T. Mandel

Performance of the high-resolution SX700/II monochromator

This article reports on the high‐resolution performance of the grazing‐incidence plane grating monochromator SX700/II, installed at BESSY by the Freie Universitat Berlin, in the photon energy range from about 40 to 900 eV. The high resolving power up to 10 000 achieved with this monochromator is based on improving the figure error of the ellipsoidal focusing mirror, on reducing the vertical dimension of the beam source, and on employing a 5‐μm exit slit. We report on high‐resolution gas‐phase studies in the double‐excitation region of He, as well as at core‐excitation thresholds of Ne, Ar, Kr, and Xe in the photon‐energy range from ≂45 eV to ≂900 eV. In addition, high‐resolution core‐excitation spectra at the K thresholds of C, N, and O are presented for gas‐phase CO, N2, and O2. In all cases, high‐n Rydberg states and/or vibrational sidebands of the electronic excitations were resolved. The various contributions to the present instrumental linewidths are discussed as well as the prospects for further imp...

Carbon and oxygen K-edge photoionization of the CO molecule

Abstract High-resolution, high-intensity photoionization studies were performed near the carbon and oxygen K-edges of gas-phase CO. At least 37 absorption lines were resolved at the carbon K-edge, and new information was obtained about the π*, Rydberg, and double-excitation resonances, including Rydberg states up to n=7 and vibrational transitions up to v′ = 3. Vibrational structure in oxygen is 1s−1π* and Rydberg states was resolved for the first time. The derived molecular structure parameters are consistent with the Z+1 approximation. High-resolution, high-intensity core-level photoabsorption promises new opportunities in vibrational state-to-state chemistry and surface science.

Photoemission study of alkali/GaAs(110) interfaces

A detailed core-level photoemission study of interfaces between thin alkali films andn-orp-type GaAs (110) formed at different substrate temperatures 85 K and 300 K) is reported. All the interfaces grown at 85 K (with Na, K, Rb, and Cs) were found to be non-reactive, while at 300 K, the interface with Na is reactive and that with Cs remains non-reactive. In case of the non-reactive interfaces, a strong band bending of ≊1.0 eV is observed forp-GaAs at alkali coverages as low as θ≊0.01 monolayers, but practically none forn-GaAs. This striking asymmetry in band bending is interpreted as a consequence of the donor character of the alkali atoms. On the other hand, an approximately symmetric band bending at low coverages is observed for the reactive interfaces of Na withn- andp-GaAs and assigned to defect states. For high alkali coverages (θ>2 monolayers), the final band bending is characterizeds by the same Fermilevel position forn- andp-GaAs, independent of the reactivity of the interface, and assigned to metal-induced gap states. Furthermore, systematic trends along the alkali series in Fermi-level position ionization energy, plasmon-loss features, and layer-dependent binding-energy shifts of alkali core levels are discussed.

Core-level binding energies and Auger electron energies in epitaxial rare-gas layers on graphite (001)

Abstract Layer-dependent shifts of core-level binding energies and Auger electron kinetic energies are reported for epitaxial multilayers for Xe, Kr, and Ar adsorbed on graphite (001). Despite negligible changes of the substrate work function, clearly resolved shifts are observed, which are described by layer-dependent hole screening effects. As with metallic substrates, the Auger electron energies shift three times more than the core-level binding energies, but the magnitudes of the shifts are reduced due to the small electron density in graphite.

Layer resolved photoemission study of the Cs/Si(111)2×1 interface

For Cs multilayers grown on Si(111)2×1 at 130 K, Cs 5p and Cs 4d core levels exhibit shifts to higher binding energies for second-layer Cs atoms as compared to first-layer atoms by 1.0 and 1.1 eV, respectively. For thick Cs layers, Cs 5p emission from bulk and surface atoms is resolved, resulting in a surface core level shift of 0.23 eV. These results are quantitatively described on the basis of the (Z+1) approximation using a Born-Haber cycle. In addition, layer dependent differences of the photoemission line widths and the Cs 5p spin-orbit splitting are reported as well as an anomalous energy dependence of the electron mean free path.

High-resolution inner-shell photoionization of NO

Abstract For the open-shell molecule NO, high-resolution photoionization studies of N 1s and O 1s core excitations to antibonding π* and nonbonding Rydberg orbitals were performed. In each case three vibrationally split 1 s 2 π*→1s(π*) 2 transitions were observed, caused by the interactions of the π* electrons, as well as two Rydberg series with 3 Π and 1 Π character. An unambiguous assignment of all observed resonances is given. A Franck—Condon analysis of some of the observed spectral profiles yields vibrational frequencies and equilibrium internuclear distances of the various core-excited states of NO.

Inverse photoemission study of copper at variable photon energy

Abstract We report on an angle-resolved inverse photoemission (IPES) study of Cu(100), Cu(110), and Cu(111) in the photon-energy range from 10–28 eV using a toroidal-grating IPES-spectrograph. An overall-system resolution of better than 0.3 eV (FWHM) was achieved. The Cu bulk bands were mapped along the high-symmetry lines of the bulk Brillouin zone. Taking into account both bulk- and surface-umklapp processes, good agreement with theoretical band dispersions was found. Unoccupied surface states are observed on all three faces and discussed.

Layer-resolved band dispersion of rare-gas multilayers on graphite(001)

Abstract Two-dimensional band dispersions were studied by angle-resolved photoemission for monolayers and bilayers of Xe, Kr, and Ar on graphite(001). Due to layer-dependent binding-energy shifts, the contributions of the two individual layers of a bilayer could be monitored separately. The widths of the resulting single-layer band dispersion curves were found to correlate strongly with the interatomic spacings within the layers, in agreement with expectations from band-structure calculations for unsupported rare-gas monolayers. For a Xe bilayer on graphite(001), the band widths of the second layer were found to be much smaller than those of the first layer, reflecting a relaxation of the interatomic spacings from the compressed first-layer towards those typical for bulk Xe. For a monolayer of Kr on graphite, rather narrow band widths were observed, in agreement with the expanded arrangement, while an Ar monolayer exhibited band widths close to those expected for an unsupported layer. An Ar monolayer adsorbed on a monolayer of Kr/graphite turned out to be expanded on the basis of the narrow band widths observed. In all Xe spectra, peaks from indirect photoemission transitions were observed and interpreted as due to disorder effects; they were found to be most intense for the bilayer spectrum as a consequence of the structural misfit between the two layers.

High-resolution vibrationally-resolved core-level photoabsorption of physisorbed CO/Al(111)

Abstract The results of a high-resolution photoabsorption study at the K thresholds of C and O are reported for CO/Al(111). Polarization effects on the C1s −1 π ∗ resonance show that the CO molecules are oriented parallel to the surface only in a relatively narrow range of submonolayer coverages around a dosage of ~ 0.6 L. At higher coverages, a second species of CO molecules is suggested oriented straight-up or tilted. The vibrationally-resolved spectra of the C1s −1 π ∗ resonance reveal a weakening of the intra-molecular bonding in the core-excited state for this second species relative to gas-phase CO.

Oxidation of alkali/GaAs(110) interfaces

Abstract Codeposition of K, Rb, and Cs with oxygen on GaAs(110) at ≈150 K results in the formation of distinct alkali oxides, and the respective O2−, O2−2, and O−2 species can be identified from their 0–2p derived valence-band photoemission multiplets. This does not apply to Na due to a more covalent bonding. While the substrate is unaffected at 150 K, it is oxidized upon annealing with an enhancement up to ⋍ 109 by the alkali atoms. The mechanism of GaAs oxidation is dependent on the thickness of the alkali layer.