Table of contents

We present angular resolved scattering (ARS) distributions of various glass mirrors and of SiC mirrors at glancing incidence. The measurements have been performed at the XUV reflectometer station at the DORIS2 storage ring (HASYLAB) between 25 and 1.2 nm wavelength. The data are compared with different scattering theories by using least squares fitting procedures. Of all the available theories the Rayleigh-Rice vector perturbation theory is based on the most realistic model and gives by far the best agreement between experiment and theory. Indeed, even near the critical angle, where anomalous scattering arises, excellent agreement was obtained. Also, convincing evidence for scattering from dielectric fluctuations inside some of the glass samples was found. The rms-roughness, autocorrelation length (measure of the mean lateral separation of the surface irregularities), and the type of correlation function could be determined. Furthermore, we show that ARS-measurements are sensitive to detect irregularities with a mean lateral separation from 50 to 1500 nm. Therefore ARS-measurements in the soft X-ray range could become an important tool to supplement other methods characterizing optical surfaces. Practically all other familiar methods are restricted in spatial frequency corresponding to a lateral resolution above a few microns.

We have developed a microchannel plate intensified, subnanosecond X-ray detector for X-ray imaging experiments. It consists of an X-ray photocathode, a microchannel plate, electrostatic focusing optics and a subnanosecond phosphor. The detector is used for one dimensional imaging and spectroscopic measurements. Signals are recorded using either a one dimensional Reticon camera or a streak camera. The microchannel plates employ an X-ray photocathode (CuI or CsI) deposited on the front surface of the microchannel plate to enhance their soft X-ray efficiency. Electrostatic focusing of the electrons exciting the microchannel plate also enhances the gain of the detector by an order of magnitude. The electrons are accelerated to 20 kV before striking a fast phosphor (indium doped cadmium sulfide). The detector output is coupled to a Reticon camera or a streak camera with a fiber optic array. We have built and calibrated more than twenty microchannel plate intensified detectors. Efficiencies in excess of 1000 (w/cm² output per w/cm² input) have been demonstrated. The time response of the detector is less than 500 ps. The efficiency if the CuI and CsI X-ray photocathodes have been measured from 450 eV to 1300 eV at the Stanford Synchrotron Radiation Laboratory. Data are presented on the efficiency, time response, spectral response and the spatial resolution of the detectors.

Normal-incidence grating optics coated with appropriate multilayers show great promise as a means of achieving high spectral resolution at x-ray wavelengths. Multilayer-coated mirrors have been made and tested successfully, but comparatively little work on such multilayer-coated gratings has been reported. We describe the results of reflectance meansurements made on a superpolished flat mirror and a Ferranti-Astron ion-etched 2000 l/mm laminar grating, which were coated simultaneously at the Lawrence Livermore National Laboratory with a 25-period Mo-Si multilayer. The multilayer was designed so that at normal incidence the mirror would have a maximum reflectance of 31% at a wavelength of 176 Å. The measurements were performed using a reflectometer and monochromator installed on the Naval Research Laboratory X24C beamline at the Brookhaven National Synchrotron Light Source.

Crystal configurations are discussed with which one can obtain a 2D image of an X-ray synchrotron source, with high spatial resolution. Cases where this may be achieved through a triple relection in one crystal are proposed.

We have developed a vacuum ultraviolet spectrophotometer with wide energy and temperature range coverage, utilizing a laser-plasma light source (LPLS), CO₂-laser sample heating and time-resolved dispersive analysis. Reflection and transmission spectra can be taken from 1.7 to 40eV (31-700nm) on samples at 15-1800K with a time resolution of 20-400ns. These capabilities permit the study of the temperature dependence of the electronic structure, encompassing the effects of thermal lattice expansion and electron-phonon interaction, and changes in the electronic structure associated with equilibrium and metastable phase transitions and stress relaxation.

The LPLS utilizes a samarium laser-plasma created by a Q-switched Nd:YAG laser (500mJ/pulse) to produce high brightness, stable, continuum radiation. The spectrophotometer is of a single beam design using calibrated iridium reference mirrors. White light is imaged off the sample in to the entrance slit of a l-m polychromator. The resolution is 0.1 to 0.3nm. The dispersed light is incident on a focal plane phosphor, fiber-optic-coupled to an image-intensified reticon detector. For spectroscopy between 300 and 1800K, the samples are heated in situ with a 150 Watt CO₂ laser. The signal to noise ratio in the VUV, for samples at 1800 K, is excellent. From 300 K to 15 K samples are cooled using a He cryostat.

The first undulator on Aladdin recently began operating as a user facility. The device is equipped with a 6-meter toroidal grating monochromator beamline. The undulator and its beamline have several unique characteristics. Furthermore, the adjacent bending magnet beamline is almost a twin of the undulator beamline, with similar grating efficiencies and reflection losses. This makes it easy to directly compare undulator and bending magnet source beamlines in a realistic environment. The results indicate that the undulator beamline produces between 20 and 100 times more flux in the first harmonic than a similar beamline on a bending magnet. The excellent polarization of 99.5% for the first harmonic makes this beamline extremely useful for polarization studies.

