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A survey of biophysical and biomedical applications of free-electron lasers (FELs) is presented. FELs are pulsed light sources, collectively operating from the microwave through the x-ray range. This accelerator-based technology spans gaps in wavelength, pulse structure, and optical power left by conventional sources. FELs are continuously tunable and can produce high-average and high-peak power. Collectively, FEL pulses range from quasicontinuous to subpicosecond, in some cases with complex superpulse structures. Any given FEL, however, has a more restricted set of operational parameters. FELs with high-peak and high-average power are enabling biophysical and biomedical investigations of infrared tissue ablation. A midinfrared FEL has been upgraded to meet the standards of a medical laser and is serving as a surgical tool in ophthalmology and human neurosurgery. The ultrashort pulses produced by infrared or ultraviolet FELs are useful for biophysical investigations, both one-color time-resolved spectroscopy and when coupled with other light sources, for two-color time-resolved spectroscopy. FELs are being used to drive soft ionization processes in mass spectrometry. Certain FELs have high repetition rates that are beneficial for some biophysical and biomedical applications, but confound research for other applications. Infrared FELs have been used as sources for inverse Compton scattering to produce a pulsed, tunable, monochromatic x-ray source for medical imaging and structural biology. FEL research and FEL applications research have allowed the specification of spin-off technologies. On the horizon is the next generation of FELs, which is aimed at producing ultrashort, tunable x rays by self-amplified spontaneous emission with potential applications in biology.

Photoelectron spectroscopy data from Bi2Sr2Ca1-xYxCu2O8+delta Single crystals, for x = 0, 0.16, and 0.55, are presented. It is shown that there are core level shifts related to the opening of a pse ...

The electronic structure of the diamond C(100)‐(2×1)/(2×2) has been investigated by means of angle‐resolved photoelectron spectroscopy for the first time. A surface‐related shift has been observed in the C 1s core‐level spectrum. The surface‐state band dispersion was measured along the symmetry axis Γ–J’ in the surface Brillouin zone. For k∥=0, there is a very pronounced surface state 1.5 eV below Fermi level EF , and it disperses downwards with increasing k∥. Near the boundary of the surface Brillouin zone J’, we find two states with binding energies of 1.9 and 2.4 eV with respect to EF .

We report the observation of electrochemical deposition and stripping of a monolayer of Pb on the surface of Au(111) using an in situ scanning tunneling microscope. Sequential STM scans taken during slow potential sweeps show that the Pb layer forms by initial growth at Au step edges followed by Pb island nucleation on Au terraces. When the potential sweep is reversed to remove the Pb monolayer, the surface is restored to a state similar to the original, but the details of the Au topography are modified

The high-resolution Ca ${\mathit{L}}_{2,3}$ x-ray-absorption near-edge-structure (XANES) spectrum of a ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ single crystal has been measured by use of a magnetic-projection x-ray microscope probing a surface area of 200\ifmmode\times\else\texttimes\fi{}200 \ensuremath{\mu}${\mathrm{m}}^{2}$. The Ca ${\mathit{L}}_{2,3}$ XANES spectrum is analyzed by performing a multiple-scattering XANES calculation in real space and comparing the results with the spectrum of ${\mathrm{CaF}}_{2}$. Good agreement between the calculated and experimental crystal-field splitting ${\mathrm{\ensuremath{\Delta}}}_{\mathit{f}}$ of the Ca 3d final states is found and the splitting is shown to be smaller by 0.5 eV than in the initial state. The Ca 3d partial density of states is found to be close to the Fermi level in the initial state. The Ca-O(in plane) distance is shown to be a critical parameter associated with the shift of the Ca 3d states relative to the Fermi level; in particular, we have studied the effect of the out-of-plane dimpling mode of the in-plane oxygen atoms O(in plane) that will move the Ca 3d states on or off the Fermi level. This mode can therefore play a role in modulating the charge transfer between the two ${\mathrm{CuO}}_{2}$ planes separated by the Ca ions.

