Studies of Surface Electronic Structure and Surface Chemistry Using Synchrotron Radiation

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Abstract
Photoemission studies for hv < 400 eV are used to illustrate some of the advantages of synchrotron radiation for studying the surface electronic structure and chemistry of solids. Studies of GaAs and Pt are used to illustrate the following advantages (all of which depend on a source of radiation with a continuously tunable photon energy): (1) both valence and core states can be examined, (2) photon energies can be chosen so that the electron escape length can be minimized, restricting the studies to the first several atomic or molecular layers of the solids, (3) since the escape depth minimum is rather shallow the matrix element for excitation from a given set of levels may be maximized consistent with probing the last few layers, (4) when two sets of levels are degenerate or near degenerate in energy, e.g. the solid valence levels and the orbits of an adsorbed gas, the photon energy dependence of the matrix element can be used to separate the photoemission from the two states, and (5) good energy resolution (often 0.2 eV or better) can be obtained. For GaAs, studies of the surface valence electronic structure (hv ≈ 21 eV) in conjunction with LEED results indicated that rearrangement of the atoms within the last unit cell of the surface plays an important role in determining the electronic structure associated with this last layer of the crystal. In fact, it appears that measurement of this electronic structure may prove to be one of the most sensitive ways of determining the details of the atomic rearrangement. Studies of the Ga and As 3d core shifts on chemisorption of oxygen indicate that electrons are transferred from the As to the oxygen. The next step in oxidation, the formation of true As and Ga oxides, has also been followed using the core shifts. CO adsorbed on metals has been studied extensively previously using photoemission methods; however, neither the CO 3σ levels nor the "shake up" structure has been clearly seen in the past due to background produced by the strong Pt 5d valence photoemission. Making use of the Cooper minimum in the 5d excitation cross section at hv = 150 eV, the CO 3σ and "shake up" structures have been clearly seen for the first time.