User programs based on the undulator radiation included angle-resolved photoemission in gas phase and tests of optical components. The beamline is now entirely dedicated to soft X-ray spectromicroscopy (project MAXIMUM).

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 T_c 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.

The advantages of multilayer coated blazed gratings are discussed, and multilayer/grating matching conditions are considered. Measurements of a W/C multilayer-coated, 2000 g/mm 3.8° blaze angle ion-etched grating and superpolished flat are reported. These include reflectometer measurements as well as operational performance of the pair as multilayer-multilayer grating diffraction elements in a two-crystal type monochromator. The results are promising for a variety of new spectrometer types.

In this paper we describe the optical, mechanical, vacuum and control design and construction of the soft X-ray microscopy beamline BL-U12A at HESYRL.

The decay processes in Ne, Ar, Kr and Xe after resonant excitation of Ne 1s, Ar 2p, Kr 3d, and Xe 4d electrons to Rydberg levels have been studied recently in detail. A comparison between the results shows a clear resemblance in the decay of the Ne 1s → np, Ar 2p → ns, Kr 3d → np, and Xe 4d → np resonances whereas the Ar 2p → 3d excitation gives rise to a more complicated structure.

It is shown that a complete photoelectron-spin polarization occurs independent of emission angle for two special sets of values of the dynamical photoionization parameters. In spin- and angle-resolved photoelectron spectroscopy of thallium the dynamical parameters were measured near the autoionization resonances at 149 nm and 82 nm. A pronounced angular variation of the polarization component A(θ) was observed at 83 nm. For both resonances the parameters were found to approach different limiting values such that the length of the spin-polarization vector assumes values close to unity at all emission angles. The implications of these results for the corresponding dipole transition matrix elements are discussed.

Triple coincidence techniques are applied to the analysis of the fragmentation of molecules following single photon and multiphoton ionisation. In the case of single photon ionisation a new wavelength-scanning photoelectron-photoion-photoion coincidence experiment is employed to determine thresholds for fragment ion pair production. In the multiphoton regime a new covariance mapping technique (photon-photoion-photoion) is described which allows unambiguous correlation of fragment ions with parent molecular ions.

The relative intensities of spectral lines from Δn = 0 transitions in 2p⁵3l configurations of neon-like Ar⁸⁺ and Cl⁷⁺ are measured in a plasma generated with a θ-pinch device. Six of these lines involving 3p-3s transitions have exhibited significant amplification in high density plasmas of higher-Z elements, but with considerable disagreement compared to numerical predictions. Agreement in the present experiment indicated that the collisional excitation, mixing, and radiative decay in the modeling is accurate, at least for the ions measured. Data supporting this on 3d-3p as well as 3d-2p and 3s-2p transitions are presented. The emphasis here is on the measurements performed. For example, an essential instrumental relative intensity calibration by a branching ratio technique is described in detail.

A discussion of the line and unresolved transition arrays emission of rare earths ions isoelectronic with PdI to CuI emitted from low (∼ 10¹³ cm⁻³) and high (∼ 10²⁰ cm⁻³) density tokamak and laser produced plasmas is presented. The spectra of prasaeodymium, europium, gadolinium, dysprosium and ytterbium have been excited in the TEXT tokamak plasma and recorded in the 50-200 Å range by means of a photometrically calibrated, time-resolving multispectral grazing incidence spectrograph. The prasaeodymium, samarium, gadolinium, dysprosium and ytterbium spectra obtained by focussing an Nd-glass laser beam on solid targets were recorded in the same spectral range on photographic plates. The relationship between the main plasma parameters, electron temperature and density, and the soft X-ray emission of rare earths is analyzed and discussed.

We have measured the time of flight mass spectra of several molecules, which include carbon disulfide (CS₂) and thiophene (C₄H₄S), in coincidence with Auger electrons which result from the decay of core excitations in these molecules. These experiments allow us to observe the behaviour of parent ions of the molecules which are selected for their excitation energy and also for the site of the original excitation.

We have observed strong state selectivity in the fragmentation of CS₂. As a result of this, there is a strong correlation between electronic states of the parent ion and electronic states of the fragments which arise from these states. This correlation can be deduced from the observed kinetic energy releases in fragmentation, which are nearly independent of the electronic excitation energy of the parent ion.

We have also observed a certain amount of site selectivity in the fragmentation of a molecule as large as thiophene. This shows up as a difference in the yields of certain fragment ions from states in the same region of excitation energy of the parent ion, depending on the site of the original excitation of the molecule. Clearly, such an observation argues against complete statistical behaviour in the fragmentation of thiophene.