We report angle-resolved photoemission data from single crystals of C60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C60. We assert that the width of the photoemission peak of C60 does not reflect the intrinsic band width because it is broadened by the non 0-0 transitions via the Franck-Condon principle. Our view point provides a possible reconciliation between these photoemission data and those measured by other techniques.

We discuss the use of synchrotron radiation in the photon energy range between 32 and 300 eV for performing very surface sensitive X-ray photoemission spectroscopy. The usefulness of this technique is illustrated with results for the chemisorption of oxygen on the (110) surface of both gallium arsenide (GaAs) and gallium antimonide (GaSb). Binding energy shifts in the 3d levels of gallium, arsenic, and antimony due to chemisobed oxygen can be seen for oxygen coverages below 5 percent of a monolayer combining the surface sensitivity of Auger electron spectroscopy with the precise chemical information of X-ray photoemission spectroscopy. When oxygen is adsorbed on GaAs, charge is transferred from the surface arsenic atoms to the chemisorbed oxygen with no breaking of back bonds and, consequently, no oxidation of the surface. In GaSb, on the other hand, there is simultaneous charge transfer from both the surface gallium and antimony atoms, indicating that back bonds are broken and oxide formation is taking place. The escape depth for GaAs (110) was also measured for electron kinetic energies between 20 and 200 eV.

Surface extended x‐ray adsorption fine structure (SEXAFS) has been used to investigate the structure of Sb on the Si(001) 2×1 surface. The coverage of Sb which remains after annealing thick layers at 375 °C, previously reported to be one monolayer (ML), is found in this work to form a disordered overlayer with three dimensional Sb clusters. This finding is concluded from the Sb L3 absorption spectra which are similar for this coverage to that of bulk Sb. After a 550 °C anneal, Auger electron spectroscopy, and scanning tunneling microscopy (STM) show that about 1 ML of Sb remains. Phase and amplitude analysis of the Sb L3 edge SEXAFS shows that the remaining Sb atoms occupy a modified bridge site with a Si–Sb bond length of 2.63±0.04 A. The Sb atoms form dimers with a Sb–Sb bond length of 2.91±0.04 A, which is almost identical to the bulk Sb–Sb bond length of 2.90 A. The Sb atoms lie 1.74±0.06 A above the Si(001) surface. STM confirms the dimer structure of the Sb overlayer.

The magnetic projection photoelectron microscope performs imaging photoelectron and photoabsorption spectroscopy with a lateral resolution of the order of microns. We review practical experience accumulated in two months of synchrotron-based operations. Theoretical limitations and practical issues related to count rates, elemental and chemical contrast and extraneous noise are discussed. A comparison is made between our experience with retarding field analysis and a conceptual imaging band-pass analyzer. Recent observations from surface diffusion experiments are presented.

Photoemission core spectra from Gd silicides (Gd 3 Si 5 , GdSi and Gd 5 Si 3 ) are presented. With respect to pure crystalline Si the Si 2 p and 2 s levels shift toward lower binding energies as follows: 0.4 eV in Gd 3 Si 5 , 0.7 eV in GdSi and 1.2 eV in Gd 5 Si 3 . On the contrary, the Gd levels (4 f , 5 p , 4 d and 4 p ) do not shift within the experimental sensitivity. The observed shifts are discussed in connection with the available information on the valence electron states of Gd silicides.

High-quality ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ superconducting single crystals, annealed at different oxygen partial pressures, have been studied using angular-resolved x-ray photoelectron spectroscopy with a resolution higher than that used in any previous study. Two states of the oxygen, separated by \ensuremath{\approxeq}0.7 eV, are unambiguously observed. Examining these components at different angles makes it possible to distinguish bulk from surface components. Using this capability we discover that annealing under lower oxygen partial pressure (1 atm) results in oxygen intercalation beneath the Bi-O surface layer of the crystal, whereas for higher-pressure anneals (12 atm) additional oxygen is found on the Bi-O surfaces. This steplike intercalation mechanism is also confirmed by the changes observed in the Cu and Bi core lines as a function of the annealing oxygen partial pressure.

