Table of contents

The VUV conferences series

The international conferences on vacuum ultraviolet radiation physics started in 1962, and are now being held every third year. VUV-8 took place at Lund University, August 4–8, 1986. VUV-9 will be arranged at the University of Hawaii, USA, August 14–18, 1989, with Prof. C S Fadley as conference chairman. Chairman of the international advisory board for the period 1986–89 is Prof. L Hedin.

The theme of the series can be summarized as experimental and theoretical progress in research fields utilizing the interaction of VUV radiation with matter. The topics cover broad areas within atomic and molecular physics, solid state physics and VUV instrumentation. The conferences emphasize interdisciplinary aspects. To these belong common experimental techniques as, e.g., synchrotron radiation instrumentation, and common theoretical foundations for the description of photon interactions with matter.

The VUV-8 conference

The VUV-8 conference in Lund was attended by 300 participants from 26 countries. An address list of the participants is given at the end of this volume. There were 33 invited papers given as plenary or key-note talks. As many as 229 posters were presented; 49 of them were also given orally. These numbers are typical for the VUV conferences, except for the number of posters, which was unusually large. In the conference planning the poster sessions were stressed, and particular care was taken to provide a good atmosphere at these sessions. Thus the posters were kept up during the whole conference, coffee was served in the hail with the posters and there were convenient places to sit down close to the posters.

Considering the wide scope of the conference it was necessary to emphasize a limited number of topics of high current interest and importance. Thus besides traditional topics, several rapidly expanding fields were discussed in special sessions. At VUV-8 there were the following sessions.

• Theory of atoms and molecules

• photoabsorption and -ionization of atoms and molecules and related phenomena

• multiphoton and other dynamical processes

• plasma physics

• VUV lasers

• time resolved spectroscopy

• instrumentation for VUV radiation

• synchrotron radiation centres

• solid state spectroscopy

• dynamical processes involving localized levels

• fundamental aspects of photoemission

• spin-polarized photoemission

• inverse photoemission

• semiconductors

• organic materials

• adsorbates

Proceedings of VUV-8

The present volume contains most of the invited papers (28 out of 33). Regarding the contributed papers, over 50 are now being published in regular issues of PHYSICA SCRIPTA. These papers will also appear in a reprint volume, PHYSICA SCRIPTA RS4, which soon will be available. Abstracts of invited and contributed papers appeared in three conference volumes as follows: Volume I: Atomic and molecular physics. Instrumentation. Volume II: Solid state physics. Volume III: Post deadline papers. These books have been registered in an international data base and can thus be cited as published documents. Copies may be received from the conference secretary.*

Acknowledgements

We would like to thank our sponsors, which are listed on the following page, the members of the international program committee, and all others who helped in the planning of the program. Above all we like to thank those who worked with the local organization. Due to their dedicated efforts the conference ran very smoothly with a pleasant atmosphere.

Recent results for the arsenic termination of the Ge(111), Si(111) and Si(100) surfaces are reviewed. Arsenic interaction with the Ge(111) and Si(111) surfaces has been found to result in the replacement of the outer Ge (Si) layer with an As layer. This arrangement has a 1 × 1 symmetry and the calculated positions of the As atoms are very close to the positions expected from bulk bond lengths. Ge(111): As and Si(111): As thus provide us with model ideal surfaces. Similar investigations of the Si(100): As 2 × 1 surface show that this system can be considered as a model for surface dimers. Experimental surface-state dispersions obtained from angle-resolved photoemission measurements and their comparison with results of total energy calculations for these surfaces are reviewed. Photoemission core level data for the As-terminated surfaces and for GaAs on Si(111) are also discussed in terms of the model surfaces.

Experiments and theories on localized excitations of electrons in solids are reviewed. The scope of the review is limited to 3d transition metals and their compounds. Two-hole states that are created in Auger decay and photoemission from metals are discussed. Configuration-interaction theories of compounds are used to describe XPS and resonant photoemission experiments. Analogies between the phenomena observed in metals and compounds are noted. The nature of the energy gap of NiO and its relationship to Mott-Hubbard insulators is explored. Recent work utilizing these ideas is summarized.

Relativistic and non-relativistic mean-field theories are discussed in the framework of quantum electrodynamics. g-Hartree mean-fields are derived using methods of the steepest descent which yields semi-classical expansions as a generalisation of asymptotic expansions of classical analysis. Our applications of the g-Hartree theory of atoms show that, in the examples considered, correlations are efficiently suppressed.