A highly resolving apparatus for molecular photoionization experiments based on resonantly enhanced sum-frequency mixing is discussed. The VUV source, based upon commercially available laser equipment, delivers more than 10¹¹ photons/second at a bandwidth of 0.38 cm⁻¹ with linear or circular polarization. Results of photoelectron yield measurements obtained for the complete (n* = 6) Rydberg order of the autoionization resonances in the region of the spin-orbit split ²Π states of the HI⁺ ion are presented. In addition, measurements of the angle-integrated spin-polarization of the photoelectrons, ejected by circularly polarized radiation (Fano effect) are discussed.

By using the technique of two laser-produced plasmas we have obtained the absorption spectra and the relative photoionization continua of ionized and neutral carbon in the soft X-ray region. In this spectral range (25-45 Å), the absorption is due to the 1s inner shell electron. The CV absorption spectrum has been completely analyzed in a previous work and is reported here only as a reference spectrum for the CIV and CIII spectra. A preliminary analysis of the CIV is reported. Finally the absorption spectra and the relative photoionization continua of neutral carbon are presented in three different states, solid, vapour (clusters) and CI atoms.

The photoabsorption cross sections of Kr i in the photoionization threshold region [845-887 Å] have been measured with the highest resolving power (⩾ 1.5 x 10⁵) ever achieved in this wavelength region. The data obtained have been analyzed by means of least squares fitting with a line shape formula based on the multichannel quantum-defect theory. The quantum defects, width parameters, and asymmetry parameters thus determined are given together with the photoabsorption cross sections for the ns' and nd' series converging to the ²P_1/2 ionization limit.

Multiple photoionization cross sections of Kr have been measured in the photon energy region of 90-260 eV. Partial cross sections for valence ionization, 3d ionization and 3d shake (shake-up and shake-off) have been derived from these multiple photoionization cross sections. The experimental results are compared with a recent theoretical calculation.

Dissociation processes following double photoionization of Al(CH₃)₃ have been studied in the range of valence and Al : 2p core-level ionization by means of the photoelectron-photoion and photoion-photoion coincidence methods. The double-ionization threshold and the Al : 2p core-ionization threshold are estimated to be about 30 and 80eV, respectively. The variation of the fragmentation pattern with photon energy is discussed in conjunction with the relevant electronic states. The relative yields of the H⁺ -Al⁺ and H⁺-CHm⁺ (m' = 0-3) ion pairs are enhanced around the Al : 2p core-ionization threshold.

Using monochromatized synchrotron radiation in the range 103 to 112 eV we have monitored the dispersed UV/optical fluorescence resulting from excitation of a Si 2p electron in SiCl₄. The main features in the fluorescence spectrum have been identified as emission from the SiCl₄⁺C state and from excited Si atoms. Features in the fluorescence excitation spectra are assigned to excitation of a Si 2p electron to unoccupied valence and Rydberg orbitals. For the SiCl₄⁺C state emission there is significant enhancement in the yield following excitation to valence orbitals and very little enhancement as a result of Rydberg excitation. The opposite is true for emission from excited Si atoms. Enhancement in the SiCl₄⁺C state production upon valence excitation results from autoionization of the core-excited state. The relatively large yield of excited Si atoms following Rydberg excitation is due to the greater probability of the core-excited Rydberg state decaying, via a resonant Auger process, to highly excited, unbound states of SiCl₄⁺. The molecular ion then fragments before the excited Rydberg electron can relax.

We discuss problems in recognizing the physically observable aspects of calculated structures in near-threshold Rayleigh scattering. Various structures in near-threshold photoionization, and irregularities in the ionic cross sections across isonuclear sequences, are understood to be unphysical. These structures are associated with the lack of smoothness in Latter-tail potentials or over-screening of inner electrons in potentials which allow these electrons to see their own charge distributions. Such problems are remedied as one goes beyond independent particle approximation (IPA) potentials or at the IPA level by using "vacancy potentials"; we find in photoionization this leads to improved agreement with experiment. In this work Rayleigh scattering cross sections for neutral neon and argon have also been calculated using such vacancy potentials for photon energies near threshold, but requiring experimental threshold energies.

We review and discuss some recent resonance enhanced multiphoton ionization (REMPI) studies of small molecules, emphasizing the role that shape resonances and Rydberg orbital evolution play in determining vibrational and rotational ion distributions. Strong non-Franck-Condon effects observed in ion vibrational distributions for REMPI of O₂ via the C³Π_g and d¹Π_g Rydberg states are partially attributed to a σ_u shape resonance, previously observed in ground state photoionization of diatomic molecules. Autoionization of repulsive valence states also induces non-Franck-Condon effects in these REMPI spectra. Significant non-Franck-Condon effects in molecular REMPI spectra also arise from a mechanism associated with rapid evolution of the resonantly populated Rydberg orbital with changing inter-nuclear distance. These effects should be most pronounced in diatomic hydrides, and are illustrated by theoretical predictions of vibrational and rotational ion distributions for REMPI of OH and CH via the D²Σ⁻ and E'²Σ⁺ Rydberg states, respectively.

