HYDROGEN INTERACTIONS WITH POLYCRYSTALLINE AND WITH DEPOSITED TITANIUM SURFACES

The room temperature kinetics of hydrogen chemisorption and adsorption on polycrystalline and on deposited (sputter-deposited and evaporatio n-deposited) titanium surfaces were studied. Measurements of hydrogen surface accumulation were performed in a combined surface analyses sys tem incorporating direct recoils spectrometry and Auger electron spect roscopy (AES). There, three different types of surface cleaning proced ure were applied: heat-flashing, sputtering and sputter-deposition of titanium on a polycrystalline titanium substrate. The surface chemisor ption kinetics obtained for the deposited samples were compared with t he total kinetics of the gas phase consumption, performed in a volumet ric Wagener system. From this comparison it was possible to distinguis h between topmost surface chemisorption and subsurface (or bulk) absor ption kinetics. It was concluded that, for all types of surface studie d, hydrogen chemisorbed according to a Langmuir-type random two-sites chemisorption model, with high (close to unity) zero-coverage sticking probabilities. The only difference between these surfaces was in thei r roughness factors, which increased going from the heat-flashed, thro ugh the sputtered, to the deposited surfaces. Following the initial st age of a chemisorbed surface layer formation, constant-rate absorption of hydrogen proceeded over a very wide range of exposures (greater th an 10(4) Langmuirs). The accommodation probability of hydrogen during this linear stage was about 10(-2). It is possible that this absorptio n process is controlled by the chemisorption of the H-2 on the surface hydride phase, formed by the earlier hydrogen chemisorption.