M.-L. Shek

Soft X-ray studies of a c(4 × 2)* β-SiC(100) surface

Abstract We report soft X-ray results for a Si-terminated β-SiC(100) surface characterized by a c(4 × 2) plus a weak possibly (2 × 2) reconstruction in low-energy-electron diffraction. The existence of two surface Si 2p photoemission core levels is confirmed, shifted by −0.5 and −1.4 eV from the bulk-like level. We further show results which indicate that the component with the large −1.4 eV shift is correlated with the surface reconstruction. Surface-sensitive Si L 23 absorption spectra suggest the existence of a high density of unfilled s-like states about 1.3 eV above, as well as unfilled s-like states near, the bulk conduction band minimum. Reconstructed surfaces beyond this “c(4 × 2)*” surface are also noted.

Interaction of alkali‐metal overlayers with oxygen

The interaction of oxygen with alkali metals (Li, Na, K, and Cs) was studied with valence and core‐level photoemission (PE) using synchrotron radiation and by multiple mass thermal desorption (TDS). During a stepwise coadsorption of oxygen at 80 K, an increase in the emission intensity, a linewidth broadening, and a negative binding‐energy shift of alkali‐metal core levels is observed. Two stages of oxygen adsorption are identified in PE and TDS. In the low O2 exposure range, an oxide species is formed; at higher exposures, peroxide and superoxide species are observed in Na, K, and Cs. The potassium–oxygen interaction is discussed in detail.

Core level binding energy shifts of K and Cs adlayers on Ru(001)

Using synchrotron radiation from the VUV ring at NSLS and vacuum ultraviolet radiation from a HeI resonance lamp, we have recorded high resolution photoemission spectra of K and Cs overlayers on Ru(001). It is found that for “thin” multilayer coverages (∼ 3 ML) the K3p and Cs5p core levels exhibit three sets of core levels which can be assigned to interface, “bulkrd and surface emission in increasing binding energy. The results are discussed in terms of the nature of electronic interaction and a thermodynamic model. The K3p core level spin-orbit splitting is also resolved in these measurements for K in the condensed phase, for the first time with photoemission spectroscopy.

Electronic structure of Mn overlayers on the Ru(001) surface

Abstract The first three monolayers of Mn grow pseudomorphically on Ru(001) at room temperature. For coverages up to 8 ML a ( 3 × 3 R 30° LEED pattern is observed which upon annealing transforms to a (1×1) pseudomorphical one. A similar transformation is observed for thicker disordered Mn films. Corresponding UPS spectra are sensitive to the above surface structural changes. XPS and AES data suggest that island formation occurs after annealing

The early stages in the oxidation of Na

We report soft x‐ray photoemission results on the early stages of the oxidation of Na at ∼80 K. The first stage (up to ≳0.5 L exposure) is characterized by a narrow O 2p level and a Na 2p surface core level shifted by −0.55 eV with respect to the bulk core level. The Na 2p surface plasmon loss intensity shifts toward smaller energies, consistent with a decrease in surface conduction electron density. The second stage (starting from ∼1 L) is characterized by the emergence of ‘‘di‐oxygen’’ valence levels and a Na 2p component shifted by ∼0.8 eV with respect to the bulk binding energy. The oxidation is strongly nonuniform, as suggested by the small oxygen doses needed for the onset of the second oxidation stage, and the continued presence of the bulk plasmon loss from metallic Na. The two new Na 2p binding energies are tentatively attributed to different final‐state screening of metallic Na adjacent to dispersed oxygen atoms in the first stage, and of the Na atoms with mostly oxygen nearest neighbors in the ...

Coverage dependence of photoemission core levels of alkali‐metal overlayers

Photoemission from alkali‐metal core levels (Li 1s, Na 2s, K 3p, Cs 5p, and 4d) has been studied for different alkali‐metal coverages on Ru(001) using synchrotron radiation from the Vacuum Ultraviolet (VUV) ring at the National Synchrotron Light Source (NSLS). The alkali atoms interacting directly with the Ru substrate show coverage‐dependent binding‐energy shifts towards the Fermi level, with the largest shift for Li (1.1 eV) and the smallest ones for K and Cs. At coverages > 1 monolayer (ML), the core‐level shift between the second (surface) and the first (interface) layer is observed in all alkali overlayers. At ‘‘thin’’ multilayer coverages (∼3 ML), all alkali overlayers exhibit three sets of core‐level photoemission bands which are assigned to the interface, the immediate ‘‘bulk,’’ and the surface in increasing binding energy. At coverages >5 ML (‘‘thick’’ multilayer) all alkali metals exhibit bulklike properties and surface–atom core‐level binding‐energy shifts are observed in all cases.