This is a preliminary report on a series of measurements, using high optical resolution, on the photoionisation of the nitrogen molecule in the wavelength region 740-780 Å where there are at least five Rydberg series converging onto the 1st excited state of N₂⁺A²Π_u. An advanced electron spectrometer system is used which provides measurements for the angular distribution parameter β for three members of the ground state of the ion, on a wavelength mesh sufficient to show variation in β over the width of the autoionising lines. This study was undertaken to assist in the identification of these series, and to encourage further theoretical work on interaction between electronic and vibrational motion in the region of autoionising resonances.

Optical properties of argon clusters and microcrystals containing between 2 and 500 000 atoms have been studied in the VUV using fluorescence excitation spectroscopy with Synchrotron Radiation. Below the ionisation limit the fluorescence yield roughly corresponds to the absorption cross section. Surface and bulk excitations as well as Rydberg states have been observed. The evolution with cluster size shows strong variations of the band intensities whereas the energetic positions are only slightly shifted compared to the solid. It turns out that the Wannier excitons appear only in clusters if the radius of the cluster is approximately four times larger than the radius of the exciton.

The anomalous scattering factors g', g" for forward scattering are generated for neutral Ne and for Ne 2+, 4+, 6+ and 7+ ions, utilizing a dispersion relation integration method, for photon energies from 1 to 70 keV. Photoionization cross sections (i.e., g") above threshold are calculated in the relativistic Hartree-Slater potential; the code of Cromer and Liberman [1] is used to obtained the corresponding bound-free contribution to g' from g". However it is found that the contribution from bound-bound transitions to g' is much more important for ions than for the neutral atom. The primary reason is that in ions there are more inner-shell vacancies for which bound-bound transitions are allowed. Near the K threshold the major bound-bound transition for the neutral neon atom is 1s → 3p, whose oscillator strength is about ten times less than that of 1s → 2p transition which is allowed in neon ions. With the contribution from the major bound-bound transitions included in g', the Rayleigh scattering cross sections obtained in this way agree well with results of direct S-matrix calculations [2].

The question is addressed of whether it is possible to obtain in a toroidal grating spectrograph, competitive performances with "Classically" ruled and aberration corrected holographically exposed gratings, the answer being a qualified yes.

Stigmatic coma, primary defocus and stigmatism were made to vanish at the shortest wavelength of interest, thus determining the toroidal surface radii. A steepest descent numerical search for minimum aberration, carried out in a multidimensional space, confirmed the analytical choice of parameters.

The photometric threshold of the detection system was estimated to be 3 × 10⁵ UV photons within a band pass of ∼ 0.4% at the entrance slit.

The tokamak scrape-off (the region between the vacuum vessel wall and the magnetically confined fusion plasma edge), represents a source/sink for the hot fusion plasma. The electron densities and temperatures are in the ranges 10¹¹-10¹³ cm⁻³ and 1-40eV respectively (depending on the size, magnetic field intensity and configuration, plasma current, etc.). Relatively low charge states of various impurities are formed here in a variety of atomic and molecular processes. Since it is very difficult to study these processes in situ, we looked for a way to simulate them under conditions similar to those of the tokamak scrape-off layer. In the work reported, the electron temperature and density have been estimated in a Penning Ionization Discharge by comparing its spectroscopic emission in the VUV with that predicted by a collisional radiative model. An attempt to directly compare this emission with that of the tokamak edge is briefly described.

An ergodic magnetic limiter (EML) creates a randomization of the magnetic field at the edge of a tokamak plasma. As a result, poloidal changes in the electron temperature and density profiles are expected. We describe an experiment in which these parameters have been measured during EML experiments on the TEXT tokamak using line ratios emitted in the VUV by intrinsic oxygen and carbon charge states isoelectronic with Be I. The spectra were recorded by means of two photometrically calibrated spectrometers equipped with time-resolving, multispectral image intensifier detectors. A scanning mirror system mounted close to the plasma at grazing incidence, enable the tangential view of the tokamak edge plasma. The reliability of the line ratio used as local density and temperature diagnostics is discussed also.

A symmetry plane for K-CO co-adsorbate at low coverage on Fe(110) surface was found using angle-resolved UPS with polarized light. This plane is parallel to the ⟨110⟩ azimuth of the crystal surface. The dependence of the CO 4σ peak intensity on incidence angle was found to be nearly the same as in the case without K, where the molecular axis of CO is perpendicular to the surface.

The investigation of electrode materials based on Li-V₂O₅/MoO₃ was performed by V Lα soft X-ray emission, V K-absorption, and photoelectron spectroscopies. During the discharge process lithium is inserted into the electrodes accompanied by a surface enrichment of LiO⁻ groups. As the X-ray absorption spectrum obtained by excitation with synchrotron radiation, the emission band recorded with a laboratory spectrometer is also sensitive to reduction processes of the vanadium atom and to changes of bond lengths in the first coordination sphere.

