Table of contents

We are in the process of developing and commissioning a scanning photoelectron microscopy (SPEM) at the X1A beamline of the National Synchrotron Light Source (NSLS). It is desigend to make use of the Soft X-ray Undulator (SXU) at the NSLS. This high brightness source illuminates a Fresnel zone plate, which forms a focused probe, ⩽0.2 µm in size, on the specimen surface. A grating monochromator selects the photon energy in the 400-800 eV range with an energy resolution of better than 1 eV. The expected flux in the focus is in the 5 × 10⁷-10⁹ photons/s range. A single pass Cylindrical Mirror Analyzer (CMA) is used to record photoemission spectra, or to form an image within a fixed electron energy bandwidth as the specimen is mechanically scanned.

As a first test, a 1000 mesh Au grid was successfully imaged with a resolution of about 1 µm and the CMA tuned to the Au 4f photoelectron peak. Once it is commissioned, a program is planned which will utilize the microscope to study beam sensitive systems, such as thin oxide/sub-oxide films of alumina and silica, and ultimately various adsorbates on these films.

We report on the normal incidence performance of multilayer coated concave gratings in spectral regions where near normal incidence reflectivities of conventional coatings are calculated to be 10⁻⁴ to 10⁻⁵. These concave gratings were designed for use in a McPherson 225 Spectrometer and were tested for resolution in this instrument using a Garton flash tube source and a film detector. Efficiency measurements were obtained for both the gratings and flat multilayers fabricated at the same time using a Penning source and an incident beam monochromator to obtain spectral purity. The incident and diffracted beams were measured using the same microchannel plate based two dimensional photon counting detector. Experimental results are compared with scalar theory.

We describe the layout and expected performance of the soft X-ray spectroscopy beamline, which is currently being installed at the 35 pole undulator on the X-ray ring at NSLS Brookhaven. We present our rationale for the choice of monochromator and discuss some of the experimental program planned for this new high resolution instrument.

Soft X-ray spectroscopy can be carried out in commercially available spectrometers if the following simple changes are made: (1) an open window gas-discharge X-ray tube (e.g. CGR Elent-10) is used for the excitation of characteristic X-rays, (2) large 2d spacing crystals or multilayers are used to disperse the X-rays and (3) a one micron (or thinner) window is used in the proportional counter. Examples are given of Al, Si, P and S L X-ray and Be, B, C and N K X-ray spectra, measured on a Philips PW 1410 spectrometer. Results using multilayers and organic ester crystals (octadecyl maleate-OHM, 2d 6.25 nm and octadecyl adipate-OAO, 2d 9.12 nm) are compared; the former shows much greater intensity, the latter much better resolution.

A soft X-ray reflectometer with a laser-produced plasma source driven by a Q-switched Nd: YAG laser or a XeCl excimer laser was constructed. By comparing the reflectances of a multilayer mirror with the data obtained using synchrotron radiation, its usefulness for reflectance measurements in the soft X-ray region (40-130 eV) is demonstrated.

We describe the development of the scanning photoemission spectromicroscope MAXIMUM at the Wisoncsin Synchrotron Radiation Center, which uses radiation from a 30-period undulator. The article includes a discussion of the first tests after the initial commissioning.

Various aplanatic flat field spectrometers fitted with spherical or aspherical holographic gratings are described. Basic conditions for stationary anastigmatism are first expressed, the object and image being located on the sagittal focus. Optical performances of some coma corrected configurations are finally described.

Optical properties of spherical or aspherical holographic gratings are investigated from the imaging point of view. Basic conditions for aplanetism are first expressed, the object image and recording point sources being located on circles centered on the grating tangent. The performances (resolution, anamorphism) of some imaging configurations are described.

This paper deals with the opto-mechanical design and some experimental results of a new anastigmatic normal incidence monochromator. The configuration involves a plane holographic grating used in convergent light, and the first part of this paper will be devoted to the optimization of the Line Spread Function.

In this paper we investigate the optical design criteria of an VUV or XUV three beam grating interferometer whose performances has been experimentally checked in the visible spectral region.

A lithography beam line, as the first of possibly six or more, has been installed in the Hefei Synchrotron Radiation Laboratory (HESYRL). A scanning mirror is used to cut off shorter wavelengths and to expand exposure dimensions vertically. The scanning mirror is oscillated by a stepping motor while an in-situ Moirè fringe grating system measures motor speed uniformity. Some testing results are given. In the first part of the beam line, there is a beryllium window to block longer wavelength light, a laser alignment unit to align the beam line and a special exposure shutter, which is controlled by another stepping motor. An exposure chamber with vacuum of 5 × 10⁻⁷ torr is located 7 meters downstream from the source point. Because there is no window at the entrance of the chamber, a differential pumping system is needed. The chamber is equipped with a mask-wafer system, driven by a third stepping motor.

A VUV Sun Spectrometer designed at Leningrad State University implements a new concept of collisional photoionization spectroscopy (CPS). While in the spectrometers of known type collisions between photoelectrons and atoms of the gas filling the internal volume of the spectrometer cause deteriorization of instrumental characteristics, the CPS-method for the first time makes use of the collision to improve sensitivity by 2 to 3 orders of magnitude.

The responsivities as well as the losses of photons through reflection and absorption of a Si photodiode with a SiO₂ film, and of a GaAs photodiode with a Au film are estimated using an optical model in the wavelength region from 9 to 800 nm. The model is compared with experiments performed in the visible-far ultraviolet region, and good agreements between the calculation and the experiments are obtained except for scale differences due to the surface recombination of carriers. It is shown that the rate of the surface recombination in a Si photodiode can be determined through a measurement of the internal quantum efficiency in the visible-far ultraviolet region. Thus, the spectral responsivities of semiconductor photodiodes in the VUV region can be roughly predicted.

