Table of contents

The best gauge of the success of a conference is the long-term impact of its proceedings. Some of the previous editions of the VUV conference have been, using this criterion, extremely successful: their proceedings still enjoy an active life over the desks of many ultraviolet scientists. As Editors of the 1989 VUV proceedings, we face the tough challenge of matching these performances. The present volume is only part of the conference production; it is a collection of invited talks, whereas contributed papers will appear in regular issues of Physica Scripta. Both invited and contributed presentations were, in our opinion, at an excellent level. We are, therefore, reasonably confident that these proceedings will meet the challenge.

The papers presented here reflect the characteristics that make the VUV conference truly unique – and explain its success over many years. First, the international character: the conference and its proceedings are a fair representation of the status of this field all over the world. Second, interdisciplinarity: under the umbrella of vacuum ultraviolet, the proceedings host research in diverse fields. This has been in the past an opportunity for cross-fertilization, and we certainly hope that the present volume will reach the same objective. In a world of topical conferences, interdisciplinarity is a rare commodity, which made our task much more enjoyable.

The proceedings of a conference are, of course, the creature of the authors of the papers, rather than the editors'. Thus, the credit for (we hope) the success of these proceedings should be given primarily to a multitude of authors from many different countries. Even the credit for the organization of the work cannot be solely taken by the editors. Extraordinary personal circumstances curtailed our physical presence at the conference, raising the nightmarish prospective of papers spread all over the world, from Honolulu to California and from Wisconsin to Sweden. The project, however, did not collapse; the merit goes to the hard work and impeccable organization of two extraordinary persons, to whom we are deeply indebted: Michelle Siggel and Patricia Rackley. We are equally indebted to Pamela Layton, for her competent help in the final stage of the editorial work. The success of this volume will be as much theirs as ours.

We particularly appreciated the help of Michelle and Pat, because it was unstintingly offered at the cost of diminishing their opportunity to experience the natural beauty of Hawaii. That beauty. and the hospitality of the Hawaiian hosts were very much part of the conference. We dreamed fantastic ways to put their flavor in this book, perhaps by a chemical concentrate of the perfume of plumeria flowers, or a clever simulation of the Hawaiian surf. Unfortunately. none of these dreams can be transferred to the harsh reality of printed stuff. The colleagues that were at the conference can use their imagination; the others will entirely miss the unique environment of the meeting. We will be forever grateful to Chuck Fadley and his collaborators, that made it possible for all of us to savor it directly.

Gerhard L Weissler, Distinguished Professor Emeritus of Physics at the University of Southern California, and founder of this series of International Conferences on Vacuum Ultraviolet Radiation Physics, died of cancer on Sunday 13 June 1989, at his home in Encino, California at the age of 71.

Professor Weissler was born in northern Germany in 1918 and, after having earned his BS degree in physics from the Institute of Technology, Berlin in 1938, he came to the United States as a graduate student in physics at UC Berkeley. He earned his MS and PhD degrees in physics from UC Berkeley in 1941 and 1942, respectively. From 1942 to 1944 Professor Weissler was an instructor of medical physics at the University of California Medical School. In 1944. Professor Weissler joined the Department of Physics of the University of Southern California as an Assistant Professor, where he was promoted to full professor in 1952. In addition to directing his primary research program in atomic and molecular physics, he also served as the Department Chairman from 1951 to 1956. and directed USC's Nuclear Accelerator Laboratory from 1955 to 1965. He retired in 1988.

In 1979, USC honored him with an Associates Award for Excellence in Teaching and in March 1989 he received USC's Special Emeritus Award. Professor Weissler was also a fellow of both the American Physical Society and the Optical Society of America.

His illustrious career was devoted to the pursuit of excellence, and he succeeded admirably. He was one of the preeminent pioneers in the field of vacuum ultraviolet spectroscopy, carrying out research in all phases of vacuum ultraviolet physics. He invented the condensed spark discharge light sources which were a key to the early investigations of both the structure and absorption cross sections of atoms and molecules. He also used these light sources to study solid state and collective phenomena. The availability of his very bright light sources, together with increasingly improved vacuum systems, made much of this work possible for the first time. The early literature on the absorption spectra of atoms and molecules in the VUV region is dominated by the work of Professor Weissler and his many students. As his research evolved, specific processes involved in the absorption of very short wavelength radiation were identified. Quantifying these processes is the subject of much current research, as is evident from the variety of papers presented in the present proceedings.