The angle-resolved He I photoelectron spectrum of the methanethiol absorbate (methyl mercaptide) on Cu(111) has been examined with both s-and p-polarized radiation. An adsorbate-induced band at 6 eV below E_F has been found to completely disappear at normal emission, for both polarizations. This implies that the emission is symmetry forbidden. In turn, this requires that the site symmetry be at least 3-fold, and that the band be of a forbidden symmetry species. No adsorbate-derived band has such a symmetry; neither does any bulk band along ΓL, nor any individual copper atomic orbital. However such symmetries do occur amongst the symmetry-adapted orbitals of the adsorbate-defined surface unit cell of a single copper atomic layer. The procedure developed for formulating these substrate group orbitals is appropriate to all adsorbate-substrate systems. The polarization-dependence of the off-normal emission further indicates that, of these orbitals, the 6 eV band can be assigned to a B₁ species associated with an atop site (in C_6v symmetry). Its energy suggests a Cu 4_s origin, which in turn implies a (√3 × √3)R30° adsorbate layer.

Due to electron diffraction in the atomic network, the measurement of core electron peak intensity or Auger electron peak intensity from a single crystal surface as a function of the electron take-off angle gives rise to plots in which pronounced fine structure is superimposed on the instrumental response function.

Of course, in amorphous samples or polycrystals, where the arrangements of atoms are macroscopically randomized, such fine structures are averaged. In these cases, the angular distribution curves (ADCs) show smoothly-varying curves containing no marked structure. Perfectly crystalline surfaces provide the other extreme situation giving ADCs with maximum anisotropy. So we can draw some qualitative conclusions from the measurement of the anisotropy degree of the XPD patterns, which is directly related to the change of surface structure and stoichiometry.

Using the XPD method, we examine the stoichiometry variations and the damage induced by different technological processes including (i) the case of the acidic etching of GaAs (001) surfaces (ii) the reactive ion etching of InP (001) substrates and the subsequent epitaxial growth of InP layer on this etched surface.

The influence of electron-phonon scattering on the structure of the photoelectron energy distribution has been studied in CsI for electron energies lower than the band gap. Photoelectron energy distributions at hv < 11.8eV have been measured as a function of CsI layer thickness. In this energy range hv is lower than the double band gap (E_g = 6.2eV) and the energy of photoexcited electrons is not enough for electron-hole pair production. The photoelectron energy distribution can then be changed only by electron-phonon scattering. Back-side illumination of the CsI layer has been used to separate the structures of the spectrum due to excitation and scattering. It has been shown that the initial energy distribution of photoelectrons is significantly distorted by the transport processes and new structure features appear in the energy distribution. These features correlate with the density of states in the conduction band: a peak in the density of states corresponds to a dip in the energy distribution at the same energy. Transport of excited electrons in the CsI conduction band has been described by solving the diffusion equation obtained on the basis of a kinetic equation. Photoelectron spectra have been simulated and compared with experimentally measured ones. As a result maxima in the CsI density of unoccupied states have been obtained at the energies of 6.7, 7.5 and 8.9eV above the top of the valence band.

Luminescence excitation spectra of a wide variety of undoped and doped (by rare-earths or transition elements) crystals with alkali-earth cations and complex oxyanions, either octahedral (ZrO₃²⁻, CeO₃²⁻) or tetrahedral (SO₄²⁻, WO₄²⁻, PO₄³⁻), have been measured using synchrotron radiation of the "Siberia-1" storage ring in the energy range of exciting photons of several bandgaps (up to 35 eV). An attempt is made to work out a uniform approach to the treatment of luminescence excitation of such systems based on the common tendencies observed in the measured spectra.

We have studied the room temperature optical reflectivity of MgO, MgAl₂O₄, and α-Al₂O₃ from 5 to 40eV using a novel spectrophotometer with a laser plasma light source. Structure in the imaginary component of the dielectric response is analysed using critical point line shapes, and the origins of the major transitions in MgO and MgAl₂O₄ are determined using an ab initio pseudofunction band structure calculation of MgO. The exciton reflectivity has been studied in the three materials at temperatures between 300 and 1500 K, and exciton-phonon coupling appears to increase from MgO to α-Al₂O₃. The temperature dependence of the higher lying interband transitions in MgO has been determined to 1100 K, and we find that while the temperature dependence of the onset transitions at Γ and X are nearly identical (− 1.22meV/K at Γ), higher lying transitions have very different temperature dependences. Furthermore with increasing temperature the X point valence band separation increases at a rate of 0.38meV/K, while the conduction band separation at X decreases at −0.41meV/K.