The Centre for Advanced Microstructures and Devices (CAMD) has been established by the Department of Energy at Louisiana State University for the expressed purpose of developing X-ray lithography based techniques for manufacturing microcircuits. The heart of the Center will be a synchrotron light source optimized for the soft X-ray region and will be the first commercially manufactured electron storage ring in the United States. The magnet lattice is based on a design developed by Chasman and Green and will be similar to the VUV ring at Brookhaven NSLS and will operate at 1.2 GeV with a critical wavelength of 9.5 Å. Straight sections will allow up to three insertion devices to be installed for higher energy and higher intensity radiation.

In addition to the lithography effort, experimental programs are being established in physics, chemistry, and related areas.

A modified optical design for a synchrotron radiation (SR) soft X-ray scanning microscopy beamline at Hefei National Synchrotron Radiation laboratory (HESYRL) has been done. On the beamline the main component is a linear monochromator which consists of a diaphragm with a diameter of 7 µm and a condenser zone plate (CZP). The CZP is selected from among three condenser zone plates with different parameters for the following three overlapping wavelength ranges: 1.97-2.78 nm, 2.74-3.87 nm, and 3.84-5.44 nm. Estimating the flux losses from the pinhole source to the end spot for 2.3 nm, for example, a photon flux of 3.1 × 10⁵ photons/s can be achieved at the end spot from a 150 mA current and a high brightness configuration at HESYRL.

Parameters of three kinds of CZPs are presented as a compromise to a variety of requirements from the experimenters. In addition, the thickness of some CZP layers, such as polyimide and gold, are calculated to be optimal. Finally, the processing steps for CZP fabrication at HESYRL are described briefly.

The principle for measuring the loss impedance of a storage ring, which is caused by the energy loss of the stored bunched beam in passing through discontinuous parts of the vacuum chamber of the ring, is discussed and the set up to measure the loss impedance of the HESYRL storage ring is described.

The measurement results of the energy loss and loss impedance of the kicker vacuum chamber, the septum vacuum chamber and other parts to be installed on the HESYRL storage ring are given. Some of the results are consistent with those measured in DESY. The results would be useful to design and improve these parts.

Gas-phase UV photoelectron spectra and pseudopotential calculations were used to elucidate the electronic structure of a series of alkyl lead acetylides, a trimethylsilyl-trimethylplumbyl-acetylide and bis(trimethylsilyl)butadiyne. The analysis of the results obtained, along with the comparison with previous data on tin analogues, confirmed the presence of a σ/π conjugation between the π_C≡C and the σ orbitals of the metal-alkyl moieties. The ionization of the 5d pseudo-valence orbitals of Pb was observed and compared with the calculated net atomic charge on Pb. A spin-orbit effect was found in the ionization of valence MO's with significant contribution of 6p Pb orbitals.

Depopulation cross sections for Na ns states with n = 6 to 11 are calculated for Na-Ar and Na-He collisions at thermal velocities using quantum defect wave functions for Na and first-order perturbation theory. The results are compared with the experimental values of Gallagher and Cooke [1].

An X-ray laser scheme was recently proposed based on the pumping of neon-like 3s-3p transitions using inner-shell ionization of sodium-like ions by suprathermal electrons. Since this laser operates in principle with no population in the neon-like charge-state power requirements can be reduced from the standard collisionally driven amplifier, and the problem of line-trapping can be ameliorated. Model calculations using a quasi-steady state collisional-radiative model indicated that measurable gains could be obtained in practical experiments. We propose a proof-of-principle experiment to demonstrate the pumping of neon-like excited states by hot electrons. A ~ 1 ns long, 0.53 µ laser beam, will be focussed on a solid dot target of dilute (5%) bromine, to create a cool, dense plasma. The pulse and target will be optimized to produce as much sodium-like population as possible in the corona. This low-fluence green pulse does not produce suprathermals. A second, ~ 100 ps, 1.06 µm pulse of 10¹⁵ W/cm² will be "piggybacked" on the first, to create hot electrons. The timing of the red beam will be optimized with respect to the sodium-like fraction of the cool plasma. The hot electrons create a transient population of neon-like ions, coincident with the red beam, that are detected by their characteristic n=3-n=2 resonant X-ray emission. If, as expected while the red beam is present, the neon-like excited states are produced by inner-shell ionization (as opposed to collisional excitation), n = 3-2 line ratios will carry a distinctive signature indicating a statistical feeding mechanism. The same signature will imply the existence of a population inversion. Proposed detection systems include space and time resolving crystal X-ray spectrometers for measuring the neon-like lines, and an array of filtered X-ray diodes to measure suprathermal electron temperature and number.

We observe bond-selective fragmentation of the nitrous oxide molecule upon site-specific 1s excitation at either the terminal or the central nitrogen atom. Our results can be interpreted as a chemical "memory effect" where the fragmentation process memorizes the inital site of energy deposition. We empty a new "Zero Kinetic Energy Photoelectron"-photoion coincidence technique using monochromatized x-ray synchrotron radiation from BESSY.

We assess aspects of the feasibility of a recent proposal to use the coincidence between a Mo¹¹⁺, 136.499 Å, line and a 4p⁶-(4p⁵)_1/26s, J = 1, transition of Mo⁶⁺, to obtain amplified spontaneous emission around 600 Å. [Feldman and Reader, J. Opt. Soc. Am. B6, 264 (1989)]. A collisional-radiative model for the Mo⁶⁺ ion, including resonant photoexcitation of the 4p⁵_1/26s level, was constructed using the HULLAC atomic physics package. The model included the 4s²4p⁶; 4p⁵4d, 4f; 4p⁵5s, 5p, 5d; 4p⁵6s, 6p, 6d; 4s4p⁶4d, 4f; 4p⁶4d² and 4s²4p⁴4d² configurations, amounting to 193 levels. All configuration interactions and collisional mixing were taken into account. A gain of 3 cm^-1 is obtained for the 4p⁵6s(J = 1)-4p⁵5p(J = 2) transition at 645 Å with T_e = 5 eV, n_e = 10¹⁸ cm^-3, and an effective radiation temperature of T_rad > 40 eV for the pump transition. Other transitions exhibit lower gains.