Using CO in its free form, coordinated to a metal atom, or adsorbed onto a surface as an example, some recent experiments are reviewed investigating secondary processes following the excitation of a core electron. These experiments deal with the electronic decay of a core hole excited state or the decay of the parent molecule into ionic fragments. The study of these secondary processes yields new insight into the formation of the molecular bond, static charge distributions, and dynamic charge transfer processes. These experiments also allow us to study energy redistribution in highly excited electronic states on a timescale equivalent to the lifetime of the core hole (10^-14 sec).

The use of core-level photoelectron diffraction for structural studies of surfaces and epitaxial overlayers is discussed. Photoelectron diffraction is found to provide several direct and rather unique types of structural information, including the sites and positions of adsorbed atoms; the orientations of small molecules or fragments bound to surfaces; the orientations, layer thicknesses, vertical lattice constants, and degrees of short-range order of epitaxial or partially-epitaxial overlayers; and the presence of short-range spin order in magnetic materials. Specific systems considered are the reaction of oxygen with Ni(001), the growth of epitaxial Cu on Ni(001), the well-defined test case S on Ni(001), and short-range spin order in the antiferromagnet KMnF₃. A rather straightforward single scattering cluster (SSC) model also proves capable of quantitatively describing such data, particularly for near-surface species and with corrections for spherical-wave scattering effects and correlated vibrational motion. Promising new directions in such studies also include measurements with high angular resolution and the expanded use of synchrotron radiation.

Dynamic effects, i.e., shake up transitions in photoemission of non-polar, polar and, in particular, dipolar organic compounds are reviewed. Gas phase spectra are contrasted with solid phase spectra. For the non-polar molecules a one-to-one correspondence between the two phases is well documented. For polar and dipolar molecules, however, this one-to-one correspondence is lost, i.e., the dynamics of the hole creation process changes. It is shown that intermolecular screening becomes important. We develop simple physical criteria that allow to rationalize and predict under which conditions such effects are to be expected. In general it is concluded, that the interpretation of solid state spectra on the basis of calculations on the free molecule is not necessarily sound. We show, that even in cases where solid and gas phase spectra appear to be similar this similarity may be accidental and that it may be caused by particularly intermolecular interactions in the solid.

It is pointed out that technical developments in synchrotron radiation, monochromators, photoyield spectroscopy and vacuum technology, together with theoretical advances, have enormously increased the potential of x-ray absorption studies up to ∼ 2 keV, where sharp core levels (FWHM ∼ 0.1–1 eV) can be found for many elements. Using examples from silicides, transition metals, and rare earths, we discuss core hole potential effects and the uses of multiplet structure to diagnose the fine details of the ground state electronic structure in narrow band materials.

Angle-resolved photoelectron spectroscopy has been used to study the surface electronic structure of overlayers of column III- and column V-atoms on semiconductor surfaces. Some well ordered and stable systems that also have been treated in theoretical calculations will be described. Monolayers of antimony on GaAs(110) or arsenic on Si(100) give surface electronic structures characterized by bands near the valence band maxima that are derived from orbitals on the ad-atoms. Three surface state bands on GaAs(110) 1 × 1–Sb and one on Si(100) 2 × 1–As have been found to be in qualitative agreement with calculated surface state bands. Column III ad-atoms form stable overlayers on Si(111) at 1/3 monolayer coverage. Photoemission spectra from Al, In and Ga covered surfaces show two filled surface state bands in good agreement with the calculated dispersion of the silicon substrate dangling bonds modified by the ad-atoms in a three-fold symmetric adsorption geometry.

With the development of BESSY a light source of circularly polarized vuv radiation with sufficiently high intensity has become available to make angle- and energy-resolved photoelectron spin-polarization studies with circularly polarized radiation feasible. This paper reviews the recent experimental activities performed with free atoms, physisorbed adsorbates and solid surfaces (metals as well insulator) in the past three years. The spin-polarization of photoelectrons measured under certain conditions to be almost complete studies atomic effects as auto-ionization resonances in the gas phase as well as even in the photo-emission of adsorbates. The cross comparison of photoemission data from Xe in its different structural phases (free atom, adsorbate in different structures and on different substrates, three dimensional crystal) yields symmetry characterizations of the electronic states (assignment of quantum numbers, determination of dipole matrix elements and phase shifts; spin resolved band mapping) and shows the ability of spin-polarization spectroscopy to build a bridge between atomic and surface physics.