Synchrotron-radiation photoemission spectroscopy and other surface-sensitive techniques have been used to study silicon chemisorption on gold and silver single-crystal substrates. In both cases, we find evidence of extended interfaces and formation of interface chemical species. This is quite interesting, since the morphology of the interfaces obtained by silver deposition on silicon is very controversial – and most authors believe that they are not sharp. The Schottky barrier heights measured for our two interfaces suggest that the role of the Schottky-like terms is independent of the deposition sequence.

A simple parallel detection system which allows several bremsstrahlung isochromat (inverse photoemission) spectra to be recorded simultaneously is described. It is based on the dispersive properties of a LiF lens in the vacuum ultraviolet previously described by Childs et al. [13] and operates in the photon energy range of approximately 8.0-11.5eV. In the instrument described three isochromats can be recorded at photon energy separations of 1.5eV. Its utility is demonstrated with measurements on Cu(111), the different photon energies permitting easy confirmation of the contributions of surface-localised states.

Ultraviolet photoelectron spectra have been measured for the radical phthalocyanine dimer, Pc₂Lu, and the fluorine bridge stacked phthalocyanine polymer (PcAlF)_n. Previous workers have shown that both materials can, under appropriate conditions, be prepared in a well characterised, highly oriented thin film form. Thus, samples for this work were prepared by in situ sublimation at very slow evaporation rates onto crystalline substrates to try to maximise the degree of sample orientation. The angle dependence of the spectra were measured and the sample structure subsequently examined using high resolution TEM. The TEM results show that the (PcAlF)_n films have a much higher level of orientation than the Pc₂Lu films and this is reflected by the angle dependences of the UPS measurements. The spectra for (PcAlF)_n are very similar to measurements on most other simple phthalocyanine compounds and have a small angular dependence. The spectra for Pc₂Lu show almost no angular dependence. Again the spectra are broadly similar to that of other Pc's with two significant differences, the lowest energy peak is split and the whole spectra is shifted to lower energy. This result will be discussed in terms of simple molecular orbital ideas. The effect of air on the spectra of both materials was examined and the spectra of (PcAlF)_n was found to be particularly sensitive. Attempts to determine the position of the lutetium orbitals by varying the light frequency around the lutetium resonance energies was attempted but no significant variation in the spectra was observed.

Experimental investigations of cleaned (0 0 1) surfaces of superconducting (SC) and nonsuperconducting (NSC) La_1.85Sr_0.15CuO₄ single crystals are carried out in the range 20-300 K by LEED, AES, UPS, XPS, and EELS techniques. Different surface cleaning procedures are used. It is found that cleaned (0 0 1) surfaces are rather stable in ultra-high vacuum and possess very low sticking coefficients for all residual gases, except oxygen. It is established by LEED that these (0 0 1) surfaces are nonreconstructed in the whole range 20-300 K. The surfaces are sensitive to the electron beam: at the exposure to an electron beam (E = 100eV, J = 10^-6 A) the sharpness of the LEED pattern increases in time. It is argued that this is due to the extra ordering of the surface stimulated by beam. The Auger and XPS peak amplitude dependencies on angle of incidence of the primary beam with respect to the surface are investigated. From the obtained results it is suggested that cleaned (and probably as grown) La_1.85Sr_0.15CuO₄(0 0 1) surfaces are formed by Cu-O planes with saturated chemical bonds. It is found that UPS and EELS spectra are practically identical for both SC and NSC single crystals.

The results of the EELS of La_1.85Sr_0.15CuO₄(0 0 1) are summarized. It is suggested, that similar to La_1.85Sr_0.15CuO₄ ceramics, the bulk plasmon energy is hW_b = 13.2eV and the surface plasmon hW_s = 8.9eV. On this basis it is argued that the one-electron approximation is not satisfactory to describe bulk and surface plasmon excitation for La_1.85Sr_0.15CuO₄ single crystals.

Effects of symmetry breaking in the 4f orbital occupancy of Ce atom in ferromagnetic Ce compounds on the multiplet structures in the Ce 3d- and 4d-core photoabsorption spectra are discussed based on the impurity Anderson model. The calculation predicts a strong circular dichroism in the spectra even for a molecular field producing the 4f magnetic moment of ~0.5µ_B. The interplay of the 4f-conduction band hybridization and the 4f spin-orbit interaction in the ground state is discussed in detail and the present dichroism is shown to be a good method to measure the spin and orbital contributions to the 4f magnetic moment.

Photoelectron spectra of the Hf 4f core levels from a HfN(100) single crystal and a polycrystalline HfC sample have been measured and analysed. Surface-shifted 4f levels could be identified for both compounds, at a smaller binding energy than the bulk 4f levels in HfN and at a larger binding energy in HfC. The surface core level shifts were extracted by a curve fitting procedure and found to be −0.65 eV and +0.41 eV in HfN(100) and HfC. For HfC the shift is very similar to the previously published surface core level shift of +0.42 eV in polycrystalline Hf metal which agreed with theoretical calculations. For HfN however the results cannot be explained in terms of d-band narrowing effects, or by a simple electrostatic picture.