Surface extended x-ray-absorption fine structure (SEXAFS) has been combined with scanning tunneling microscopy (STM) to determine both the local and long-range bonding properties of the Si(001)2{times}1-Sb interface. Sb {ital L}{sub 3} edge SEXAFS shows that Sb dimers occupy a modified bridge site on the Si(001) surface with a Sb-Sb near-neighbor distance of 2.88{plus minus}0.03 A. Each Sb atom of the dimer is bonded to two Si atoms with a Sb-Si bond length of 2.63{plus minus}0.04 A. STM resolves the dimer structure and provides the long-range periodicity of the surface. Low-energy-electron diffraction of vicinal Si(001) shows that the Sb dimer chains run perpendicular to the original Si dimer chains.

It is pointed out that the change in the oxidation state of Cu in YBa2CU3O7-x with increasing x vitiates the trend in the Cu(2p) satellite intensity and hence the Cu-O charge-transfer energy. When Y is partly replaced by Ca, however, the satellite intensity and T(c) decrease with the increase in Ca content or hole concentration, just as in other cuprates.

We report the observation of electrochemical deposition and stripping of a monolayer of Pb on the surface of Au(111) using an in situ scanning tunneling microscope. Sequential STM scans taken during slow potential sweeps show that the Pb layer forms by initial growth at Au step edges followed by Pb island nucleation on Au terraces. When the potential sweep is reversed to remove the Pb monolayer, the surface is restored to a state similar to the original, but the details of the Au topography are modified

We present results of resonant-photoemission and high-resolution x-ray-photoemission-spectroscopy (XPS) studies on single-crystalline CuO, which confirms most of the earlier data obtained from polycrystalline samples. However, some minor differences with earlier results were also observed, which are concentrated on a mainly oxygen-related feature. The important first ionization state is much better resolved in our high-resolution XPS data, and is found to have both copper and oxygen orbital character. This is consistent (though not a proof) with the theoretical prediction that the first ionization state is a {sup 1}{ital A}{sub 1{ital g}} singlet with holes on both oxygen and copper sites. The 16-eV satellite is found to show a Cu antiresonance, and is assigned to a {sup 1}{ital A}{sub 1{ital g}} singlet with both holes on the copper sites.

Description de la technique utilisee pour obtenir la section efficace differentielle de photoionisation (σ) et le parametre d'asymetrie (β) du photoelectron, a partir des spectres de photoemission. On montre que dans la phase condensee, le parametre d'asymetrie peut etre determine experimentalement avec plus de precision que la section efficace differentielle, a cause de plusieurs incertitudes experimentales

We have performed resonance photoemission, angle-resolved photoemission, and core-level photoemission studies of single-crystalline CoO. On the one hand, strong correlation effects among the {ital d} electrons are observed, as signaled by a strong reduction of Co 3{ital d} bandwidths and satellites in both the valence band and the cation core levels. On the other hand, the oxygen states are found to be very bandlike, as indicated by strong dispersions of oxygen states in the valence band and the lack of oxygen satellites. We give estimations of 30 and 1.5 for {ital U}/{ital W} (Coulomb interaction divided by bandwidth) ratio of Co 3{ital d} bands and O 2{ital p} bands, respectively. By comparing the experimental and theoretical {ital E} versus {bold k} relation, we show that the density-functional band calculation works well for the oxygen bands but not for the Co bands. We argue that CoO is not a band insulator, but a charge-transfer insulator. We have also observed the effects of local magnetic order on the electronic structure. Finally, we suggest a guideline on calculating the band structure of CoO: introducing a mechanism that reduces the Co 3{ital d} bandwidth by 25% while still retaining the other essential features ofmore » the band calculation.« less