The photoionization cross section and angular distribution asymmetry parameter of the 4d¹⁰ subshell of atomic barium have been calculated using many-body perturbation theory and the relativistic random-phase approximation modified to include relaxation effects. Our main purpose is to determine whether relaxation and polarization effects can account for the large difference between recent 4d photoemission experiments and total absorption experiments. Polarization effects have been included by evaluating many-body diagrams. Results are compared with experiments and other calculations.

The energies and intensities of X-ray emission diagram lines associated with the methanol molecule have been determined using ab initio molecular orbital procedures. Absolute transition probabilities are sensitive to the inclusion of electronic relaxation effects and inter-atomic moment contributions. Relative intensities determined from frozen orbital and relaxed orbital models are in better agreement with one another than are the absolute values. To achieve a realistic basis for comparison with experiment, calculations for molecules containing first-row atoms must be undertaken at least at the multi-centre relaxed Hartree-Fock level. Good agreement is obtained between the theoretical and experimental O-K spectra for the CH₃OH molecule. A significant discrepancy is reported in the C-K spectral comparison. It is concluded that the most probable causes for the differences are satellite contributions and contamination in the experimental spectrum from other molecular species.

The observation of C-H bending, and C-C and C-H stretching vibrational levels in high resolution C 1s⁻¹ π* photoabsorption spectra of ethylene and benzene demonstrates that both molecules have broken symmetry in this core excited state. A simple model based on the antibonding character and the out-of-plane charge distribution of the π* electron is utilized to explain our observations and to show that both core excited molecules become non-planar. The model is also consistent with the vibrational structure associated with the transitions of C 1s → Rydberg states in ethylene, which is found to be very different from that of the C 1s⁻¹ π* state.

Angle-resolved photoelectron spectrometry employing synchrotron radiation is used to study oxygen atoms generated in a microwave discharge. The energy region below the ⁴P^e threshold associated with removal of a 2s electron is addressed. Photoionization cross sections (σ) and asymmetry parameters (β) are obtained for both the O⁺ (⁴S⁰), εl and O⁺ (²D⁰), εl channels and evidence for autoionizing Rydberg series with ³S⁰, ³P⁰, and ³D⁰ symmetries, converging to the ⁴P^e ionic state, is observed.

Many-Body Green's function calculations which employ large configurational expansions and which are accurate to third and fourth order of perturbation theory are reported for the valence ionic states of HCl and H₂S. Large polarized basis sets including several sets of Rydberg functions of s-, p-, and d-type symmetries have been used. Third order calculations are sufficient in the outer valence region, but in the inner valence region where the breakdown of the one-particle picture of ionization is observed a fourth order theory is in general necessary in conjunction with large basis sets. The synchroton and X-ray excited spectra can be assigned up to the double ionization threshold. No indication of strong outer valence satellite lines is found.

A quantitative analysis of the NEXAFS for C₂H₄ molecules on Cu(100) in a single layer coverage is given. A simple quantum mechanical scattering model is used in order to reproduce the asymmetric profile of the σ*-shape resonance using only three independent parameters, that appear to possess a physical meaning. As a result small threshold resonances can be quantitatively analyzed. The angular dependence of all the molecular resonances is presented.

The production of specific Ar⁺ satellite states has been studied with synchrotron radiation at wavelengths between 300 and 350 Å with an effective energy resolution of 20 meV. The specific states studied were the (³P)4p(²P_3/2), (¹D)4p(²F_7/2), and (¹D)4p(²P_1/2) states. The fluorescent radiation emitted from these excited ionic states was measured at 4766, 4611, and 4133 Å by the use of narrow band interference filters. The variation of the fluorescence intensity was measured as a function of wavelength. this provided a measure of the relative cross section for production of the satellite states. Each satellite state was found to the completely dominated by autoionization of the neutral doubly excited states (3s²3p⁴)nl, n'l' found in this spectral region.

A survey of the experimental method of photon induced fluorescence spectroscopy applied in the VUV is given. Results obtained at BESSY for photoionization of the Ar 3s-electrons are presented in an exemplary fashion in a comparison with published results obtained by photoelectron spectroscopy. For this comparison resonances close to threshold and the energy dependence in the threshold range have been selected.

The effects of rotation-vibration interaction and of rotational coupling between the B 2p¹Σ_u⁺ and C 2p¹Π_u electronic states on the B-X continuum of H₂ were investigated spectroscopically using monochromatized synchrotron radiation for selective excitation of rovibronic states. The perturbed continuum intensities observed for the first time agree well with close-coupling calculations also performed.

Intercombination transitions imply a change of spin which is not allowed in pure LS coupling. Forbidden transitions connect levels for which electric dipole transitions are not possible because of the parity selection rule. Both transition types are of much lower probability than allowed E1 transitions. They have been studied by time-resolved spectroscopy of foil-excited fast ion beams. The method is outlined in brief, and data are presented for highly charged ions of Ag and Xe.

The theory of linear response of the Thomas-Fermi atom to an external monochromatic field is developed in some approximation. This approximation corresponds to the high frequency limit of the exact theory and can be formulated in terms of the classical problem of polarizability of inhomogeneous particles. The main specific features of this problem as applied to atomic polarizability in the XUV are: (1) the wavelength is much larger than the particle's size; (2) there is a point inside the particle where the dielectric constant is zero. The resulting equations have been solved for various atomic electron density distributions. The photoabsorption cross section and the real part of atomic polarizibility are obtained. It can be shown that calculated in such a way they automatically satisfy the Kramers-Kronig equations. This theory can be considered as a natural generalization of S. Lundqvist's approach.