For almost 30 years, the large and active group of faculty members, students, and research associates assembled by Professor Weissler made USC one of the world's centers in ultraviolet atomic and molecular spectroscopy. Many members of this group are well known scientists who have established distinguished careers in major universities and industrial and government laboratories; one of his post-doctoral fellows is at this time President of the University of Munich, one of the leading universities in the world. The highly successful careers of his many graduate and postdoctoral students were surely given a running start by the quality of their research experience under his guidance. His enthusiasm and extraordinary energy level were highly infectious, and he will be sorely missed by all of us who knew him.

Professor Weissler is survived by his wife, Claire, and two sons, Roderick and Robert.

Soft x-rays are attractive for microscopy because they tend to be less damaging to specimens than charged probes. In addition their interactions with specimens can result in element-specific information and the penetration depth is adjustable by choice of the beam energy. Resolution on the order of 50 nm has been demonstrated, and further improvements are anticipated.

The experimental program at the NSLS X1A beamline is dedicated to soft x-ray microscopy. We are developing two types of instruments, both dependent on the remarkable brightness of the undulator source. One of these uses a Fresnel zone plate to focus the beam to a small size. This microprobe is used either to study biological specimens, or, in a different apparatus, to study surfaces by photoelectron microscopy. The other type of instrument makes use of x-rays diffracted by the specimen. In this category we are involved in the development of Gabor holography, Fourier transform holography, and in imaging by soft x-ray diffraction.

There are two central motivations for developing new scientific methods. One is, of couse, to accomplish what established methods cannot. A second is for comparison: To verify the conclusions of established methods. That is, are results obtained by one method congruent with those obtained by another independent means of measurement? In regard to microscopic imaging in biology, this means that we seek to ground our view of microscopic structure on more than a single methodological standard, with whatever particular uncertainties that standard presents. These are the motivations that underlie the current impetus for the development of x-ray microimaging methods.

Our knowledge of the internal structures of biological cells has been shaped in great part by 40 years of study applying and developing the methods of electron microscopy. This has led to the evolution of a model of the cell that contains defined structures with established details and known spatial relationships. Belief in the fidelity of this model to the natural cell rests in great part on the understanding that the preparative procedures commonly used in electron microscopy, procedures that greatly modify the natural object, do not alter or distort intracellular structure as to form, location or high resolution detail. Even though the cell as seen in the electron microscope most certainly resembles the natural object, important questions of the faithfulness of the image often remain.

The electronic structure of oxide thin films on silicon single crystals has been measured with high spatial resolution by x-ray absorption near edge spectroscopy (XANES) using a photoelectron microscope and monochromatic synchrotron radiation. Native oxide overlayers on Si(100) and Si(111) crystals were studied at various stages during thermal desorption of the oxide. The spatial resolution capability of the microscope is used to perform selected-area XANES measurements of silicon and silicon oxides, which reveal for the first time the absorption characteristics of intermediate oxidation states of silicon (Si²⁺ and Si³⁺), and identify the spin-orbit splitting of the silicon 2p core-exciton structure in Si²⁺, Si³⁺, and SiO₂. Our results confirm photoemission measurements which find Si²⁺ and Si³⁺ states existing at vertical interfaces between Si and SiO₂. Unlike the photoemission spectra, however, selected-area XANES can resolve the spectroscopic signatures of these intermediate oxidation states in laterally inhomogeneous interfaces.

The combined use of synchrotron and laser radiation for photoelectron spectroscopy of free atoms is described with special emphasis on the polarization of both radiation sources. By the absorption of the polarized laser radiation an atomic alignment or orientation is produced which has a strong influence on the subsequent absorption of the polarized synchrotron radiation and on the angular distribution of the photoelectrons.

Recent work using high resolution angle-resolved photoemission to determine the Fermi surfaces for surface-localized electronic levels on several metal surfaces is reviewed. We examine our fundamental motivation for these studies and their relation to phenomena observed in other branches of chemistry and physics.