We have examined In–GaAs(110) contacts fabricated in situ both by depositing In at room temperature followed with annealing and by depositing In on heated surfaces [n‐ and p‐type GaAs(110) surfaces (250 and 300 °C, respectively) using synchrotron radiation photoemission. It is found that In deposition on GaAs(110) surfaces with temperature below room temperature does not induce measurable reaction, whereas reaction takes place with InAs as product when the room‐temperature‐formed interface is annealed or when In is deposited on the heated substrate. However, there is no evidence of InxGa1−xAs graded heterojunction formation at the interface in either case. Outdiffusion of the excess As from the substrate is suggested to provide a source for InAs formation instead of the previously proposed In–GaAs replacement reaction. Electrical measurements were also performed on in situ In/n‐GaAs(110) contacts before and after annealing. Reproducible high‐barrier heights were recorded (1.02±0.02 eV) with good ideality ...

Palladium deposits supported on amorphous alumina were prepared in ultrahigh vacuum and studied using transmission electron microscopy (TEM), x‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and energy loss spectroscopy (ELS). The growth of clusters with deposition time was followed using TEM. The XPS, AES, and TEM results support a consistent model for the distribution of palladium on the alumina substrate, with the amount of material within the clusters accounting for the majority of the palladium deposited. The combined results of XPS, ELS, and TEM indicate that the deposit growth with deposition time can be tentatively separated into the following size regimes: monolayer islands, ‘‘small’’ clusters and metallic three‐dimensional clusters. The size‐dependent XPS Pd 3d binding energy shifts are analyzed within the initial‐state‐final‐state framework. Component fitting of the Pd 3d spectra is consistent with assigning one of the components to the layer of interfacial Pd atoms.

GaAs band bending in the low adatom coverage regime (

We present Yb L III X-ray absorption spectra (XAS) from a series of valence fluctuating Yb 1- x Sc x Al 2 ( x = 0; 0.2; 0.4; 0.6; 0.8) isostructural pseudobinary alloys. Yb mean valences, as derived via a conventional intensity analysis of the Yb L III double-peak singularities, result to monotonically increase upon decresing the lattice spacing, i.e. as the crystal is progressively shrinked by increasing the Sc-content. The Yb valence vs lattice spacing trend is found to be strictly similar to that already reported on the YbAl 2 binary compound while spanning different conditions of external pressure. Discrepancies in the Yb valence evaluation outcome when comparing Yb L III XAS versus Yb 4ƒ XPS results, XAS valences being significantly higher than XPS values in the case of Sc-rich alloys ( x = 0.6 and 0.8).

Interfaces between UHV-cleaved GaP (110) and overlayers of Sn, Sb, and Bi have been studied using soft X-ray photoemission spectroscopy. The substrate core level attentuation indicates that these three interfaces exhibit Stranski-Krastanov growth. The overlayer core level spectra (Sn 4d, Sb 4d, and Bi 5d) exhibit two components for coverages up to one monolayer, indicating that the adatoms form a zigzag chain overlayer as has been reported at the Sb/GaAs interface. The Ga 3d and P 2p surface components disappear as the first monolayer develops, and the remaining bulk components are used to determine various lineshape parameters of these core levels. The Schottky barrier formation at these interfaces has also been examined. The band bending at these interfaces occurs chiefly for coverages exceeding one monolayer, when islands are forming on the ordered layer. For all three of these interfaces the final surface Fermi level position is in the neighborhood of 1.0eV above the valence band maximum.

We have performed angle-resolved photoemission experiments and density-functional band calculations for NiO and CoO. The measured oxygen bands are in excellent agreement with the calculation, but the measured cation 3d dispersion is only about 25% of the calculated one. We have also demonstrated the effects of the antiferromagnetic order on the electronic structure of these materials

