Table of contents

We describe the present capabilities and future plans for soft X-ray optics characterization at the NIST storage ring, SURF II. The existing facility is made up of a monochromator and a reflectometer capable of characterizing the reflectivity of soft X-ray devices, including multilayers, gratings and grazing incidence optics. This system can also be used to characterize the transmission of components such as filters. The present capabilities include a wavelength range from 80 Å to 600 Å. The reflectometer will handle components up to several cm in diameter and its detector can be scanned in two independent angular directions. This allows the mapping of scattered photons from the sample, as well as the measurement of reflectivity as a function of angle and wavelength. Results of measurements on users' samples will be shown.

We plan to modernize and expand the capabilities of this facility in the following ways:

(1) Construct a new monochromator capable of scanning wavelengths from 30 Å to greater than 600 Å.

(2) Construct a new reflectometer with independent, computer-controlled angular and translational motion for the sample and detector. This will permit automated wavelength scans and Θ/2Θ scans at fixed wavelengths, as well as mapping of the sample surface. The reflectometer will have a multidetector capability. It will also allow the measurement of optical constants of thin films.

(3) Construct an optical test bench for characterizing the imaging properties of focusing optics.

Other considerations for expanded capabilities, depending on the needs of the soft X-ray user community, include:

(1) In-situ deposition of thin films with a vacuum transfer capability.

(2) Provisions for other types of characterization, such as scanning tunneling microscopy (STM) and interferometric profilometry.

Free-standing and membrane-supported transmission gratings are very useful for applications in VUV, X-ray, and matter-wave diffraction and spectroscopy. We will describe the techniques used to fabricate deep-submicron-period transmission gratings and review some of the applications for these diffractors. Our fabrication process begins with holographic lithography to define a master grating pattern. Spatial periods as small as 0.2 µm (5000 lines mm⁻¹) are routinely achieved, and a new technique, called achromatic holographic lithography, has achieved a period of 0.1 µm (10 000 lines mm⁻¹). Holography is followed by SiO₂ shadow-evaporation, reactive-ion etching, and gold liftoff or electroplating to transform this pattern into absorbers for C K (45 Å) or Cu L (13 Å) X-rays. This patterning is performed on the membrane of an X-ray mask, which is then printed using X-ray lithography. The X-ray lithography process transfers the master pattern into a thick-resist coated substrate, which is then gold, silver, or nickel electroplated to form the final grating pattern. These structures can be supported on 0.5-1.0 µm-thick polyimide membranes, or made free standing by the support of a metal mesh. Progress in this area has been fueled primarily by the requirements of the Advanced X-ray Astrophysics Facility (AXAF) X-ray telescope, for which thousands of square centimeters of high-quality transmission gratings are required. However, other applications, including X-ray nanolithography development, solar astronomy, X-ray and matter-wave interferometry, and laboratory VUV/X-ray spectroscopy are also driving this technology.

Reflectance degradation of soft X-ray reflection filters of Mo/Si and Rh/Si multilayers fabricated on SiC substrates was studied after exposure to Synchrotron Radiation for 26 and 140 hours, respectively. For both mirrors, darkening was seen at the exposed area, where peak reflectances around 100 eV photon energy were decreased from 45% to 35% in the Mo/Si and from 40% to 9.8% in the Rh/Si mirror. Unexposed areas retained much higher peak reflectances of over 30% in spite of the temperature increase of the substrates to over 110°C. At the exposed area of the Mo/Si mirror, a carbon layer was found and the attenuation factor in peak reflectance vs. photon energy was primarily explained by absorption in this layer.