A soft X-ray study of a LiS surface compound

Abstract We have studied a “Li2Sx” compound (x ≈ 0.79to0.95) with soft X-ray excited valence and core-level photoemission and Auger electron emission. The Lils core level is found at a binding energy of 56.9 eV, shifted by ∼ 1.9 eV relative to the bulk ls level of Li metal. The Li KVV Auger transition gives two peaks at kinetic energies 44.9 and 41.2 eV with respect to the Fermi level, corresponding to final states with two valence holes on different sites or on the same site, respectively. The S2p photoemission doublets are observed at 162.7 and ∼ 163.7 eV, their de-excitation giving rise to the SL2.3VV Auger peak at 149.2 eV. We discuss the electronic interactions in the Li2Sx surface compound, and make a comparison with those in a Li O surface compound.

Cs5p and 4d photoelectron linewidths of CsRu(001): implication for CsRu interaction at the interface

Abstract Photoelectron linewidths of Cs5p and 4d levels of Cs adsorbed on Ru(001) for a series of coverages varying from submonolayer to multilayer have been examined. It is found that linewidth broadening for Cs atoms in direct contact with Ru occurs for both levels when compared with that of bulk Cs atoms, and that while Cs4d levels broaden slightly but persistently over the entire range of coverage, the 5p linewidth exhibits a sharp change between Cs atoms that are in direct contact with the Ru substrate and those that are not; that is at submonolayer, monolayer and thin multilayer coverage, the Cs5p peak of the Cs atoms which are in direct contact with the Ru substrate broadens considerably in comparison with the Cs5p peak of the Cs atoms in the second overlayer and in Cs metal. The Cs5p linewidth broadening is attributed primarily to the relatively short lifetime of the core hole resulting from a fast Coster-Kronig/super Coster-Kronig decay channel facilitated by CsRu interaction at the interface. This channel, which is absent in the primary decay of the Cs4d core hole, has little effect on the 4d linewidth. This Cs5p linewidth broadening appears to be general for Cs in direct contact with any metal substrate and is discussed in terms of a s-d charge redistribution (rehybridization) model with the assistance of relativistic Dirac-Fock calculations. The less dramatic Cs4d broadening is attributed to a combined effect of interfacial phonon, chemical inhomogeneity and crystal filed broadening.

XAFS of Mn multilayers on Ru(0 0 1)

Abstract Mn K-edge XAFS together with LEED and thermal desorption has been used to investigate a series of Mn multilayers deposited on a Ru(0 0 1) substrate under UHV conditions. It is found that a meta-stable phase with a (1 x 1) LEED pattern can be prepared at thin as well as thick layers. The XAFS of such a system appears to exhibit an intense HCP-like EXAFS pattern indicating that the as-deposited Mn overlayer is pseudomorphic with the substrate. Upon annealing, the XAFS of the meta-stable phase collapses to a pattern similar to that of the bulk Mn.

Coverage dependent core-level shifts of Cs adsorbed on Ru(001) and observation of surface defects with a Cs probe

Abstract Adsorption of Cs on Ru(001) with coverages varying from multilayer to less than 10 −3 of a monolayer has been studied with Cs4d core level photoemission. Using photon energy optimized for adsorbate and substrate cross sections, detection sensitivity of 10 −3 of a monolayer has been obtained. The most important finding is that as the Cs coverage decreases from a full monolayer (√3 × √3R30° LEED) to 10 −3 of a monolayer, the Cs4d level shifts slightly away from the Fermi level (the entire range is 0.25 eV) until the Cs atoms on the surface are farther apart than they are in the bulk; this shift is accompanied by the appearance of a low binding energy shoulder which becomes a dominant peak at coverages below 0.02 ML. These results, together with LEED and thermal desorption observations are discussed in terms of charge redistribution (hybridization), volume, and relaxation effects in the spirit of recent theoretical and experimental results. In this model, the charge state of the alkali atoms on the surface is essentially neutral (metallic), and is insensitive to coverage. The low binding energy peak emerging at lower coverages appears to have its origins in surface defects of a good Ru(001) single crystal with a well-defined LEED pattern.