High resolution HeI (580 Å) photoelectron spectra of As₂, As₄, and P₄ were obtained with a newly-built high temperature molecular beam source. Vibrational structure was resolved in the photoelectron spectra of the three cluster species. The Jahn-Teller effect is discussed for the ²E and ²T₂ states of P₄⁺ and As₄⁺. As a result of the Jahn-Teller effect, the ²E state splits into two bands, and the ²T₂ state splits into three bands, in combination with the spin-orbit effect. It was observed that the ν₂ normal vibrational mode was involved in the vibronic interaction of the ²E state, while both the ν₂ and ν₃ modes were active in the ²T₂ state.

Using synchrotron radiation as a continuum light source, the photoabsorption cross section of PH₃ in the 1600-2100 Å region has been measured at 155 and 295 K. The peak cross sections measured at 155 K were found to be significantly larger than the corresponding peaks measured at room temperature. The spectrum obtained at low temperature shows more structures than that obtained at 295 K. Many broad features collapse into several distinct peaks thus permitting other previously obscured features to be identified at low temperature. As a result, we were able to identify two more vibrational peaks on the long wavelength side of the band origin of the state of PH₃ previously assigned by Humphries et al. The band origin and the vibrational constants were accordingly revised. Several Rydberg series converging to the ground state, òA₁, and one converging to the first excited state, ²E, of the PH⁺₃ ion have been identified. Most of the observed electronic transitions show ν₂ vibrational progressions and negative anharmonicity. The results of the tentative vibrational and electronic assignments, the derived molecular constants and the cross section data are presented.

Ab initio cluster calculation which model the adsorption of Al atoms on the basal plane of graphite are presented. One layer clusters find the Al prefers to adsorb at the center of the carbon rings. A two layer cluster determines the most stable Al adsorption site to be above the carbon atom in agreement with recent Scanning Tunneling Microscopy experiments. The charge distributions on the clusters are oscillatory and largely independent of the Al position. The form of the lowest unoccupied pi orbital for each cluster essentially determines the cluster charge distribution. From a constrained space orbital variation we find the on top site to be favored when Al and cluster wavefunctions are superimposed. However when the Al and pi orbitals interact the relaxation favors the open site.

The use of low-energy electron beams to initiate X-ray spectroscopic analysis of surface layers, 10-1000 nm thick, is described. The probed depth is controlled by the energy of the bombarding electrons in the range 3-12 kV and the characteristic soft X-rays emitted by the sample are analysed by a wavelength dispersive spectrometer [Telsec Betaprobe]. Soft X-rays can exhibit large `chemical shifts'. This property is used to determine the chemical state of elements such as; aluminium in alumina layers on aluminium metal; iron and chromium in oxide layers on steel, and boron in silica layers containing traces of boron and phosphorus.

The electronically-stimulated dissociation of NO₂ on Pt(111) has been studied through state-selective, time-of-flight detection of the NO product above the surface. The NO leaves as a direct dissociation product resulting from 5-800 eV electron impact, whereas the O atom remains on the surface. The quantum-resolved analysis of the NO reveals fundamental aspects of the stimulated surface process such as dominant excitation channels and dynamics. Because of rapid decay via resonant tunneling from substrate levels, the 3 eV shallow valence excitations prominent in gas-phase photodissociation of NO₂ have lifetimes that are too short to produce observable dissociation on the metal surface. Instead, we find a 10-15 eV threshold which corresponds to ionization of 3b₂ and double ionization of 1a₂ levels which cannot be resonantly filled by substrate electrons and thus have the longest lifetimes relative to Auger decay. Screening of the hole(s) by the metal through the 6a₁ orbital of the molecule not only determines the lifetime, but is found to influence the extent of internal excitation in the NO product.

GaAs band bending in the low adatom coverage regime (<10¹⁴ cm⁻²) has been investigated using photoelectron spectroscopy. Surface states originating from adatoms chemisorbed on the GaAs surface are responsible for this band bending, which can be described quantitatively by the depletion approximation. A relationship between the energy levels of surface donor states and the first ionization potential is established and is supported by recent theoretical calculations.

We here report core level photoemission spectra of mass selected monodispersed Pt clusters. The samples were produced from a mass selected cluster beam and deposited onto various substrates such that each sample contained clusters having an identical number of atoms. In this series of experiments we have followed the development of the Pt 4f core electron binding energy for different size clusters having between one and six Pt atoms. By comparing the results for two different substrate materials, naturally oxidized Si wafers and amorphous C films, we can evaluate the support interaction for these small clusters.

The MBE growth and interface formation of thin Ca_0.43Sr_0.57F₂ films deposited on GaAs(100) has been studied using synchrotron radiation photoemission spectroscopy. Surface sensitive core level spectra and RHEED suggest an extended fluorine-deficient interface for growth at 325°C. Annealing experiments confirm the loss of fluorine at temperatures equal to or greater than 325°C. As 3d spectra did not provide evidence for Ca-As bonds. The fluoride interface formed under these conditions apears to be atomically abrupt but contains a considerable amount of fluorine vacancies. Despite the reduction in interfacial strain by using a lattice matched mixed fluoride of composition Ca_0.43Sr_0.57F₂, RHEED images indicate growth of epitaxial islands.

High resolution polarized Cu L₃ X-ray absorption spectra of single crystals and of highly oriented pellets of high T_c superconductors of the bismuth kind have been measured by using synchrotron radiation at Super-ACO. We have investigated the energy splitting Δ between the in-plane and out-of-plane 2p → 3d transition. We find that the splitting Δ as well as the intensity of the E∥z component is correlated with the critical temperature of the superconducting systems for each class of superconductors.

High resolution angle resolved photoemission measurements of critical points show asymmetric lineshapes that are interpreted in terms of a second order calculation of the joint density of states.