We have investigated the interesting alloy system Ce(Ru{sub 1{minus}{ital x}}Rh{sub {ital x}}){sub 3}B{sub 2}, where the physical properties change from superconducting to ferromagnetic as {ital x} changes from 0 to 1. We report the results of transport, magnetic, crystallographic, and electron-spectroscopy measurements, and a theoretical analysis of the electron-spectroscopy data using the impurity Anderson Hamiltonian applied to the Ce 4{ital f} electrons. Superconductivity exists in the range 0{lt}{ital x}{lt}0.38, while ferromagnetism occurs in the range 0.84{lt}{ital x}{lt}1. Our studies of superconductivity and ferromagnetism in Ce(Ru{sub 1{minus}{ital x}}Rh{sub {ital x}}){sub 3}B{sub 2} show that ferromagnetism is associated with nearly trivalent Ce ions. We find that the alloying mechanism controlling the low-energy properties of this system is the transition-metal 4{ital d} density of states at the Fermi level, rather than any parameter directly connected to the 4{ital f} states, such as its binding energy, Coulomb interaction, or the average hybridization strength between the localized 4{ital f} and conduction electrons. We use this result to further elucidate our previously proposed exchange split-Kondo resonance model of the ferromagnetism near {ital x}=1.

UV and X-ray photoelectron spectroscopic methods are presently used to study the band-bending behavior and interfacial chemistry of Al, In, Ag, and Pt overlayers on vacuum-cleaved p-CdTe and p-ZnTe (110). All four metals are found to yield Schottky barriers on CdTe and ZnTe. The metal-induced gap states model prediction of a difference in barrier heights for two semiconductors which is dependent on their band lineup is borne out by the results for Ag, Pt, and Al, but not for In. Reaction and intermixing for Al, Ag, and Pt overlayers on CdTe and ZnTe indicate that these interfaces are not ideal.

A technique is demonstrated for measuring photoemission and photoabsorption spectra from areas as small as 20 µm in diameter. A magnetic projection photoelectron microscope relying on the electron imaging and magnifying properties of a diverging magnetic field is described. This photoelectron microscope is of the type originally conceived by the Turner group at Oxford University. XANES spectra from 20 µm diameter regions of high Tc superconductors and from 40 µm diameter regions at the bottom of vias etched through photoresist are presented. Small area UPS spectra of In deposited locally on GaAs are also shown. Potential exists to measure photoabsorption spectra from areas on the order of one micron square.

A review of photoemission measurements on high-temperature superconductors is presented. Earlier photoemission results and interpretations are critically examined, providing the reader with an overview of the present status of the field. Special emphasis is paid to recent progress, in particular the revelation of the superconducting gap, and angle-resolved photoemission studies of high-quality single crystals of the Bi2Sr2CaC2O8 material. Comparison with predictions of band calculations and impurity model calculations as well as the electronic structure of some transition-metal oxides is made. Finally, comments on the present status of photoemission studies of the high-temperature superconductors and future implications are given.

We present calculations of resonance photoemission spectra for Nd{sub 2}CuO{sub 4}. The calculations use a model Hamiltonian for which all parameters for the valence electrons are obtained from {ital ab initio} calculations or atomic data. Most features of the calculated sp agree well with experiment where comparison is possible. A substantial dependence on the polarization of the light is predicted for the occurrence of resonance behavior, and for the off-resonance intensity of the local singlet and one of the satellites.

Photoemission results from Al and Rb interfaces with single crystals of Bi2Sr2CaCu2O8 high‐temperature superconductors are reported. The Al and Rb adsorbates are found to react quite differently with the Bi2Sr2CaCu2O8 substrate. While adatoms of Rb significantly affect only the Bi and O atoms in the top atomic layer, the Al adsorbate profoundly disrupts the bonding character of the whole Bi2Sr2CaCu2O8 material. For Al, the Bi and Cu states are strongly reduced, and the Sr and O states show evidence of oxidized components. In addition, Al causes a strong out‐diffusion of oxygen from the bulk. The differences in the reactivity of Al and Rb are discussed in terms of the different mobility of the two atoms.