Quantum wells and superlattices are examples of artificially-produced material structures exhibiting novel electronic properties. The ability to grow such structures with the atomic precision afforded by molecular-beam-epitaxy gives the solid-state experimentalist exciting opportunities to study interesting quantum-electronic effects and to gain new fundamental understanding of material properties. There are many examples of such work involving semiconductor materials, such as GaAs and Al_xGa_1-xAs; however, the basic electronic effects associated with these artificial structures should be found in metallic systems as well. We have examined a variety of such metallic systems with angle-resolved photoemission, a direct probe of electronic band structure. Quantum-well electronic states were observed for Ag(111) overlayers on several different metallic substrates, including Au, Cu, and Ni. In these cases electron confinement is made possible by band structure mismatch between the overlayer and the substrate. The states were observed in Ag films as thick as 40 monolayers (ML), indicating that they have a rather long coherence length. Advantage can be taken of this to apply photoemission to the study of buried interfaces. This is demonstrated through an analysis of data from Ag/Cu systems, in which the reflection phase shift of Bloch electrons at the Cu/Ag interface is determined. For superlattices, systems of alternating slabs of Ag(111) and Au(111) of various configurations with periods ranging from 8 to 12 monolayers were studied. The superlattice bands derived from the sp states of the constituent materials were directly mapped along the (111) direction. They clearly exhibited the fundamental electronic effects of superlattice formation, that is, folding according to the expanded periodicity and the formation of energy gaps at the new zone boundaries. The bands were dispersive even above the Au band maximum, where electronic states would be nonpropagating in bulk Au, indicating that the system cannot be regarded as a collection of noninteracting quantum wells of the type mentioned above.

We demonstrate that the absorption and emission of VUV radiation by atoms and ions is determined to a large extent by many-electron or correlation effects. Some of these were discovered a number of years ago, while the others have been observed only recently.

We illustrate the discussion of correlation effects, considering the case of photon absorption. Then demonstrate that almost the same effects which show up in VUV absorption are also very important in emission. Any deviations from the predictions of the simple hydrogen-like picture in photoabsorption cross-sections are due to the many-electron nature of the atom and in this sense are many-electron effects.

Threshold and near threshold satellite intensities are examined with respect to threshold behaviour, specifically for certain electron correlations contributing to the intensity of these satellites. It is assumed that the different dominant electron correlations give rise to at least four types of threshold behaviour, making it possible to differentiate to a first approximation among them. These correlations are subdivided phenomenologically into two classes: "intrinsic" and "dynamic" correlations. Experimental evidence is presented for both inner- and outer-shell photoionization, substantiating this semiempirical concept. Our results are also compared with first ab initio calculations. The effect of resonances is emphasized as part of the threshold behaviour but also with respect to their deexcitation into various decay channels. Multielectron processes such as resonant shake-off, as well as two step autoionization and spectator Auger decay, are shown to play important roles in the decay of resonances and satellite states.

This paper presents a new design for a helical permanent magnet undulator. it can be easily implemented on a synchrotron source without engineering difficulty. The effect on the electron beam is discussed and found to be acceptable. Flux and brilliance is similar to those from conventional planar undulator. Heat load can be considerably reduced which makes it a very attractive device for the new generation of 6-8 GeV light sources. On the ESRF, very brilliant helical radiation is expected in the 0.5-10 keV photon energy range.

Two topics of new developments in the theory of microclusters will be discussed. One is on melting and magnetic ordering of transition-metal microclusters of sizes N = 7–17 studied by the extensive Monte Carlo simulation of H T Diep, S Sawada and S Sugano (1989 Phys. Rev. B 39 9252), and another on fragmentation of metal microclusters showing the shell effect of valence electrons studied by the use of the theory of shell corrections of M Nakamura, Y Ishii, A Tamura and S Sugano. In the Monte Carlo studies, the interplay between magnetic and cohesive interactions is shown to give rise to sharper melting as well as a sharp magnetic transition. In the studies of the fragmentation, the shell effects are shown to play an essential role in determining how the deformation of metal clusters is developed in the process of the fragmentation.

The photoionization of small aggregates of medium-sized molecules is discussed taking into account 1-photon and 2-photon excitation. The appearance energies of ionic aggregates are considered in terms of thermochemistry. Intracluster Penning ionization, autoionization and dissociative electron transfer as well as dissociative proton transfer are elementary follow-up processes of photoionization. The chemical reactions observed in ionic aggregates are influenced by a size effect. The mechanisms of these reactions will be discussed.