We outline an extension of the previously-developed zero-temperature spherical-wave single-scattering-cluster theory of spin-polarized photo-electron diffraction (SPPD) to finite temperature. We show explicitly how the spin-spin correlation function ⟨s(r₀) · s(r)⟩ enters into the thermal averages over near neighbors, thus allowing SPPD spectra to be computed as a function of temperature if the correlation function is known. The spin-dependent part of the diffraction may be isolated by defining a spin asymmetry for a 3s emission process S_3s, which is the ratio of spin-up to spin down intensities as normalized to the high-temperature or paramagnetic limit. A computation of S_3s for a standard correlation function derived from a Monte Carlo analysis shows a complex dependence on temperature, but does not exhibit the step-like behavior at temperatures of several times T_N where a step has been observed experimentally for KMnF₃ and MnO. These results thus confirm that the steps observed in SPPD represent a new effect that goes beyond present theoretical models of the disappearance of shortrange magnetic order.

Azimuthal X-ray photoelectron diffraction (APD) experiments have been carried out on a sample of Hg_0.78Cd_0.22 Te (MCT) epitaxially grown by liquid phase epitaxy (LPE) on (111) CdTe. To our knowledge this is the first XPD study of the (111) face of a II-VI semiconductor. Reproducible diffraction features are seen for emission from core levels of Hg, Cd and Te both before and after minimum ion bombardment, with bombardment being done to remove the native oxide overlayer. Single scattering cluster (SSC) calculations have been carried out using a spherical wave (SW) model. From comparison between experiment and theory an absolute determination of the surface polarity is possible.

LaF₃ layers ranging in thickness from submonolayer to several monolayers were deposited onto room temperature Si(111)-7 × 7 clean surfaces, and subsequently annealed at temperatures from 400°C to 750°C to promote ordering. The overlayers and the detailed interface reactions were probed with high resolution XPS in our home laboratory, and by soft XPS using the synchrotron source SRS at Daresbury laboratory, both systems used in conjunction with LEED. The experiments show that though high quality epitaxial layers may be grown, the interfaces, nevertheless, are complex with a preference for the formation of La-Si bonds. Upon further annealing to temperatures in the range 800°C to 1200°C, strong chemical reactions were observed leading to the total desorption of fluorine and the formation of lanthanum silicide. This was confirmed by detailed studies of the interactions of La metal with Si under similar annealing conditions, and by LEED. Fluorine could also be removed from the epilayers by exposure to high intensity synchrotron radiation, leading to the possibility of exploitation in the fabrication of 3D device structures.

Finally a value of 7.9 ± 0.1 eV was measured for the valence band offset of the LaF₃/Si(111)-7 × 7 system, compared with 7.75 eV predicted by the Anderson model when used in conjunction with Nethercote's equation for the photoelectric threshold.

The Cu (Ni) 3d and O 2p electronic structure is obtained via soft X-ray emission spectroscopy for La₂CuO₄, La_1.85Sr_0.15CuO₄, YBa₂Cu₃O₇, Bi₂Sr₂ CaCu₂O₈, La₂NiO₄, Nd₂NiO₄, and the simple oxides CuO and NiO. The information is discussed in relation to photoelectron spectroscopy data and densities of states calculated within the framework of the local density approximation. It is shown that the trends observed experimentally for the O 2p band are the same as predicted by theory. The Cu (Ni) 3d band dispersion depends significantly on the compound, but as expected is not well described by the ground state band structure. A qualitative explanation is given.

Measurements of X-ray photoemission from magnesium hydride MgH₂ were made in an effort to further the fundamental understanding of the electronic structure of this metal hydride. The polycrystalline MgH₂ was compressed onto a metal holder to provide a smooth solid surface in a dry nitrogen box and then transferred into a ESCA system without exposure to air. Measurements were made immediately after the surface was scrapped in the vacuum. The binding energies of the photoelectrons from Mg 2s and 2p states were found to be 88.9 ± 0.2eV, and 50.1 ± 0.2eV, respectively. The valence band spectrum shows an effective base width of approximately 8.8eV, which is in reasonable agreement with the recent band structure calculation for MgH₂. The photoelectron spectra also display features that can be identified as volume plasmon energy loss of 14.6 ± 0.2eV. The data can be shown to be useful in calculating the average band gap energy (5.80eV) for MgH₂.

The adsorption of Cs and H on n- and p-InP(110) surfaces at 170 K were studied by using a Kelvin probe (CPD) and ultraviolet photoemission spectroscopy (UPS). For p-InP it was found that both Cs and H induce surface Fermi-level pinning at about 1.3 eV above the valence-band maximum for low coverages and low exposures, respectively, while for n-InP the Fermi-level position within the band gap does not change as a function of coverage in both cases. This means that Cs as well as H are inducing surface states of donor type. These absorbates exhibit one s-valence-electron and the formation of adatom-induced surface donors may be understood in the surface-molecule picture. This model also suggests that both cesium and hydrogen are preferentially bound to surface indium atoms. The present study also shows that CPD combined with UPS is a very useful tool to measure surface band-bending.

We present the Photon Stimulated Desorption (PSD) of H⁺ ions to monitor the catalytic oxidation of Cs/Si(100) 2 × 1. Both the bonding site selectivity and the sensitivity to the free surface charge of PSD are used. The presence of 1 Cs monolayer decreases H⁺ yield from Si sites by one order of magnitude and results in new H⁺ desorption of comparable intensity originating at the Cs sites indicating the formation of Cs-H bonds. After exposure to O₂, the H⁺ yield increases by one order of magnitude from both Cs and Si sites due to the decreased reneutralization of H⁺ as a result of the charge transfer from the substrate to oxygen sites. The H⁺ desorption from both sites is interpreted in terms of the core level excitations (the Knotek-Feibelman mechanism). The H⁺ desorption from Cs sites in this work is particularly interesting in view of our previous results indicating that Cs overlayer on a transition metal surface results also in desorption of negative hydrogen ions H⁻.