We consider interfacial chemistry and band bending behavior for Al, In, Ag, and Pt overlayers on vacuum‐cleaved p‐CdTe and p‐ZnTe (110) in a comparative study using ultraviolet photoelectron spectroscopy and x‐ray photoelectron spectroscopy. A range of metal–substrate reactivities have been considered: Al reacts strongly with Te, Ag moderately, and In minimally, with no evidence seen for In reaction on ZnTe. Pt exhibits strong alloying behavior with both Cd and Zn. These results for the binaries are compared to metal/HgCdTe interface formation. We find that Hg loss can significantly influence the extent of reaction and/or intermixing for these overlayers, with resulting disruption either inhibiting or facilitating chemical interaction. Reaction and intermixing for Al, Ag, and Pt overlayers on CdTe and ZnTe indicate these interfaces are not ideal. The possible role of defects at these four metal/CdTe and metal/ZnTe interfaces is considered, and provides a consistent explanation for the final Fermi level po...

Gd{sub 3}Si{sub 5}, GdSi, and Gd{sub 5}Si{sub 3} were investigated with photoemission spectroscopy in the photon-energy range 40.8--149 eV by exploiting the energy dependence of the photoemission cross sections and the valence resonance at the crossing of the Gd 4{ital d}-4{ital f} threshold. The modification of the spectra versus photon energy, along with their stoichiometry dependence, show the relevance of covalent mixed Gd 5{ital d}--Si 3{ital sp} states in the formation of the chemical bond. In the region close to the Fermi level an increase of the {ital d} contribution is observed. These points are discussed in connection with the existing models of the silicide bond.

The novel properties of heavy-fermion materials involve rare earth 4f electrons or actinide 5f electrons. For the 4f electrons the spectrum measured in photoemission and bremsstrahlung isochromat spectroscopies is dominated by single-site energetics and the impurity Anderson Hamiltonian gives at the semiquantitative level a unified description of the 4f spectrum and certain low energy properties characterized by a single-site Kondo temperature TK. Current efforts for 4f electron materials are directed at finding the quantitative limits of this impurity approach by detailed fitting of spectra using hybridization matrix elements calculated within the local density approximation (LDA). Examples presented herein are CeCu2Si2 and the α and γ phases of cerium metal. For the most commonly studied 5f case of uranium no such unified picture exists. Various current approaches to understanding uranium spectra include analysis and modeling of differences between measured spectra and LDA calculations, and purely experimental approaches such as alloy and dilution studies of UxY1-xPd3 and UxY1-xAl2.

Through the use of synchrotron radiation photoemission spectroscopy (PES) and related techniques, we have gained detailed knowledge of Fermi level pinning, interfacial chemistry and disruption of GaAs for coverages up to several monolayers (ML). A link has been made between these data and that in the thick layer regime (hundreds of ML), which characterizes practical Schottky diodes. PES results for thin layers deposited at room temperature (RT) and low temperatures of about 80 K (LT) as well as thick films deposited at RT and annealed to higher temperatures are considered. At LT where GaAs disruption is minimized for thin films, metal-induced gap states seem to dominate the Fermi level pinning process except where GaAs metal reactions are strong. For RT thin and thick films, the effects of defects must be considered, and the advanced unified defect model (AUDM) is applied. In the AUDM the key defects are identified as the As Ga (double donor with levels at 0.75 and 0.5 eV above the valence band maximum) and the Ga As antisite (double acceptor) with the As Ga normally dominating due to the excess As which characterizes LEC GaAs crystals. The literature is reviewed and a number of phenomena are explained in terms of this model including the Fermi level position on MBE grown GaAs observed by Svensson et al. and the anomolously high Schottky barrier height (SBH) of thick Ga on n-GaAs observed by several groups. By performing electrical, TEM, and chemical studies of thick diodes and by evaluating the changes upon thermal annealing of diodes it is found that the AUDM successfully predicts the increase or decrease of barrier height on annealing.

Recent instrumental developments have extended the spatial resolution of photoelectron spectroscopy into the micron range. In this paper, one approach for high lateral resolution photoelectron microscopy which employs a superconducting magnet as a magnifying/projection lens is described along with recent results on photoemission (core level and valence band) and photoabsorption (near edge) spectroscopies.