Angle-resolved photoemission and inverse photoemission spectroscopy have been applied to investigate the occupied and unoccupied bulk and surface band structure of semiconductors. Together with the in situ combination of both techniques, applicable to a direct determination of surface band gaps, and with angle-resolved constant-initial-state spectroscopy, applicable to the study of empty states above the vacuum level, detailed information on the electronic structure is provided over a wide common energy scale. Here we give a brief review of our recent experimental results on III-V semiconductors, the cleaved (110) surfaces of several compounds and the GaAs () surface prepared by molecular beam epitaxy. The results will be discussed in view of the available surface models and calculations of the electronic structure.

Electronic and atomic structures of ordered metal-overlayers on Si(111) and Si(001) surfaces have been investigated by angle-resolved photoelectron spectroscopy. In the VUV photon energy region, angle-resolved photoemission has given us information about surface-state dispersion, and in the soft x-ray region, angle-resolved photoemission has given us information about surface atomic geometry. Metal overlayers treated are those of elements in the periodic table of Columns Ia, Ib, IIIb, IVb, IVb/Vb and VIb; surface orders treated are √3 × √3 on the Si(111) surface and 2 × 1 on the Si(001) surface.

Recent developments in the field of ultra-soft x-ray emission spectoscopy have opened up several new possibilities for research in atomic, molecular and solid state physics. Monochromatized synchrotron radiation has become available for selective excitation of ultra-soft x-ray fluorescence spectra, recorded in high resolution. Substantial advancements have taken place in the experimental techniques used to record highly resolved emission spectra of free molecules, excited by electron beam. This progress is discussed with regard to the new experimental developments and to current research in various fields. Some lines of direction, along which ultra-soft x-ray spectroscopy is likely to develop further are pointed out.

While x-ray emission spectroscopy is a relatively old technique, it takes on important new characteristics when samples are selectively excited by a carefully prepared beam of x-rays. This method is made practical by the availability of synchrotron radiation sources and it extends the capabilities of the traditional technique. Both the energy and the polarization of the excitation x-rays can have significant effects on the emission spectra. Selective control of these parameters can provide specific information on both the electronic structure and the atomic structure of molecules or solids. Examples of the new types of studies include: Satellite free x-ray emission, multi-vacancy excitation, polarized x-ray excitation and emission, and state selective excitation of neutral atoms and molecules.

Novel spectral features and new information regarding core-excitation processes are revealed in the photoabsorption and photoemission experiments performed with a newly constructed synchrotron radiation beamline. This beamline, dubbed Dragon, has an unprecendented resolving power and superior transmission in the soft x-ray region. Several experimental results on gas phase, solid and biological systems are presented to exemplify the scientific applications of high resolution soft x-ray spectrocopies. The possible application of photoabsorption spectroscopy as a tool for chemical analysis is also discussed and a few examples are given.

The performance of a 55-meter Spherical Grating Monochromator (SGM) is described. A resolution of 60 meV has been achieved at 400eV, inferred from the linewidths of the nitrogen 1s-π* resonance. A photon flux of 4 × 10¹⁰ photons/s has been observed at 440eV (and with 0.5 eV resolution). An initial experiment has studied the core-shell resonances of gas-phase ethylene, C₂H₄. Vibrational fine structure was resolved both for the carbon 1s-π* and carbon 1s-Rydberg excitations. Comparison with the vibrational frequencies of ground state ethylene implies that the ν₁ (C-H stretch) and ν₂ (C-C stretch) or ν₃ (H-C-H bend) are excited. It is suggested that the lower Rydberg orbitals, 3s and 3pσ, have molecular, anti-bonding character.

The fine structure of C₂H₄ molecules in the (1s^-1, π*) state is resolved using a plane grating ellipsoidal mirror grazing incidence monochromator at the storage ring BESSY. Two vibrational series can be identified, assigned to C-C and C-H vibrations. The natural line width of the (1s^-1, π*) state was found to be 160 meV for condensed C₂H₄ multilayers. For submonolayers of C₂H₄ molecules on Ag(100) and Cu(100) this value was found to increase to 280 and 700 meV respectively. A shift to lower energies of the ground state transition is observed for the adsorbed molecules with respect to the multilayer case.