The electronic nature of transition metal oxides and their role in changing the electrical conductivity of glasses has been investigated by X-ray photoelectron and X-ray excited Auger Spectroscopy. Evidence for the presence of copper in the Cu⁺ oxidation state in copper-sodium-germanate glasses has been obtained from the "shake up" satellite structure of the Cu 2p line. The observed Cu LVV Auger line shapes are significantly different in glasses and this indicates a different bonding environment of Cu in the glass matrix. The reduction of electrical conductivity with the increase of copper contents has been explained in terms of cluster formation (phase separation) and to some extent due to the bonding of copper atoms with glass matrix. Similarly, we have found that in nickel doped vanadium phosphate glasses, nickel may exist in two different phases, viz. nickel aluminate and nickel oxide.

Angle resolved synchrotron radiation photoemission spectroscopy (ARPES) was used to determine the valence band structure of CdTe(110). CdTe surfaces of high structural quality were obtained by homoepitaxial MBE. The experimental valence band dispersion along the ΓKX direction was determined by using a direct transition model to free-electron-like final state bands. The experimental band structure agrees well with pseudopotential calculations if the final state band is lowered by a muffin tin potential of E₀ = 5.7 eV. A photoemission structure ~ 1 eV below the Fermi level which was previously assigned to surface states is related to the emission from the top of the valence band at Γ₈. No evidence was found for surface state emission at normal as well as off-normal emission at high symmetry points of the surface Brillouin zone.

Multiple-scattering effects in Auger electron diffraction and photoelectron diffraction are assessed using a new spherical-wave multiple scattering formalism which is based on a separable approximation to the scattering Green's function. Results obtained in both single-scattering and fully-converged multiple-scattering limits are presented for linear chains of Cu atoms. Simulations of some Auger electron diffraction and photoelectron diffraction experimental results with this new theory are also presented for the first time.

Xe-adsorbates on Pd(111) in the coverage region from one layer up to a Xe(111) crystal with first layer Xe-Xe distances comparable to bulk Xe are studied by spin-resolved photoemission. Normally incident circularly polarized synchrotron radiation from BESSY (Berlin) is used and normally emitted electrons are analyzed. No dispersion with photon energy can be found for a system of two Xe-layers. The spin-polarization component measured along the direction of light points to a changing symmetry of the final states. Their symmetry is atom-like for a one-layer system and bulk-like for two and more layers. The initial state symmetry is examined by excitation and decay of excitonic states and is found to be the same for two Xe-layers as for one single Xe-layer. The degeneracy of the valence levels connected with the high symmetry of the Γ-point in bulk Xe can be observed for a system of much more than 5 Xe-layers thickness.

The surface local structure of (√7 × √7)R19.1° S/Cu(111) was studied by the polarization-dependent surface EXAFS method. The results clearly indicate the existence of two or more types of copper coordinations around sulfur atoms and they are incompatible with the previous models proposed by LEED experiments. In order to explain the surface EXAFS results, a new model is proposed, where the Cu(111) surface layer is drastically reconstructed with both lateral and vertical movements, and the sulfur atoms are located at three types of 3-fold hollow sites of the reconstructed surface.

Desorption of neutral rare gas atoms following selective excitonic excitation of rare gas solids (Ne, Ar, Kr, Xe) by VUV photons underlines the crucial role of excitons concerning (i) transport of excitation energy from the bulk to the surface, and (ii) the microscopic desorption mechanism itself. All rare gas solids yield desorption of ground state atoms which stem from relaxation of molecular-type excitons after their radiative decay. In addition, solid Ne and Ar yield desorption of ³P₁, ¹P₁, and metastable (³P₂, ³P₀) atoms. Total desorption yields as well as partial yields of excited atoms will be presented. The dominating role of primary excitation of surface excitons is obvious for excited atoms. Primary creation of free electron-hole pairs favours desorption of ground state atoms. The kinetic energy distributions of metastable Ne and Ar atoms were determined by a time-of-flight technique. Maxima were found at ≅ 0.2 eV (Ne) and 0.04 eV (Ar) which establish a new desorption mechanism of excitonic origin.

The nonselfconsistent layer KKR method was used to calculate both electron reflectivities up to 12 eV above the vacuum level and angle resolved photoemission for hydrogen on Pd(111) modeled by an ordered adlayer. Spectral features sensitive to the hydrogen adsorption are related to the band structure in the particular direction in the reciprocal space in both the occupied and empty state regions. Special attention was dedicated to the relation between the input potentials and the structural parameters. In particular we have confirmed the existence of the surface resonance near the point in the surface Brillouin zone with the specific dispersion related to H coverage.

The boron Kα X-ray emission spectra from a series of borates and other compounds has been measured. The main peak profile is determined by the participation of B 2p orbitals in different molecular orbitals and thus reflects the electronic structure in the neighbourhood of the boron atom. Low energy satellites are observed when boron is in combination with other first row elements: the energy separation of the satellite from the main peak can be used to identify the ligand atom. A high energy satellite peak (at the B K absorption edge) is also sometimes observed. The presence of this resonance peak can be correlated with trigonal coordination about the boron.

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

We present partial density of states measurements for the occupied electronic levels around oxygen, strontium and bismuth in Bi₂Sr₂CaCu₃O₈. The data were obtained using soft X-ray emission. We find that the oxygen 2p density of states peaks 4.7eV below E_F and is roughly symmetric in appearance. We also observe a p-type density of states around the strontium sites centered near 5.3eV below E_F. States of d-type symmetry are observed around the bismuth near the binding energy of the bismuth 6s level. We compare the measurements with calculations that have been presented in the literature and with published photoemission results.