A large area disc shaped vacuum ultraviolet (VUV) light source, from 2 cm to 20 cm in diameter, has been developed. It employs a soft vacuum electron beam emitted from a cathode of ring shape to excite the disc shaped plasma. The Lyman alpha emission 2p²P⁰-1s²S at the wavelength of 121.6 nm, dominates the emission spectra when running a hydrogen discharge while the N I line 3s⁴P-2p⁴S⁰ at 120.0 nm, dominates in a disc shaped nitrogen discharge. The 121.5 nm 4d²D-2p²P⁰ line of He II is observed to dominate in the helium discharge when measurements are made through a 10-15 cm long column of neutral helium. The intensity of VUV emission has a measured uniformity of ±6% over the central 15 cm of the 20 cm diameter disc using atomic helium, molecular N₂ and H₂. The windowless disc plasma is also a source of radical and excited atomic gas species. VUV photons, excited species, and radicals can all assist dissociation of CVD feedstock reactants via volume photo-absorption and sensitized atom-molecule collisions, respectively. In addition, the excited radical flux and VUV impingement on the film may also assist heterogeneous surface reactions and increase surface mobility of absorbed species. Thin films of aluminum nitride, Si₃N₄, and hydrogenated amorphous silicon have been deposited at temperatures between 100-400°C. The deposited films show significant improvement over other photo-assisted CVD processes in the film quality achieved, the substrate temperature required and the maximum deposition rates achieved.

The recent progress of the construction of light source, beam lines and stations are briefly described as well as the experiments proposed.

Using the dual laser produced plasma technique photoabsorption spectra of core excited autoionizing states in Al²⁺ (160-175 Å) and Si³⁺ (117-128 Å) have been recorded. Lines belonging to the 2p⁶ 3p-2p⁵ 3s3p and 2p⁶ 3d-2p⁵ 3s3d transition arrays have been assigned with the help of MCDF atomic structure calculations and previously classified transitions arising from the 2p⁶ 3s ground state.

Photoion spectra as a function of photon energy have been measured around the Kr 3d and Xe 4d delayed onset regions using a threshold-photoelectron photoion coincidence technique. The spectra of doubly charged ions show structures corresponding to the shake-up states of Kr 3d⁻¹4p⁻¹nl and Xe 4d⁻¹5p⁻¹nl. It is clarified that the spectra of triply charged ions are composed of the shake-off processes and the double Auger process after the Kr 3d and Xe 4d ionization.

The spin-polarisation parameters for photoelectrons from lead atoms (6s²6p²)_{J = 0} were measured in the wavelength range between the second threshold [Pb⁺ (6s²6p) ²P_3/2] and 100 nm in an angle-resolving experiment using circularly polarised vuv-radiation from the storage ring BESSY. Beyond this threshold (λ = 135.3 nm), the photoionisation cross section is strongly perturbed by autoionisation resonances. Especially, a strong enhancement of the generally weak Pb⁺ (6s²6p) ²P_3/2 photoionisation cross section occurs. Theoretical calculations by Radojevic [15] attribute these resonances to the 6s6p³-configuration of lead. By use of the measured spin-polarisation parameters A, ξ and α which show pronounced variations in the region investigated for both final ionic states ²P_1/2 and ²P_3/2 a detailed discussion of the resonances is performed.

Photoelectron spin-polarisation measurements of In(5s²5p) ²P_1/2 were performed in the autoionisation region of the 5s5p²-configuration using monochromatic circularly polarised synchrotron radiation at the electron storage ring BESSY. The autoionising levels are analysed in terms of partial cross section, dipole matrix elements and phase-shift differences. In addition the measured photoelectron spin-polarisation data were used for an independent check for the assignments of the autoionising levels.

A comparative study of multiple photoionization in the 4d ionization region (78-270eV) of M = Xe, Cs, Ba, Sm, Eu and Yb atoms is reported. Partial photion-yield spectra for Mⁿ⁺ (n = 1-4) ions have been measured by using synchrotron radiation as a light source. Main products in Xe, Cs and Ba are Mn²⁺ and M³⁺ ions due to the creation of a 4d vacancy. In Sm and Eu, a large fraction of M⁺ ions are populated via the 4d-4f resonant excitation and subsequent autoionization to the 4f ionization continuum, which is squeezing the population of multiply-charged ions to about 50%. In Yb, a large amount of M⁺ ions are also populated, but are due largely to the direct 4f ionization. In all of the atoms studied, an appreciable or considerable amount of M⁴⁺ ions are observed, indicating the existence of photoelectron and/or Auger shake-off processes.

Photoionization efficiency curves for Bi_n (n ⩽ 4) in their threshold region (7.05 to 8.40 eV) have been measured by using synchrotron radiation, which crosses a molecular beam coupled with a time-of-flight mass spectrometer. Resonance lines due to autoionization are observed only for the monomer. The photoionization spectrum of Bi₂ shows a depletion of intensity, which indicates a predissociation of Bi₂ to Bi⁺ + Bi. Bi₃ and Bi₄ show only slowly varying profiles.