Metal/InP(110) interfaces have been systematically studied using photoelectron Spectroscopy. This work aims at understanding the physical nature of interface states and Fermi level pinning mechanisms. Clear-cut experimental evidence shows that two types of interface states are responsible for Fermi level pinning. At the disrupted interfaces which occur at room temperature, a large amount of defects are generated during various interfacial processes and pin the surface Fermi levels around 0.95 eV above the valence band maximum (VBM). On the contrary, much fewer defects are present at the nearly perfect interfaces which are obtainable at reduced temperature. In these cases, the metal induced gap states play an overwhelming role, and the Fermi level is pinned around 0.75 eV above the VBM. It is also concluded that interplay between the two mechanisms should be taken into account in order to understand the process of Schottky barrier formation.

The use of synchrotron radiation in the soft and hard X-ray spectral region received major impetus with the start of parasitic operation of the Stanford Synchrotron Radiation Project (SSRP) in 1974. This was the first time that synchrotron radiation from a multi-GeV electron storage ring was made available in a user facility for studying the structure of matter. Here we review the early work at SSRP as well as the activities that preceded it, highlighting the scientific accomplishments (soft X-ray photoemission, EXAFS, protein crystallography), beamline instrumentation developments and source improvements. The early work using bending-magnet radiation led to the funding of several dedicated facilities in the US and elsewhere in the world – the so-called second-generation light sources. Early work with wiggler and undulator insertion devices led to funding of third-generation sources better optimized for insertion device sources, particularly undulators.

A study of the potential for the development of the Linac Coherent Light Source (LCLS) beyond the specifications of the baseline design is presented. These future developments include delivery of X-ray pulses in the 1 fs regime, extension of the spectral range, increase of the FEL power, exploitation of the spontaneous emission, and a more flexible time structure. As this potential is exploited, the LCLS can maintain its role as a world-leading instrument for many years beyond its commissioning in 2008 and initial operation as the world's first X-ray free-electron laser.

Semiconductor surface core level shifts are studied by use of selected overlayers. It is shown that certain nonreactive overlayers fully quench the surface core level shifts of the substrate without introducing additional interfacial components. The unique possibility of separately characterizing the bulk core level lineshape allows a precise determination of the surface core level shifts. This method is illustrated for the cleavage faces of Si and InP.

The idea of LiF lenses as monochromators for inverse photoemission spectroscopy (IPES) was introduced recently. Here we report a simple, versatile, inexpensive and very efficient IPES system. The high‐yield LiF monochromator system consists of a custom‐designed LiF biconvex lens as a UV photon dispersement element (∼128 mm/eV). It focuses light below 11.8 eV into a tunable KBr‐coated spiraltron detection system. With the detector set at 11.6 eV, experiments on n‐GaAs(110) and Au/n‐GaAs(110) yield 2400 and 6000 counts/μA s, respectively, with a resolution of 0.4–0.6 eV. Investigation into the lens quality and other components suggests that better yield and resolution are achievable in the future.

Resonant photoelectron spectroscopy studies have been performed on the NiO valence band at photon energies corresponding to the Ni $2p$,$3p$,and O $1s$ absorption thresholds. Strong resonances are seen in the vicinity of the Ni $2p$ threshold, which confirm earlier conclusions from the weaker resonances seen at the Ni $3p$ threshold. No valence-band resonance is observed at the O $1s$ threshold. The analysis of this data confirms the picture of NiO as a strongly correlated charge-transfer insulator by identifying the highest-lying states as being of mainly $3{d}^{8}\mathit{L}$ final-state character. The existence of localized excited Ni states, as well as the delocalized nature of the O states, are confirmed. Comparisons with the configuration-interaction model and quasiparticle calculations are also made.

Synchrotron radiation photoemission spectroscopy suggests that the room temperature grown Au/Si(111) interface is an abrupt interface with the contact made by metallic Au and bulk Si. Si 2[ital p] core level spectra show no sign of an interface component, but only the surface reacted and bulk Si components during the growth of the interface. A surface Au-Si alloy film is first formed for Au coverages below 3 monolayers. The alloy is then stabilized into an Au[sub 3]Si-like film and detached from the Si substrate when metallic Au starts nucleation in between.