At the Electrochemical Laboratory (ETL) four compact storage rings, TERAS, NIJI-I–III, have been already constructed and a compact storage ring, NIJI-IV, with long straight sections for FEL experiments will be constructed in 1990. TERAS is an 800-MeV storage ring. It has been operated since Oct. 1981 for research on atomic, molecular and solid state physics and ULSI lithography, generation of polarized SR and gamma-rays and for a free electron laser (FEL) experiment. NIJI-I (NIJI means rainbow) is a 270-MeV compact ring and has been operated from Feb. 1986 to March 1989 for machine studies of beam storage higher than 0.5 A at low energies below 150 MeV. NIJI-II is a 600-MeV conventional type compact ring and was operated in July 1989.

NIJI-III is a 620-MeV superconducting type ring. NIJI-II and NIJI-III will be used for FEL and ULSI lithography experiments. Before the superconducting magnets are installed, NIJI-III with conventional type magnets has already been constructed and operated in June 1989. The conventional type magnets of NIJI-III will be replaced by superconducting type ones, starting in the fall of 1989 one by one. NIJI-IV is a 500-MeV conventional type compact ring with straight sections longer than 5 m where an optical klystron can be installed to achieve a high gain FEL experiment.

To extend the use of synchrotron radiation outside the field of fundamental research, superconducting compact storage rings are under development worldwide. The basic concepts of such SR sources are presented with a brief outline of the main development problems. The COSY ring at BESSY has been designed as a prototype of a compact source for x-ray lithography. After detailed studies with normal conducting magnets, commissioning with the superconducting magnets began in spring 1988. The first electron beam was stored in this machine in November the same year, a milestone in the development of superconducting compact storage rings. In the meantime the energy has been raised up to 360 MeV. The main characteristics of COSY are reported together with the latest results.

Multilayer structures are now of sufficient quality to be applied as optic elements in high performance instrumentation. Additionally, the range of materials used in multilayer construction has significantly broadened so that optimum performance is achievable in the soft x-ray and extreme ultraviolet. Calculated ideal reflection efficiencies for the spectral range 100 to 2000 eV and examples of the predicted performance of several specific material pairs are presented first. A short summary of multilayer synthesis procedures is then given. A simple schematic model of a multilayer structure is described and used as a basis for discussing the variety of imperfections that limit multilayer performance. Application of multilayer diffraction gratings to the determination of soft x-ray and ultraviolet optical constants is then discussed. Recent advances in multilayer synthesis technology are then described. The paper is summarized with a consideration of advanced multilayer optics, optical systems and new directions.

Two photochemical reactions of molecules on alkali halide surfaces induced by undulator light are described. The first investigation involves the formation of CN^- -centers. When some alkali halides were irradiated in the atmosphere of N₂ gas and gases containing C, luminescence attributed to CN^- ions was observed, the intensity of which increased with the increase in irradiation dose. It is proposed that sources of the CN^- ions are generated in the gas phase and the CN^- ions, with the aid of undulator light, stick on the surface or are adsorbed in the bulk near the surface, resulting in the formation of CN^- -centers. These centers are stable up to 400 K. The second investigation involves dissociation of H₂O molecules. The OH band and Lyman α line of H were observed from a KBr surface, when it is irradiated with undulator light in the atmosphere of H₂O vapor. It is proposed that the H₂O molecules are physisorbed on the KBr surface and dissociate due to the presence of undulator light.

A progress report is given on recent experimental studies of core excitation and inoization of free-molecules in the VUV and soft x-ray (50 to 2600 eV) by electron impact and x-ray photoabsorption (XAS) techniques. The current capabilities of inner shell electron energy loss spectroscopy (ISEELS) and XAS for gas phase studies are compared, with focus on: Dipole versus non-dipole excitations, electric dipole oscillator strength determination, and bond length correlations. Recent studies of ionic fragmentation of molecules induced by tuned x-rays are discussed with regard to the possibility of site-selective fragmentation via core excited state. Using organometallic compounds as an intermediate, connections are made between the core excitation spectra of free molecules and those of surface absorbed molecules, as measured by near-edge x-ray absorption fine structure (NEXAFS) or reflection near-edge energy-loss fine structure (NEELFS) spectroscopies.

The mechanism of inverse photoemission is explored by comparing inverse photoemission data from free-electron-like materials, such as Al, with first principles calculations of the cross section. Thereby, one can identify the inverse of the surface photoelectric effect and the decay of bulk plasmons into light. Plasmon decay can strongly affect inverse photoemission from thin film systems and lead to striking resonance phenomena.