Ionic photofragmentation of SiH₄ has been studied in the vicinity of the Si: L-edge using synchrotron radiation and time-of-flight mass spectrometry. Relative ionization efficiencies for more then 10 ionic fragment species and pair-ionization efficiencies for the pairs H⁺ + SiH_n⁺ (n=0, 1 and 2) have been obtained as a function of the photon energy from 100 to 200 eV from the photoelectron-photion and the photoion-photoion coincidence measurements. Ionic dissociation of the continuum Si: 2p hole states may occur mostly via double ionization of the parent molecule giving rise to two types of dissociation channels; (type A) SiH₄⁺⁺ → H⁺ + H⁺ + neutrals and (type B) SiH₄⁺⁺ → H⁺ + Si⁺ (or SiH⁺) + neutrals. The average kinetic-energy release given to H⁺ is much higher in the dissociation channel B than in the dissociation channel A suggesting temporal existence of localized and delocalized valence-two-hole states.

The general form of the angular distribution of the photoelectrons ejected by linearly polarized radiation is presented which includes the contributions of all multipoles of the ionizing radiation. Explicit results for photon energies less than 3 keV are presented calculated on the basis of the independent particle relativistic Hartree-Slater theory. It is found that for energies as low as 500 eV, ten percent asymmetries can be expected from the multipoles beyond the electric dipole approximation. In general, for photon energies less than 3 keV, three energy-dependent parameters are sufficient to describe the angular distribution. For some ranges of atomic numbers, a few subshells have 5 percent contributions from two additional parameters.

Angle-resolved photoion spectroscopy has been applied to molecular fragments emitted after Auger decay following photoexcitation of the N₂ and O₂K-shell in the gas phase. Prominent anisotropic angular-distributions of the fragment ejection, depending strongly on the photon energies, have been observed in the vicinity of the K-shell ionization thresholds.

Ionic fragmentation of SiH₄ has been studied in the vicinity of the Si: K-edge in the photon energy range 1800-1900eV. Ionic fragments observed at photon energies above the Si: ls → σ* excitation are mostly atomic ions Si^q+ (q = 1-3) and H⁺. Ratios of the abundances for Si⁺, and Si²⁺ and Si³⁺ are roughly 4:4:1, and the abundance of H⁺ is much higher than the sum of the Si⁺, Si²⁺ and Si³⁺ abundances. The averaged kinetic energy release given to H⁺ is estimated to be about 12eV. This complete decomposition of SiH₄ with the ejection of energetic H⁺ is interpreted as Coulomb explosion of the multiply-charged (mainly +4 or +5) parent ion produced via the vacancy cascade (successive Auger and/or Coster-Kronig transitions).

Vacuum ultraviolet (VUV) light radiation was used to produce electronically excited KrF excimers (in D-, B- and C-states) by the photolysis of KrF₂ and F₂/Kr mixtures at various excitation wavelengths. The excited KrF photoproduct quantum yield was measured over the excitation wavelength range of 120 to 200 nm, and a quantum efficiency of 0.11 was estimated at the peak absorption wavelength of 159 nm for KrF₂. The collision-free fluorescence lifetime of the B-X transition near 248 nm was determined to be 9.5 ± 0.6 ns when the KrF₂ was excited with the 159 nm light. Near gas kinetic rate constants were measured for the quenching of KrF B-X emission by KrF₂ and CO₂. Using the threshold wavelength needed for the production of excited KrF photofragments, an upper bound for the bond dissociation energy of KrF₂ was determined to be 1.03 ± 0.05 eV.

The absorption spectrum of atomic Ca has been observed photographically in the wavelength region between 1410-1330 Å with the high-resolution spectrograph at the Photon Factory. Complex structures of the observed spectrum just below the 4p²P_1/2 threshold are found to be well reproduced by the recent ab-initio calculation done by Kim and Greene.