The matrix element for photoemission is written in terms of the spectral function for the hole states, and a generalized quasi-particle amplitude for the photoelectron. This amplitude satisfies a coupled-channel Schrödinger equation, which can be re-written as a single-channel equation containing a complex self-energy. The formalism is used to discuss valence electron photoemission and plasmon satellites in core-level and surface state emission.

An overview is given on the recent progress in UPS studies of organic molecular materials taking the examples mainly from the results obtained in the authors' laboratory. We discuss molecular solids with and without strong intermolecular interactions, molecular complexes and polymers.

The advent of ultraviolet observations from space has led to rapid advances in our knowledge of the conditions in the chromospheres which surround cool stars. The solar spectrum has been studied at high spatial and spectral resolution for many years and can be used as a model for understanding the spectra of other high gravity (main sequence or dwarf) stars. The most recent spectroscopic developments in this area have come from the flights of US Naval Research Laboratory's High Resolution Telescope and Spectrograph (HRTS) on a series of rockets and on the Spacelab 2 Shuttle flight. Ultraviolet observations of other cool dwarf stars made with the International Ultraviolet Explorer (IUE) satellite have shown that their spectra are broadly similar to that of the sun and that the atomic excitation processes causing the emergent spectra are similar to those in the sun. The main requirement regarding atomic data is for electron-ion collision strengths.

In contrast to the solar chromosphere and corona, where electron temperatures reach T_e > 10⁶ K, observations with IUE have shown that stars with low surface gravities have only cool chromospheres with T_e ≲ 2 × 10⁴ K. The gas density regime is also lower and the excitation of many of the ultraviolet lines is by radiative processes rather than by ion/electron collisions as in the sun. In particular, higher opacities lead to multiple photon scattering and line leakage in semi-forbidden transitions. Several cases of accidental overlap with strong emission lines of H I and O I result in fluorescent excitation of species such as S I, Fe II and CO. Quantitative analyses of such spectra are at an early stage and a wide variety of atomic rate coefficients need to be known more accurately. Further laboratory spectroscopy in the VUV would be valuable for some species.

A review is given of many-body calculations of photoionization cross sections, and various theoretical techniques are compared. Experimental results are compared with calculations for both closed-shell and open-shell atoms. Particular attention is given to resonance strength. Multiple excitations due to photon absorption are also discussed.

Recently several spectroscopic techniques have been newly developed for probing the electronic structure of organic molecules, molecular crystals and surfaces over large binding and excitation energy ranges. We discuss recent photoemission experiments from organic molecular crystals, organometallic compounds (metal-phthalocyanines) and metal-organic interfaces with emphasis on the important extensions of the photoemission technique made possible by using synchrotron radiation.

Experimental aspects such as sample preparation, characterization, sample charging and decharging by a second light source (laser) as well as experimental aspects of C1s and N1s core level spectroscopy will also be briefly considered.

The theory of multilayer optics relevant to the following fields of application is given: The choice of materials for efficient XUV reflectors, Obtainment of high spectral resolution, Construction of polarizers and collimators for XUV beams.

In the past 10 years, photoionization cross sections and photoelectron angular distribution parameters have been measured for the s, p, d, and f subshells of closed-shell atoms and a good understanding has been achieved, in interplay with theory, for the single-electron photoionization process in these atoms. Current research is directed mainly toward the elucidation of two-electron processes, autoionization resonances and toward photoionization processes in open-shell atoms, excited atoms, and ions. Most of the current studies are closely related to many-body effects in the dynamic behavior of the electronic structure.

A review is given of spectroscopic studies of the x-ray emission from tokamak plasmas of hydrogen or deuterium. The characteristic line emission for highly ionized impurity elements reflects the plasma conditions under which the ions are produced and their excited states populated. This makes it possible to deduce plasma diagnostic information from measurements of x-ray line spectra. The conditions encountered in a tokamak plasma of relevance to the x-ray emission are discussed as well as the crystal spectrometers used for the measurements; comparisons are made with observations that can be made in the visible/UV and VUV regions.

Extensive studies have been made of the H- and He-like isoelectronic sequences for elements up to Ni (Z = 28) as limited by the maximum plasma electron temperature which has reached the 5 keV level up till now. For elements of higher Z, x-ray spectra have been measured especially for multi-electron systems around the Ne-like charge state. The use of high resolution x-ray spectra for some typical and some novel plasma diagnostic applications is discussed.