Photoabsorption cross sections have been studied extensively for many years, both experimentally and theoretically. Detailed tabulations1-4 are available over large energy regions. In the vacuum ultraviolet and soft X-ray spectral regions, however, much less attention has been paid to the energy dependence of photoionization cross sections of the atomic subshells. From a spectroscopic point of view, this is a very important spectral region,5 and knowledge of the energy dependence of subshell photoionization cross sections is of great value and necessity for a large number of spectroscopic experiments on atomic and molecular gases, physi- and chemisorbed atoms and molecules, solids, and liquids.

We report angle-resolved photoemission data from single crystals of C60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C60. We assert that the width of the photoemission peak of C60 does not reflectthe intrinsic band width.

Diamond is an important semiconductor which has great potential in high temperature, high power device applications. In the fabrication process of diamond electronic device, doping ofdiamond and understanding of diamond/metal interfaces are important. As a Column V element, Sb is a possible dopant for diamond. Early work reported that Sb is incorporated into diamond by ion implantaion [1]. In addition, Sb plays an important role in Si and Ge heteroepitaxial growth. On the Si or Ge surface one ordered monolayer of Sb occupies the epitaxial sites and saturates the surface dangling bonds, which leads to uniform epitaxial growth. While diamond has the same crystal structure as both silicon and germanium, it has a drastically smaller lattice and much stronger bond. This makes it very difficult to extrapolate antimony's behavior on diamond from its behavior on either silicon or germanium. In this work, we have studied the electronic and geometric structure of Sb on diamond surfaces using photoelectron spectroscopy and low energy electron diffraction. While the exact adsorption sites could not be determined, we find that antimony strongly bonds to the diamond surface. Further, antimony behaves very differently on the diamond(100) face as compared to the diamond(111) face. We also find that neither Sb/diamond system behaves like antimony on either silicon or germanium. We attribute these results to the drastically smaller diamond lattice and the stronger C-C bond.

Photoelectron spectroscopy (PES) has been one of the primary techniques with which Schottky barrier formation on III-V semiconductor substrates has been measured and correlated with overlayer morphology and chemistry. One important class of overlayers is the unreactive materials such as In, Ga and Ag, which cluster on III-V substrates kept at room temperature. Partially reactive materials such as Al and Au have also been observed to cluster on these substrates. Clustered systems have a nonuniform distribution of pinning sites on the surface, and as a result, nonuniformities in the surface potential are also anticipated.1 Such nonuniform surface potentials complicate the interpretation of band bending measurements from these systems. It is the purpose of this paper to describe a model treating the clustering problem.

Photoemission spectroscopy is commonly used to study band bending at clustered metal-semiconductor interfaces. However, these band-bending data have been difficult to interpret because of the nonuniform distribution of pinning sites. In this paper the interpretation of these measurements is investigated in detail. Using three-dimensional Poisson integration programs, we examine the extent to which the band bending can be ascribed to states at the cluster-semiconductor interfaces. First, the potential variations in the semiconductor near an individual cluster are calculated. The depletion of the semiconductor along the surface is found to be significantly less than the one-dimensional depletion length. Next, the surface-potential variations are computed for systems of known cluster morphology, assuming that the pinning states are restricted to the cluster-semiconductor interfaces. From these potentials, the expected band-bending measurements are extracted and compared to experiment. At submonolayer coverages, we find that the experimental band bending is greater than can be attributed to pinning states under the clusters. Thus, this low-coverage band bending is interpreted in terms of surface states between the clusters. Defects, chemisorbed adatoms, and surface unbuckling are discussed as possible sources of these states. Above one monolayer coverage, the clusters cover enough of the surface to uniformly pin the Fermi level. Experimentally, however, a separation between the n- and p-type surface Fermi-level positions persists to the highest coverages. This separation is also interpreted in terms of intercluster surface states.