Ionic fragmentation following the photoionization of Sn(CH₃)₄ (TMT) has been studied in the photon energy range of 60-600 eV using synchrotron radiation and time-of-flight mass spectrometry. Each of the Sn: 4d, 4p, 3d and C: 1s photoionization leads to a type of ionic fragmentation that is characteristic of each ionized core. The Sn: 4d photoionization above 60 eV predominantly produces the doubly-charged TMT which dissociates into two singly-charged ions and some neutral fragments. The ions produced in this pathway are CH₃⁺, C₂H₃⁺, C₂H₅⁺, SnCH_m⁺ and/or Sn⁺. The Sn: 4p photoionization produces the triply-charged TMT and enhances the production of H⁺, CH_m'⁺ (m' = 0-3) and Sn⁺ significantly. The Sn: 3d photoionization produces multiply-charged TMT whose charges are 3-5 and enhances the production of H⁺, CH_m'⁺ (m' = 0-2) and Sn⁺ significantly. The C: 1s photoionization produces doubly-charged TMT via the KVV Auger transition and enhances the production of CH₃⁺, C₂H₃⁺, SnCH_m⁺ and/or Sn⁺.

The initial stages of interface formation between In, Sb, Sn, and Ag adsorbates and the Si(100)-(2 × 1) surface have been examined with high resolution core-level photoemission spectroscopy. In each case, the Si 2p surface-shifted core-level component is seen to be converted into a component which is indistinguishable from the bulk component through adsorption. The average number of Si surface dimer atoms modified in the presence of an adatom, which is referred to as the adsorbate-to-Si bonding coordination number (BCN), is obtained for various coverages. The relative homogeneity of the adsorbate site bonding is evaluated by examining the line shapes of the adsorbate core-level spectra. The In-to-Si BCN is 3 for very low coverages and decreases to 2 for 1/2-monolayer coverage. The results are consistent with an In/Si(100)-(2 × 2) structural model which involves sp² hybrid bonding of In dimers. The Sn 4d core levels reveal the existence of two different Sn bonding sites which form sp³ and s²p² hybrid bonding configurations; a structural model for the Sn/Si(100)-c(4 × 4) is presented. The Ag-to-Si BCN measurement indicates that adsorbed Ag forms linear sp hybrid bonds between two neighboring Si dimers along the direction of dimerization at low coverages. Sb chemisorption is found to yield a BCN behavior similar to that of In.

The ultraviolet photoelectron spectroscopy (UPS) method was used in an investigation of the processes of silver film growth with thicknesses in the range θ = 0-10 monolayers (ML) at temperatures of 10 and 300 K on the (110) faces of A³B⁵ cleaved in high vacuum. A study was also made of the influence of heating the sample to room temperature after evaporation of Ag at 10 K. For Ag/GaAs and Ag/InAs the chemical interaction between deposited atoms and substrate is suppressed at LT. In contrast, for InP the interaction weakly depends on temperature. The Ag 4d peak obtained at LT from Ag/InSb is probably composed of two components corresponding to quasiamorphous and b.c.c. Ag.

The (1 1 1) face of Ag has been studied by angle-resolved photoemission spectroscopy utilizing synchrotron radiation as the excitation source (25 ⩽ hv ⩽ 50eV). The overlapping Ag 4d bands were deconvolved by the modified FIRO method. The peak positions thus determined are used to map the dispersion curves along the ⟨1 1 1⟩ (Γ−L) direction. The results show general agreement with calculated band structure, so far as the energy levels and symmetries are concerned. However, it is found that the density of state effect is dominant in the spectra obtained in the present photon energy region. The emission from the Ag 5s, p bands is observed to be broadened due to the indirect transition process.

Optical absorption spectra of quantum well excitons in KCl-KBr and RbCl-KBr alternating multilayer structures are studied. The absorption bands of excitons confined in the well exhibit blue shifts and steeper rise as compared with the bulk exciton band. These spectral characters are explained in terms of the quantum confinements. A sinusoidal potential profile of the multilayer is proposed to explain the interface-mixture effect and is confirmed by the observed spectra. A simple model of the sinusoidal well is also proposed.

The N_4,5(4d) spectra of the atoms of Rare-Earth elements in metals and oxides have been examined under monoenergetic electron beam-excitation with energies varying in the region containing 4d thresholds. The full pattern of the radiative decay of the states of Rare-Earth atoms with a vacancy in the 4d shell have been obtained. Besides, the process of radiative electron capture into free 4f states (inverse photoemission, IPE) has been studied. Attention has been paid to the results of the competition between the capture and 4d shell excitation. The results obtained have been used for the analysis of the structure of 4d appearance potential spectra.