The time evolution of events connected to photoabsorption in adsorbate layers can be subdivided into primary (photoexcitation/ionization including multiple excitations and screening), secondary (Auger decay in the case of core excitations), and tertiary (bond-breaking) processes. On all these stages, manybody interactions are important. Improved understanding can be gained, if all these processes are viewed together. For adsorbate core excitations on metal surfaces, these connections are examined in some detail, using recent data. Auger spectra for the decay of well-defined primary excitations show that essentially all higher primary excitations have decayed by the time Auger deexcitation occurs; an exception is briefly discussed. On the other hand, the strong influence of delocalization upon the probability of successful bond-breaking leads to an excessive amplification of surviving higher excitations in photodesorption, particularly of fragment ions. To somewhat lesser extent, this is also the case for primary valence excitations and for desorption of neutral and ionic molecules. It is concluded that competition–and possibly even interference–between many-body interactions (screening vs. Coulomb localization) determines this selective branching as seen in electron emission and in desorption.

The excitation of a core electron into an antibonding empty valence orbital of a molecule leads to a quite intense resonance compared to Rydberg transitions in the photoabsorption spectrum. We show the direct experimental evidence of its repulsive character by photoelectron spectroscopy in the case of a d-shell excitation of halogen compounds such as HBr, CH₃Br, HI and CH₃I. The first step of the resonance decay is a very fast dissociation process followed by autoionization of the core excited fragment as a second step. Competition with usual molecular autoionization is found when the mass of the radical linked to the excited atom is increased.

The pulsed nature of synchrotron radiation permits time resolved spectroscopy in the VUV spectral range. In the foreground of discussion is the use of time resolved techniques in fluorescence spectroscopy. The time constants presently accessible extend from the ps to the ms regime, over nearly nine orders of magnitude. The experimental techniques which fit best to the properties of the excitation source, as well as an advanced experimental set-up for time resolved fluorescence spectroscopy, are described. Various results are presented demonstrating the unique possibilities for time resolved VUV spectroscopy offered by synchrotron radiation excitation.

The properties of the angle resolved photoemission spectra from nearly free electron (NFE) metals have been investigated for a wide range of electron densities, from K (1.4 × 10²² electrons/cm³) through Be (24.7 × 10²² electrons/cm³). These data are used in conjunction with theoretical calculations to illustrate the importance of many body effects in the excitation spectra. Characteristic features of the emission from the bulk and surface of NFE metals will be described as well as the possibility for interference between the emission from the bulk and surface.

The availability of extraordinarily bright femtosecond ultraviolet sources is rapidly extending the study of atomic responses into an unexplored regime for which the electric field strength is considerably in excess of an atomic unit. As this regime is approached, experiments studying multiple ionization, photoelectron spectra, harmonically produced radiation, and fluorescence all exhibit strong nonlinear coupling. Peak total energy transfer rates on the order of ∼ 2 × 10^-4 W/atom have been observed at an intensity of ∼ 10¹⁶ W/cm², and the removal of an electron from an inner principal quantum shell has been demonstrated in xenon. Measurements of the radiation produced by the high field interaction with the rare gases have revealed the presence of both copious harmonic production and fluorescence. The highest harmonic observed was the seventeenth (14.6 nm) in Ne, the shortest wavelength ever produced by that means. Strong fluorescence was seen in Ar, Kr, and Xe with the shortest wavelengths observed being below 10 nm. Furthermore, radiation from inner-shell excited configurations in Xe, specifically the 4d⁹ 5s5p → 4d¹⁰ 5s manifold at ∼ 17.7 nm, was detected. The behaviors of the rare gases with respect to multiquantum ionization, harmonic production, and fluorescence were found to be correlated so that the materials fell into two groups, He and Ne in one and Ar, Kr, and Xe in the other. These experimental findings, in alliance with other studies on innershell decay processes, give evidence for a role of atomic correlations in a direct nonlinear process of inner-shell excitation. It is expected that an understanding of these high-field processes will enable the generation of stimulated emission in the x-ray range.

The Göttingen x-ray microscope at the electron storage ring BESSY in Berlin and x-ray microscopy experiments with biological specimens are described. A short view on future developments - optics for higher resolution and with better efficiency, detector devices with higher detective quantum efficiency is given.