The application of inverse photoemission to semiconductors yields two- and three-dimensional band dispersions of surface states and conduction bands. It turns out that inverse photoemission measurements provide a stringent test of the state-of-the-art quasiparticle calculations, because the self-energy correction to the conventional (local density) bands is much larger for unoccupied than for occupied states. Examples include Si, Ge, III-V's, dangling bonds and ordered overlayers.

Future directions for inverse photoemission are also discussed, including the measurement of holes that are responsible for superconductivity in high T_c superconductors.

Recent progress is reported in theoretical, computational, and semiempirical studies of shape resonances in the partial photoionization cross sections of diatomic and polyatomic molecules. A multi-channel Feshbach-Fano formalism is described which is particularly well suited for the analysis of resonance ionization channels. The development allows calculations in non-degenerate point-group-symmetry scattering waves, provides compact expressions for partial-channel cross sections, and gives generalized Fanolike absorption lineshapes of parametrizable form. An appropriately selected Q space of zeroth-order molecular configurational states, corresponding to the (σ, σ*) charge-transfer excitations of Mulliken, is seen to account for the appearance of prominent features in the cross sections of simple molecules. These zeroth-order states further provide a basis for understanding and elaborating upon semi-empirically determined correlations between bond lengths and resonance energies in the valence-and K-shell cross sections of linear and quasi-linear molecules. A universal correlation is derived theoretically in these cases which relates resonance energies to effective bond lengths, square-well quantum numbers, and effective potentials, each of which is obtained a priori from elementary considerations. Recent studies of C₂H₂, C₂H₄ and C₂H₆ photoionization cross sections illustrate the complexities that can arise in compounds which exhibit two σ* resonances (C-C and C-H) in some channels. Mulliken configurational states underlying resonance features in these compounds are seen to account satisfactorily for the natures of calculated and measured partial cross sections in the absence of simple square-well correlation behaviour. Connections of the development with conventional computational methods based on K-matrix, S-matrix, or eigenchannel states are indicated, and cautionary remarks are made on the use of well models in describing photoionization resonances.

Although x-ray absorption spectroscopy is now applied frequently to the study of the orientation of molecular adsorbates on single crystal surfaces, little attention has been paid to the spectroscopic aspects of the technique. The experimental data obtained hitherto have usually been interpreted in terms of resonances associated with isolated functional groups within the molecular species. We argue, however, that the number of allowed resonances, their degeneracy and polarisation dependence should be predicted by reference to the molecular orbital scheme and the appropriate point group symmetry. Further, the effects of symmetry lowering, due to the bonding to the surface, and of symmetry breaking, due to core hole localisation, may have to be considered. A fruitful proving ground for these ideas is provided not only by adsorbed molecules, but also by molecular fragments bound to the surface following simple heterogeneous reactions. Because of the polarisation dependence of the resonances x-ray photoabsorption proves to be a useful technique for determining approximate molecular orientation in many cases. On the other hand, core-excited states provide only an indirect probe of the surface chemical bond.

For the past forty years the sun has been the object of extensive observation at ultraviolet (100 to 300 nm), extreme-ultraviolet (20 to 100 nm), and x-ray ultraviolet (1 to 20 nm) wavelengths. The temperature structure of the solar atmosphere, and perhaps that of most stellar atmospheres, display the unlikely characteristic of rising outward from a relatively low temperature near their base to exceedingly high values, greater than 10⁶ K. Strong emission lines from hydrogen, helium, and the heavier elements, together with most of their ionization states, dominate the solar VUV spectrum. Due to its proximity, the sun is easier to observe than its stellar counterparts. It is an extended object such that individual features can be resolved and therefore processes occurring in this low-density, collisionally excited plasma have been studied in great detail. Observations of solar phenomena have provided new insights not only into the sun's own unique character but have also improved our understanding of other astrophysical plasmas. Most solar processes are best studied with high spatial and high spectral resolution. In addition, the sun's radiation is the dominant energy source to our solar system. The dynamics, temperature and photochemistry of the Earth's atmosphere are driven by solar radiation with wavelengths below 300 nm. Variability of the sun's output, which is known to exist on all time scales, will produce associated changes in our atmospheric system. Recognition of the effects that human activity has on our atmosphere lends new urgency to understanding its photochemistry, including the time-varying solar VUV irradiance that drives it.