We theoretically analyze the spectra of X-ray photoemission, X-ray emission and X-ray adsorption associated with the Cu 2p core level in high-T_c superconducting materials. We use the single-site impurity Anderson model with five Cu 3d orbitals and O 2p valence bands specified by the irreducible representations of the D_4h symmetry around the Cu atom. We also take account of Coulomb and exchange interactions between Cu 2p and 3d states, those interactions between Cu 3d states, and the spin-orbit interaction.

Magnetic probing by X-ray absorption is discussed. Various techniques, such as X-ray scattering, spin dependent X-ray absorption, magnetic X-ray dichroism (MXD) and spin-orbit branching ratio measurements are evaluated.

The occupied and unoccupied electronic structure of Bi₂Sr₂CaCu₂O₈ single crystals has been investigated by high resolution angle-resolved photoemission, applying synchrotron radiation of 18 eV, and angle-resolved inverse photoemission. The electronic states far from the Fermi level are almost dispersionless due to correlation effects. Near the Fermi level, a band reveals weak but distinct k_∥ dispersion, crossing E_F at about ½ ΓX. Starting for T > T_c on the Fermi surface, for T < T_c spectral weight is shifted from E_F to higher binding energy related to the opening of the superconducting gap of Δ = 30 meV. These findings strongly favour Fermi liquid based theories for high-T_c superconductivity in this material.

The valence states of vanadium nitride have been studied by X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) as a function of the nitrogen vacancy concentration. Vacancy induced states are observed in the V K emission band (and in the XPS valence band spectrum) ~2eV below the Fermi energy suggesting that the V p states are strongly affected by the presence of nitrogen vacancies. This is confirmed by simulations of XES and XPS spectra from Korringa-Kohn-Rostoker coherent potential approximation densities of states.

Prior work has shown that excitation of a core electron into an antibonding molecular orbital of adsorbed and condensed H₂O/D₂O (O 1s → 4a₁) leads to an unusually high efficiency of H⁺/D⁺ formation. This effect and the unexpected angular behaviour observed for it have been tentatively explained by comparable time scales of electronic decay and nuclear motion, and by symmetry breaking through the influence of hydrogen bonding and/or the coupled excitation-deexcitation-dissociation sequence. We have now investigated the corresponding behaviour in multilayers of the isoelectronic molecules NH₃ (at N 1s) and CH₄ (at C 1s), including mixed NH₃/CH₄ layers. While pure NH₃ shows essentially the same behaviour as H₂O, CH₄ does not; and in mixed CH₄/NH₃ layers the H⁺ "threshold peak" is quenched even for NH₃ (at N 1s). This suggests that hydrogen-bonding is of importance for this interesting behaviour. The possible physical effects involved are discussed.

Slab calculations of the surface electronic structure of Rh(001) predict surface states. However, they disagree on their relative strengths and, in particular, whether the surface states near the point are located above or below the Fermi level. This has ramifications as regards the value of the surface charge density in relation to the magnitude of the work function values. We present angle-resolved photoemission evidence for a strong narrow spectral peak located just below E_F about the point which is independent of photon energy, indicative of an occupied surface state, in agreement with one of the calculations.

The electronic structure of the high temperature superconductors, YBa₂-Cu₃O_7-x and Bi₂Sr₂CaCu₂O₈ have been studied by high energy electron energy-loss spectroscopy in transmission mode. We could derive the dielectric functions from the low energy loss spectra below ∼ 40eV. These dielectric functions give information on plasmons, interband transitions and low-lying core levels. From the core electron excitation spectra, we obtain information on the unoccupied electronic states.

The influence of electron correlations on the temperature dependence of ultraviolet photoemission and bremsstrahlung isochromat spectra of ferromagnetic Ni is investigated. The calculations are performed within the framework of the one-step model of photoemission using an expression for the energy-, spin- and temperature-dependent self-energy of the hole state, which rests on a combination of the results of density functional theory and those of a generalized Hubbard model. The model contains only two parameters, the Hubbard U and the interband exchange J. It is approximately solved by use of a selfconsistent moment method and has so far resulted in an excellent description of macroscopic magnetic properties, like the magnetic moment at T = 0, the Curie temperature, a Brillouin-like magnetization curve and a strict Curie-Weiss behaviour of the paramagnetic susceptibility. In this contribution we demonstrate the virtues of this theory for quasiparticle properties in Ni by calculating spin-polarized and temperature-dependent photoemission spectra and comparing with experimental results available in literature. The theoretical photoemission results for the occupied d-bands are shown to be in excellent agreement with the experiment. Since the ferromagnetism and the many body effects in Ni are mainly caused by the uppermost d-band, which is only partly occupied, we also calculate the spin-polarized inverse photoemission spectra for this subband and compare with the most recent experimental results obtained in bresstrahlung isochromat spectroscopy.