Based on time correlated single photon counting the time structure of MAX operating in the multibunch mode at 500 MHz between 100–500 MeV was investigated. Dependent on electron current and energy the pulsewidth of the synchrotron radiation was found to lie between 35–160 ps (σ).

The beam line for time resolved spectroscopy (beam line 32) which is completed by now will permit collection of time resolved excitation and emission spectra from VUV to IR with a time resolution of better than 10 ps.

Data collection in the PHA and MCS mode is based on a MicroVax II computer and permits on line evaluation of excited states' decay as well as parallel recording of up to eight time resolved excitation or emission spectra. The system will be used to analyse molecular structure and dynamics of biopolymers in solution.

Comparison of x-ray fluorescence and electron-yield surface extended x-ray absorption fine struction (EXAFS) above the S K-edge for c(2 × 2)S on Ni(100) reveals an order of magnitude higher sensitivity for the former technique due to a much lower background. The higher sensitivity of soft x-ray fluorescence EXAFS opens the possibility of structural studies on very dilute atomic species on surfaces (< 0.1 monolayer) or in the bulk (< 10¹⁹ atoms/cm³) of a different material. An EXAFS study on the local structure of S impurities in GaAs is presented as an example of such studies. We derive the local geometry around the S atoms finding two competing configurations of approximately equal population: (1) substitutional S on an As site (2) a complex formed by S on an As site and an As vacancy on the second shell with a S-first neighbor distance relaxation of 0.14 ± 0.04 Å. The two configurations explain the disparity between implanted S concentration and net electrical activity.

Ultraviolet inverse photoemission spectroscopy has revealed an abundance of unoccupied surface states associated with the s, p band gaps of noble and transition metals. These surface states include the Rydberg series of image states which arise through self-trapping of an electron by its own image potential. The systematics of surface-state and image-state occurrence are well described, both qualitatively and semiquantitatively, by a simple adaptation of multiple reflection theory. This model is described, and its implications with regard to image-state effective masses, surface corrugations and the determination of the surface barrier potential are discussed. The connection between inverse photoemission and two-photon photoemission studies of image states is also mentioned.

The role of electron correlations in nonresonant multiphoton processes of atoms is presented. The evidence for the validity of a lowest order perturbation theory (LOPT) approach for the laser field in the case of nonresonant processes is sketched. The growing consensus for a LOPT interpretation of even the multiply-charged ion spectra obtained with high intensity lasers is reviewed. Finally, a review of those specific electron correlation processes within LOPT that are important for the quantitative prediction of two-photon ionization cross sections is presented and some of their effects on calculated generalized cross sections are exhibited.

Ultraviolet photoemission spectra have been measured for single crystals of Sm₄As₃, Sm₄Bi₃, Yb₄Sb₃, Yb₄Bi₃, U₃P₄ and U₃As₄ by use of synchrotron radiation. Constant initial state (CIS) and constant final state (CFS) spectra have been measured as well. The complicated electronic structures are elucidated by considering the p-f mixing and Coulomb-exchange interactions.

Sm₄As₃ is a predominantly trivalent material, which shows the Kondo behaviour. The spectral shape near the Fermi level (E_F) has shown a drastic hv dependence below 60 eV. The structure at E_B (binding energy from E_F) = 1.0 eV is found to be dominated by the Sm²⁺ 4f states, for which the p-f mixing effect is essential and provides the valence fluctuation (VF). The sharp structure intersecting E_F is assigned to the Sm 5d conduction band states. Sm₄Bi₃ has shown the mixed valence, whereas the density of divalent Sm states is small at E_F. The effective correlation energy U(4f, 4f) is evaluated as 5.6 eV.

Yb₄Sb₃ and Yb₄Bi₃ have also shown the VF, with appreciable density of states (DOS) of the divalent Yb4f states as well as of the anion p states at E_F. The contribution of the surface 4f states is resolved. The upper limit of the mean valence is evaluated as 2.36 and 2.37 for Yb₄Sb₃ and Yb₄Bi₃.

In the cases of U₃P₄ and U₃As₄, the 5f features are observed around E_B = 0.7-0.8 and 1.9-2.0 eV reflecting the strong p-f mixing effect, whereas the 6d states are observed closer to E_F. The 5f states are partly mixed with 6d band, providing the VF. Many Auger structures are resolved in both materials, from which U(5f, 5f) is evaluated as 1.2 eV.