A relatively new technology for fabricating multilayer optics that operate in the vacuum ultraviolet (VUV) at near normal incidence is finding broad application in both solar and galactic astronomy. These optics provide a number of advantages for use in astronomical instruments that make them highly complementary to traditional grazing incidence VUV optics. Five separate multilayer telescopes have already been used for VUV observations of the solar corona and five more astronomical instruments involving multilayer optics are presently in process of design and development. Multilayer fabrication technology is still rapidly developing and further improvements promise to provide even greater capabilities for future instrumentation in astronomy. This paper summarizes the present VUV observations obtained with multilayer telescopes, decribes instrumentation now under development and discusses further improvements in multilayer performance that can benefit future astronomical observations.

A review is given on the methods of VUV radiometry and the uncertainties that can be achieved. The properties of radiometric standards, detectors as well as radiation sources, are described and various calibration techniques are discussed. Details are given on the uncertainties of the electron storage ring BESSY as a primary radiation standard. The responsivity of semiconductor photodiodes as transfer detector standards in the spectral range around 1 keV photon energy is determined from model calculations and the result is compared to a radiometric calibration based on the known spectral photon flux of the electron storage ring BESSY.

Dissociative photoionisation of methane is studied in a wide range of internal energy (12.6–28 eV photon energy) by a threshold electron–ion coincidence technique. Whereas the CH₄⁺X ground state is statistical, the A (2a₁)^-1 state (22.4–25 eV) and the doubly excited states (26–32 eV) show direct dissociation mechanisms and inefficient internal conversion to the CH₄⁺ ground state. In the Franck–Condon gap between the X and A states, we observe a 1.7 eV shift in the sequential dissociation of CH₃⁺ into CH₂⁺ + H. This is interpreted as the result of dissociation of Rydberg states into neutrals followed by resonant autoionisation of the CH₃^** fragment into a stable excited electronic state of CH₃⁺.

By calculating an exchange-correlation potential from the self-energy operator, we show that interpretation of the one-electron band structure appearing in density-functional theory (DFT) calculations as quasiparticle energies is seriously invalid. For example, the well-known error in the minimum band gap of semiconductors and insulators is not caused by the use of the local density approximation (LDA), but is inherent to DFT. Furthermore, the metal-insulator transition undergone when a semiconductor is compressed is not described correctly within DFT, showing that the DFT Fermi surface is not necessarily that of the real system. However, excited state properties can be calculated correctly, by using computational many-body theory. The GW approximation for the self-energy operator gives quasiparticle energies in excellent agreement with experiment. It may also be used to obtain the one-particle Green's function, from which other properties of the system may be found.

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 T_K. 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 U_xY_1-xPd₃ and U_xY_1-xAl₂.

The knowledge of the crystallography of metastable cobalt and iron films prepared by deposition on suitable substrates is the key for the understanding of their unusual magnetic properties; their local structure is determined precisely by SEXAFS. This technique allows a measure of the asymmetry of the first shell radial distribution function in monometallic cobalt and iron films prepared by vapor condensation on a low temperature amorphous substrate. In both cases the measured profile allows to conclude the existence of true amorphous monometallic samples and compares well with the calculated one assuming a random packing of hard spheres. Following the evolution of the local order with increasing temperature we show that the cobalt amorphous films crystallize abruptly at a temperature dependent on their thickness, while the iron amorphous films crystallize slowly. Moreover EXAFS has been used to investigate the local structure of f.c.c. iron films which have puzzling magnetic properties. These metastable films grown on Cu(100) have a structure strongly dependent on their temperature of growth: Both the short range order and the amplitude of the thermal vibrations measured at room temperature are higher in films prepared at higher temperatures. These different crystallographic structures can be the origin of the conflicting results produced in the study of their magnetic properties.

