisorption states, respectively. Adsorption energies 0,07, 0,08, 0,13 eV (0,17 eV for W (111)) are attributed to these states. Their occupation proceeds sequentially. The molecular beam technique has been applied to study a adsorption kinetics at effusive sourse gas temperature of 78-550K. A Similarity of oxygen adsorption kinetics on the (100),(112), (111) and vicinal tungsten faces and qualitative by different initial adsorption kinetics on the (110) face were observed. The probable mechanisms of adsorbed film formation for the investigated systems has been analyzed. A role of intrinsic and extrinsic precursor states during adsorption process has been elucidated on the ground of two-stage model of adsorbed layer population and experimental date on dependences of sticking probability on coverage, substrate temperature and kinetic energy of impinging molecules. An existence of dissociative character of adsorption at 5K and effect of surface structure and defects on adsorption kinetics have been prooved. The obtained results manifest a capability of the adsorption kinetics data to serve as a informations sourse concerning the potential barrier nature and involvement of precursor states in dissociative adsorption.