The electronic structure of semimagnetic semiconductor ternary alloys Cd_1-xM_xSe (M = Mn, Fe, Co) crystals was investigated by photoemission. The ground state of impurity level of Fe 3d⁶ and Co 3d⁷ were found to determine the position of the edge of the valence band of the ternary alloys crystals Cd_1-xFe_xSe and Cd_1-xCo_xSe. The core level Se 3d spectra measured for Cd_1-xFe_xSe crystals with different composition x correspond to the appearance of local structure in the crystal. The valence band density of states distribution was estimated for MnSe, FeSe and CoSe with CdSe structure as a difference between valence band density of states distribution for Cd_1-xMn_xSe, Cd_1-xFe_xSe or Cd_1-xCo_xSe respectively and CdSe.

In an attempt to assess the relative importance of recent improvements in the theoretical description of X-ray photoelectron diffraction (XPD) data we consider an extensive set of Ni 2p_3/2 azimuthal measurements from a clean Ni(001) surface, taken at a very high angular resolution of approximately ±1.0°. In particular, we discuss the sensitivity of the theory-experiment agreement to the variation of the following parameters: cluster size, electron inelastic attenuation length, substrate inner potential, photon polarization, angular broadening due to the finite acceptance of the electron analyzer, and the absolute accuracy in the experimental emission directions relative to the crystal axes. It is then demonstrated how different levels of scattering cluster theory, including single scattering (SS) of plane p-waves, SS of spherical p-waves, SS of spherical waves with proper angular momentum final states, and finally multiple scattering (MS) of spherical waves, affect the calculated XPD curves. Experiment and theory are compared by means of R-factors, and we are thus able to assess the reliability of structural information that one can expect at different levels of theory, thereby minimizing computational complexity.

The changes of work function of GaAs(110) after various chlorine exposures are obtained from UPS. It is shown that when more Cl was adsorbed, the work function increased which may be caused by the changes of the ionisation energy.

The behaviour of chlorine adsorption at room temperature was investigated by means of UPS and XPS on Co₆₅ Ni₃₅ (111) and (110) surfaces. The work function shows a linear steep increase at the initial adsorption and after reaching a plateau it decreases slowly. The kinetic curves of the adsorption are somewhat different for the two different surfaces. No obvious chemical reactions were observed on the surface within the range of our chlorine exposure.

Thin cerium overlayers, 3-10 Å, on aluminum are found to enhance dramatically the oxidation of the substrate at room temperature. Two logarithmic oxidation regimes are observed in the growth curve, one is assigned to the formation of a thin layer of a mixed Ce-Al oxide, and the other to the growth of bulk Al oxide.

The Au/P3MT/Al sandwich structure has been studied by AES and XPS as well as I-V and C-V analyses. It is shown by experimental results that the properties of a conducting polymer are obviously different from that of conventional semiconductors. The characteristics of the conducting polymer/metal interface are remarkably dependent on the work functions of the materials. The density distribution of ionized dopants at the surface of a conducting polymer will change as it makes contact with a metal. The time constant of transient characteristics of conducting polymer/metal contacts is much longer than that of conventional semiconductor/metal contacts.

High resolution 4f photoemission spectra from clean W(110) show that the natural lifetime width and the singularity index, which characterizes the conduction electron screening, are both larger in the first atomic layer than in the bulk. The phonon broadening of the surface and bulk components are smaller than earlier theoretical estimates. The excess broadening at the surface is compatible with a simple Debye model. These findings are very different from the interpretation previously given surface-atom core-level line shapes and have implications extending to other systems.

In order to investigate the modification of excitation and decay of core holes by condensation and adsorption in the simplest possible cases, as well as the coupling to atomic motion, we have studied the Ar2p and Kr3d excitation regions in Ar and Kr mono- and multilayers on Ru(001). Using synchrotron radiation from BESSY (Berlin), total and Auger electron yields and yields of desorbing ions and neutrals, as function of photon energy, as well as decay electron spectra for specific primary excitations have been measured. The main results are:

Multilayers: Energies for resonant core excitations are shifted to higher values by 0.6 to 1 eV compared to free atoms; for Kr, surface (smaller shifts) and bulk excitations can be distinguished. Autoionization and normal Auger spectra are clearly different, as in the gas phase. Besides desorption of neutral atoms as in the valence region, singly and doubly charged ions as well as ionic clusters are desorbed.

Monolayers: The shift of resonant excitation energies relative to the gas phase is smaller than for condensed layers. Decay spectra for resonant and non-resonant excitations are identical, as for strongly coupled chemisorbates, proving that charge exchange with the metal is fast compared to core decay. As in the valence region, only neutral atoms desorb.

The results shed light on the screening and charge transfer behaviour and on the mechanisms of stimulated desorption which are operative in them.

The time resolved spectra of intrinsic luminescence at 300 nm and 220 nm in BaF₂ crystals following F_2p⁻ valence electron and Ba_5p⁺⁺ core electron excitations have been studied. Based on the results the luminescence process related to starting and intensity increase of the emission at 300 nm after stopping excitation has been discussed.

Results of photon-stimulated desorption (PSD) measurements of H⁺ from solid Si(CH₃)₄ in the photon energy range 15-40 eV are presented. The H⁺ PSD spectrum has a weak threshold at 17(±0.2) eV and a stronger threshold at 21.5(±0.5) eV. The H⁺ desorption threshold at 17 eV is interpreted in terms of excitation of an inner valence 4a₁ (Si 3s/C 2s) electron to an unoccupied orbital (u). This excited state decays to 1t₁⁻²u or 5t₂⁻²u final states resulting in C-H and Si-C bond breaking, respectively. The desorption threshold at 21.5 eV is due to excitation of an inner valence 3t₂ (C 2s) electron to an unoccupied orbital, which decays to a 1t₁⁻²u final state, resulting in C-H bond breaking.

