Auger neutralization spectroscopy of clean and adsorbate-covered transition-metal surfaces by deexcitation of metastable noble-gas atoms

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Abstract
The extreme surface sensitivity of metastable noble-gas atom deexcitation spectroscopy is demonstrated with clean Pd(111), Cu(110), and W(polycrystalline) surfaces as well as for the adsorption systems Pd(111)/H, W(polycr.)/H, Pd(111)/O, Cu(110)/O, and W(polycr.)/O. Proper analysis of the data leads to information on the electronic surface density of states not available with spectroscopic techniques of finite sampling depth such as uv photoelectron spectroscopy. The results are compared with calculated electronic surface densities of states if available. Chemisorption of hydrogen and oxygen strongly affects the electronic surface density of states because surface states and resonances are shifted to higher binding energies or lose their surface state character due to the formation of the adsorbate—metal bond. In addition, occupied bonding as well as antibonding adsorbate-metal states are observed. Based on the specific sensitivity of this spectroscopic technique for the outermost atomic layer, it can be shown that H on Pd(111) and O on Cu(110) adsorb at 300 K above the first metal layer, in contrast to earlier conclusions which favored subsurface sites.