THERMAL-DESORPTION STUDY OF BA AND HYDROGEN COADSORPTION ON NI(110) SURFACE

In this work, we study the coadsorption of barium (Ba) and hydrogen (H ) on Ni(100) surfaces using mainly thermal desorption spectroscopy (TD S) in correlation with Auger electron spectroscopy (AES), low-energy e lectron diffraction (LEED) and work function (WF) measurements. Two di fferent processes have been used for the coadsorption experiments: (1) Ba deposition on hydrogenated Ni(110) and (2) H adsorption on Ba-cove red Ni(110). In both cases, H gives two different TD energy states, be ta(1) and beta(2). The first state is the same with that of H adsorpti on on clean Ni(110), pointing to a direct H-Ni bonding, while the seco nd, which is attributed to a H-Ba interaction, shifts to higher temper ature as Ba-coverage increases up to Theta(Ba) = 0.9 ML. This shift in dicates a gradually developing attractive H-Ba interaction leading to BaH2 formation. The formation of BaH2 does not depend on the sequence of Ba and H deposition. For Theta(Ba) greater than or equal to 0.3 ML, H adsorption on Ba/Ni(110) does not induce any WF change, as H, does on alkali-covered surfaces. This means that the Ba/Ni(110) surface mig ht be a stable negative H ion source. (C) 1998 Elsevier Science B.V. A ll rights reserved.