For submonolayer (⩽0.2 ML) coverage of N₂ on Fe(111), photoabsorption measurements (NEXAFS) at the N K-edge show π* (401 eV) and σ* (419 eV) intramolecular resonances. Photoemission spectra confirm that all the N₂ is adsorbed in the γ-state. The angular dependence of the NEXAFS indicates that the N₂ molecules are oriented normal to the surface. All resonances appear at the same energies as in gas phase N₂; in addition Rydberg-like states at 406.5 eV and 410.2 eV are found, which are not quenched by the adsorption. These observations are consistent with a weak N₂-Fe chemisorption bond.

X-ray Si K-emission bands of amorphous SiO_x: H alloy films (0 ⩽ x ⩽ 1.7) are presented. The samples were prepared by rf glow discharge decomposition of SiH₄-CO₂-H₂ gas mixtures in a capacitive reactor system. For x < 0.5 the Si K-emission bands are very similar in shape and peak position to that of a-Si: H, for 0.5 < x < 1.7, however, a dramatic reshaping of the Si K-emission bands occurs. For x = 1.7 the spectra resemble that of crystalline SiO₂ (β-crystobalite). The resulting Si K-emission bands are compared with UPS/XPS spectra of a-SiO_x in which, contrary to XES spectra, no impressive changes of spectral features are observed. Our measurements demonstrate that the Si K-emission bands of the system a-SiO_x: H are more sensitive to sample composition than UPS/XPS spectra. The results delineate the nontrivial behaviour of Si p-like electrons evolving from covalent Si-Si to rather ionic Si-O bonds.

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

Adsorption of oxygen and hydrogen on the GaP () surface prepared by ion bombardment and thermal annealing has been studied by XPS, UPS and HREELS. Information about the chemisorption reaction, surface band bending, and surface barrier build-up, etc. is obtained by measuring photoelectrons at different kinetic energies. Early saturation stages of adsorption are studied carefully. It is found that in these early stages oxygen atoms are adsorbed both associatively and dissociatively. Such adsorption is found related to surface defects.

In this paper a Si/Ge (111) heterostructure grown by rapid lamp heating and very low pressure chemical vapor deposition (RLH/VLP-CVD) has been studied. X-ray diffraction and Raman scattering spectra have shown that the epilayer is a single crystal Si film with good structure. The results of XPS and AES analysis indicate that there is a broad transition region composed of Ge_xSi_1−x (0 ⩽ x ⩽ 1) alloy material between the epilayer and the substrate, so that a Si-Ge_xSi_1−x-Ge multilayer heterostructure has been obtained. In the transition region the oxygen concentration is very low, which is perhaps caused by Ge atoms in the layer. In the Si epilayer oxygen atoms are found in the interstitial sites in the Si lattice.

Aqueous agarose gels containing ferrous ions sustain a radiolytic chain reaction, producing Fe⁺³. Longitudinal proton magnetic relaxation rates are increased in proportion to ferric production. The distribution of Fe⁺³, hence the dose, can be revealed as a scanning curve at 304 nm dependent on its extinction, coefficient, permitting visualizing of dose in these gels by a dual wavelength thin layer chromatography scanner. There is a potential for application to radiation therapy treatment planning.

There is considerable current interest in the passivation of single crystal silicon surfaces, particularly with reference to the fabrication of epitaxially grown solid state device structures.

In this paper, we discuss ordered Si(100) and (111) surfaces prepared by chemical etching in solutions of hydrofluoric acid and studied by use of Soft XPS, XPS and LEED. Despite transfer through the atmosphere before being loaded into the UHV of our analysis instruments, the hydrogen terminated surfaces observed are shown to be among the most perfect yet produced by such chemical means. Under optimum conditions, the etch can yield levels of carbon and oxygen impurities of less than 2% of a monolayer and lead to surfaces where the Fermi level is not pinned.

The etched surfaces are modified by the hydrogen termination such that they are electrically passivated and ordered into a (1 × 1) periodicity. Upon annealing at (520 ± 10)°C in UHV, the hydrogen is removed, the surface changes to a (2 × 1) reconstruction and also exhibits band bending due to Fermi level pinning by surface strates.

The relationship between residual contamination and etching parameters such as time and etchant concentration has been examined, together with the stability of the passivated surface under exposure to wet and dry environments. This has shown that the surface condition is sensitive to etching procedure and that the optimum surface may be obtained by using a 5% non-aqueous solution, or by pre-cleaning a surface using a UV generated ozone exposure prior to an aqueous etch. It has also been shown that a controlled reoxidation of an etched surface using UV-ozone can produce a non-abrupt Si-SiO₂ interface. This may form a useful part of a reduced temperature surface preparation, compatible with any subsequent epitaxial overlayer growth.

We have obtained high resolution (~1 arcsec) X-ray and EUV images of the solar corona with two rocket-borne normal incidence Cassegrain multilayer telescopes. The Cassegrain images correspond to material at ~1 000 000 K (λ ~ 173 Å) and ~ 80 000 K (λ ~ 256 Å). Other images, at somewhat lower resolution, were obtained with Herschelian multilayer telescopes and Wolter-Schwarzschild hybrid grazing incidence/multilayer optical systems. We discuss the Cassegrain and Herschelian images, emphasizing the analysis of polar plumes observed at the limb of the Sun. Polar plumes are thought to represent structures that contribute significantly to the flow of coronal material into the solar wind. We will also comment briefly on other coronal features observed, including coronal holes, the chromospheric network, active region loops, and prominence cavities. Multilayer technology represents a major breakthrough in astronomical X-ray and EUV imaging; we will discuss the application of specific multilayer techniques to astronomical observations, emphasizing observations of the sun. We will also report on current efforts in our laboratories to develop a comprehensive Multi-Spectral Solar Telescope Array rocket instrument for the study of a wide variety of problems related to the solar atmosphere.

Corrections to Physica Scripta41 95 (1990)

Unfortunately there were some errors in the printing process. In the references at the page 98,

1. The line of reference number 6 should be deleted.

2. All numbers more than 7 should be subtracted unity and become a series from 6 to 20.