Ultraviolet photoelectron spectroscopy (UPS) has been applied to the investigation of the electronic structure of oligothiophenes with 4-8 thiophene ring. In a series of α-linked oligomers (α_n, with the number of rings n = 4-8), a systematic evolution of the π band forming levels are observed in the region of 0.7-3eV below the Fermi level (E_F) and the bandwidth becomes broader with n. The non-bonding π band is observed at 3.5eV below E_F and its energy is almost independent of n. UPS spectra of α₇ and α₈ are fairly similar to the spectra of polythiophene, showing that these oligomers are good model compounds of the polymer. The effect of irregularity on the π-electron system was also studied by using oligomers which contain a β linkage or a vinylene group in the middle of the molecule. The UPS spectra showed that the β linkages significantly affect the electronic structure of polythiophene, but vinylene does not. In order to analyze the UPS spectra and to investigate the electronic structures of oligomers, the orbital energies and the geometry of these oligomers are calculated by the semi-empirical modified neglect of diatomic overlap self-consistent field molecular orbital (MNDO-SCF-MO) method. The calculated spectra using the obtained orbital energies agree well with the observed ones, particularly in the π region. It is shown from the optimized geometry that (1) α_n's have planar structures and π electrons can be delocalized, (2) the oligomer with β-linkages have nonplanar structure leading to limited delocalization of π-electrons, and (3) the oligomers with a vinylene group are almost planar and the disturbance by the vinylene group on the delocalization is small.

Synchrotron radiation measurements of near-threshold and broad-range (400-1500eV) absolute photoabsorption cross sections were made for five transition metals with ±10% overall uncertainties. Fine structure details of 2p-3d autoionising resonances are shown with better than 1.0eV resolution for solid metals: Ti, V, Cr, Fe, Ni, and Cu. Fine structure similar to what we measured can be produced using a multi-configuration Dirac-Fock (MCDF) model if a statistical distribution is assumed for the initial atomic states. Calculations were performed in intermediate coupling with configuration interactions by Mau H. Chen. The results are compared with other experimental work and theoretical methodologies.

La or Ce 3d- and 4d-core photoemission spectra are discussed for La and Ce compounds on the basis of the impurity Anderson model taking into account the atomic multiplets due to electron-electron interaction. A consistent explanation of 3d and 4d XPS of La and Ce compounds is presented. We also give an interpretation of the multiplet structures in the spectra. By transforming the Hamiltonian, we show that conditions that promote the "quenching" of the multiplet structure in the solids are small electron-electron interaction, large hybridization between the localized orbit and the valence band, and small energy difference between different fⁿ configurations in the final state.

The combination of single-bunched light pulses (duration: 0.5 ns, interval: 177.6 ns) from UVSOR (Okazaki, Japan) with a method of time-correlated single-photon counting was applied to study decay behaviors of intrinsic luminescence in nine typical alkali halide crystals (Na, K, Rb) × (Cl, Br, I). Under excitation into the band-to-band transition, two emission bands, so far called σ emission (fluorescence with a short lifetime) and π emission (phosphorescence with a much longer lifetime), were observed in general at LHeT. The decay behaviors observed were essentially the same as those reported in previous studies where pulsed particles were used as an excitation source. A novel situation has, however, been recognized in the π emission in both NaBr and NaI: A fluorescent component was found clearly to coexist with the long-lived phosphorescent component. This suggests that the initial state of the π emission in NaBr and NaI is a singlet-triplet pair slightly split by the exchange interaction.

3d → 4f resonant photoemission (PE) data are presented for Sm and Gd metal as well as for the intermetallic compounds CeAl₂, EuPd₃, TmAl₂, and YbAl₂. The observed effects are generally similar as in the corresponding 4d → 4f resonances, they are, however, considerably different in magnitude. For Tm³⁺ and Yb³⁺, enhancement factors of 40 and 48, respectively, are derived for the 4f PE cross-section. In case of Gd³⁺, quintet states are observed that resonate at the M₅ threshold through a separate spin-flip channel. Differences between constant-initial-state and total-electron-yield spectra of Tm, Eu, and Sm reveal that resonant decay channels involving 4f electrons are blocked at certain excitation energies.