We have used x-ray photoelectron diffraction (XPD) to study adsorbate structure on Pt and Ni surfaces. Temperature-dependent XPD was also used to investigate surface order on Pb(110) as the bulk melting point is approached. For CO on Pt(111) or Ni(110) at 300 K, the C 1s peak has a forward scattering (FS) intensity enhancement along the surface normal, implying perpendicular adsorption. CO adsorbed at 120 K on Ni(110) is more complex: Adsorbate crowding induces a transition from perpendicular at low coverage to tilted at high coverage (polar tilt angle 21 ± 1° along [001]) via an island growth mechanism. Tilting is also found for CO coadsorbed with K on Ni(110) at 300 K (polar tilt 27 → 32° in [001]); it is associated with a strong attractive interaction between the alkali metal and the molecule. This stabilizes the molecule (relative to perpendicular CO) and reduces thermal broadening of FS peaks. The broadening is caused in part by adsorbate vibrational motion; measurements of the effect for CO on clean Ni(110) were used to show the azimuthal anisotropy of the molecular vibration. In the Pb(110) study FS peak intensities along low-index crystal directions were measured. For T < ~500 K the peaks decrease only weakly with increasing temperature. Around 500 K (101 K below the bulk melting temperature) a much sharper decrease sets in, interpreted as the onset of surface disorder. Scanning different azimuths ([001], [10], or [12]) and looking at different FS peaks measures order in various directions. Such experiments show anisotropic behavior: Surface order at a given temperature is greater along [001] than along [10], in support of recent LEED measurements. An advantage of XPD compared to LEED for this system is the large mean-free-path of core-level photoelectrons and an intrinsic sensitivity to short-range order. Debye–Waller attenuation is also less drastic than in LEED, in spite of the larger kinetic energy. FS peaks thus remain appreciable (and measurable) to higher temperatures than do LEED reflexes.

Spin-resolving techniques in photoemission are presently attracting wide interest because they can provide a detailed insight into specific aspects of electronic structure and the dynamics of the photoexcitation process. Recent experimental work in the field of non-magnetic systems is reviewed with emphasis on the complementary nature of the information gained by exploiting the photon- and/or electron-spin. A very promising achievement is the verification of circular dichroism (CDAD) in the angular distribution of photoelectrons. It arises without spin-orbit interaction through an interference of final-state partial waves. Every aligned or oriented target (molecular adsorbates, single crystals) should in principle show this effect in photoemission with circularly polarized light. Electron-spin polarization (ESP) in photoemission is induced by circularly polarized light as a direct consequence of spin-orbit interaction. This type of spectroscopy thus provides a unique method for determining the symmetries of electronic bands of high-Z materials. Recent examples of both categories are discussed.

There has been considerable effort expended by theorists to explain the occurrence and nature of high temperature superconductivity (HTS) in oxide systems. Although it is generally believed that electron pairing is consistent with all of the measurements, there is no consensus on the nature of the pairing or the microscopic mechanism for HTS.

Two areas of focus are the mechanism for binding the pairs and the characterization of the paired state. It has been argued that the pairing boson can be a phonon as in the standard BCS theory and its refinements. However, objections have been raised about various models based on phonons. Other proposals suggest using plasmons, excitons, demons, and magnons for pairing. Many of the models proposed have attractive features, but again, there are shortcomings or insufficient experimental support. Some suggestions such as the RVB model claim that the normal state is not a Fermi liquid and that the standard approaches are not applicable. This has provided added motivation for studying the normal state in more detail.

Because of the complexity of the oxide superconductors and the observation of strong phonon, electronic, and spin effects, it is still difficult to provide verdicts for the proposed pairing mechanisms except in those cases where detailed calculations have been done. The expected short coherence lengths put restrictions on the nature of the pairs. Because of this, HTS theories range from BCS pairing to Bose-Einstein condensation.

An overview consisting of some history, background, and a discussion of some specific theoretical proposals is given here. Some suggestions are made for using experiment to reduce the number of promising theoretical choices. For non-oxide systems, a discussion is given of the possibility of phonon-induced superconductivity in metallic hydrogen in the range of 230 K.

We have performed angle-integrated and angle-resolved photoemission measurements at 20 K on well-characterized single crystals of high T_c cuprates (both 1:2:3-type and 2:2:1:2-type) cleaved in situ, and find a relatively large, resolution limited Fermi edge which shows large amplitude variations with photon energy, indicative of band structure final state effects. The lineshape of the spectra of the 1:2:3 materials as a function of photon energy are well reproduced by band structure predictions, indicating a correct mix of 2p and 3d orbitals in the calculations, while the energy positions of the peaks agree with calculated bands to within ≈0.5 eV. This may yet prove to reflect the effects of Coulomb correlation. We nevertheless conclude that a Fermi liquid approach to conductivity is appropriate. Angle-resolved data, while still incomplete, suggest agreement with the Fermi surface predicted by the LDA calculations. A BCS-like energy gap is observed in the 2:2:1:2 materials, whose magnitude is twice the weak coupling BCS value (i.e., 2Δ ≈ 7kT_c).