Ultra-thin metal overlayers on single crystal metal surfaces are novel, stable surfaces with physical properties which may be tailored for specific applications. The growth, structure and thermal stability of chromium and iron pseudomorphic monolayers and multilayers on W(110) are characterized by temperature programmed desorption, low energy electron diffraction, Auger electron spectroscopy and valence band photoemission. Surface-sensitive Cr, Fe and W core-level synchrotron photoemission is used to probe the local electronic changes at clean and oxygen-modified bi-metallic interfaces. Nitrogen chemisorption is used to illustrate the unique chemical properties of these overlayer systems.

The electronic properties of Rb/Ge(111) and Na/Ge(111) interfaces are investigated by soft X-ray photoemission spectroscopy using synchrotron radiation. On both systems, alkali metal deposition results in the formation of a new well defined interface state below the Fermi level. This interface state is highly sensitive to oxygen exposure. For the Na/Ge(111) system, this interface state is found to resonate around a photon energy of 82eV. This resonance occurs within the adsorbate/substrate complex involving hybridization of their valence electrons. These results indicate that the nature of alkali metal-germanium bond should be understood in terms of covalent bonding, as observed for alkali metal-silicon surfaces. The existence of this interface resonant state brings the first example of an extra-atomic resonance involving both substrate and adsorbate electronic levels.

Angle resolved photoemission measurements covering the photon range 10.0 to 50.0 eV have been maade on MBE grown In_0.27Ga_0.73As(100) layers using the Berlin synchrotron radiation source BESSY. The use of a toroidal energy analyser has enabled us to acquire extensive off-normal data with angular resolution close to ± 1°. Films of In_0.27Ga_0.73As were grown on GaAs by MBE in both strained (<100 Å thick) and relaxed (>1000 Å thick) forms. Differences in the raw photoemission data between strained and unstrained layers were observed. This data has been analysed using a free electron final state model to determine experimentally the detailed shape of the valence bands in the Γ-X direction in both cases. These K-dependent effects of strain are discussed.

Spectator and participator Auger decay processes have been observed in CaF₂ under resonant excitation of the L_2,3 core exciton transitions with synchrotron radiation. The CaF₂ thin films were deposited on clean GaAs (100) substrates. At the interface the valence band offset is 7.7 eV, with the Fermi level a few tenths of an electron volt above the top of the GaAs valence band.

Evidence of itinerant (band-like) behaviour of the Co 3d states in paramagnetic CoO has been adduced from CoO(001) angle-resolved photo emission spectra. Two d bands are identified, which disperse 0.4 eV and 1.7 eV between Γ and X. In broad terms the results are consistent with a recently proposed model of the electronic structure in which the insulating gap arises largely from an exchange potential perturbation of the one-electron band structure.

The valence bands and conduction bands of about 30 transition metal silicides (of which we concentrate on 4 here) have been investigated by measurements of Si X-ray emission bandsspectra, X-ray absorption spectra near the Si K (1s) edge, photoemission spectra, and Bremsstrahlung Isochromat spectra. The densities of states have also been calculated for the materials in their real crystal structures. The influence of the core hole on some spectra has been investigated using supercell calculations, a (Greens function) generalized Clogston-Wolff model, and Auger spectroscopy. A selection of results is presented to illustrate the utility of site and selective methods in investigations of the electronic structure of silicides and the nature of the "quasi-gap" of the partial density of Si p states in the region of the transition metal d bands.

We present a new, unified model describing the influence of the temperature-induced lattice and the spin disorder on the angle resolved photoelectron spectra (ARPES); we also test this model by analysing the temperature evolution of the line shape for ARPES obtained at Cu(110), Ni(110), and Ni(111) surfaces, using the synchrotron radiation of BESSY

A synchrotron radiation-excited photoemission study of the adsorption behaviour of Mo(CO)₆ on Si(111)2 × 1 is reported. The compound has been investigated in the temperature range 50 K-room temperature (RT), and in the exposure range 0.1-200 L (1 Langmuir = 10^-6 Torr s^-1). Both core (Mo 3d, C 1s, Si 2p, O 1s) and valence levels were investigated.

At T ⩽ 100 K and exposures ⩾0.5 L, the adsorption of Mo(CO)₆ on cleaved Si is molecular, as clearly indicated by a 1: 1 correspondence of the features in the valence band (VB) spectrum with those already reported for free Mo(CO)₆, at the same photon energy.

At T < 100 K and very low exposures (0.1 L) we found evidence of a decomposition process taking place. At T ⩾ 150 K, the adsorption is dissociative at exposures < 120 L and molecular at higher exposures. As the temperature increases up to 200 K, only dissociative processes are detected, also at high exposures. At RT, no adsorption at all was detected.

Reacted surfaces were exposed to zero-order light to promote metal deposition on Si. A metal layer on top of Si was obtained after short exposures. CO adsorbs on this layer and is partially dissociated. Molecular CO is removed by further exposure to zero-order light. The presence of atomic C and O from CO partial dissociation and of carbide species was detected.