We present new XPS, UPS and XAES results of dispersed Ti on Pd and Pd on Ti at room temperature. Both metal evaporation and photoemission experiments were performed in a UHV system at background pressure lower then 10⁻¹⁰ Torr. The concentration of deposited metal for both Pd and Ti varies from 1 × 10¹⁴ atom cm⁻² up to 3 × 10¹⁶ atom cm⁻². Within this range of concentrations Pd 3d core lines shift by 1.2eV towards higher binding energy monotonously with decreasing cluster size; the width of Pd core lines remains constant within the accuracy of the binding energy measurements, i.e. ±0.1eV. No shift of core levels of Ti substrate and no significant change of their widths were observed. On the other hand, in the case of Ti evaporation onto a Pd sheet the substrate 3d core levels split into two lines; one remaining at the Pd clean substrate position and the other is shifted by 1.2eV for all concentrations of deposited Ti. The Ti 2p binding energy of the dispersed Ti increases monotonously from the value corresponding to the highest coverage to the one for the lowest coverage by 1.3eV. We conclude that evaporation of Ti on a Pd surface results in the formation of bimetallic islands, which is in contrast with the results of Pd dispersed on a Ti surface, where weak bonds between Pd and Ti atoms in the interface were interpreted.

We have studied the photochemistry of NO and NO₂ on Ni(100) using 193 nm light from an excimer laser. The experiment is complete in the sense that we characterize the desorbing particles by their rotationally and vibrationally resolved time of flight spectra via LIF (Laser Induced Fluorescence)- and REMPI (Resonance Enhanced Multi Photon Ionization)- techniques in the gas phase, and we characterize the solid surface before and after irradiation by electron spectroscopic methods, i.e., AES, LEED, and XPS.

We find the build up of NiO after irradiation of the molecular adsorbates. The structure of the oxide is characterized by LEED. The electronic and geometric structure of NO and NO₂ adsorbed on NiO is studied using angle resolved photoelectron spectroscopy (ARUPS), electron energy loss spectroscopy (HREELS) and near edge x-ray absorption fine structure (NEXAFS) measurements and the results are compared with those for NO and NO₂ on clean Ni(100). For the ARUPS and NEXAFS measurements we have used synchrotron radiation from the storage ring BESSY I in Berlin.

As expected, on NiO the desorption process has a much higher cross section than on the clean metal surface. A "photodesorption" channel of NO desorbing from NiO is clearly identified by the rotationally resolved time-of-flight spectra. In addition to the photodesorption channel a "thermal" channel is observed. The influence of the change of the adsorbate's geometric and electronic structure on desorption will be discussed.

Photoconductivity threshold energy and photoionization quantum yield of xenon supercritical fluids doped with TMAE (tetrakis-dimethylaminoethylene) were measured as a function of xenon density at 300 K. Density dependence of the photoionization threshold energy was successfully described with the continuum model where the fluid is treated as a uniform continuum.

Variation with X-ray excitation energies of spectral shapes and intensities of autoionization electron emission coexisting with the M_2,3-VV Auger line is discussed theoretically. Autoionization spectra are obtained as a one-step process of X-ray scattering, while M_2,3-VV spectra are obtained as a two-step process of a resonant electron emission following the X-ray absorption. The present theoretical result agrees qualitatively with the experimental result of Brener et al. [Phys. Rev. B37, 1387 (1987)].

The threshold exponent α₀ of anomalous soft X-ray absorption spectra of metallic Cs in the theory of Nozières and De Dominicis is calculated. The value of α₀ for Cs is obtained experimentally as 0.06 or 0.07 which is considerably smaller than those for K and Rb and could not have been explained even qualitatively by predecessors. By a proposed model the localized 4f-orbital is pulled in by the 5p-hole that is created by absorption of a soft X-ray, resulting in the screening of the 5p-hole. From this model we obtained the value of α₀ = 0.05, in agreement qualitatively with the experimental results. For K and Rb, the obtained values for α₀ in the present study are 0.27 and 0.25, respectively, in satisfactory agreement with the experimental results.

Polarization dependent XANES spectra were obtained by use of synchrotron radiation for oriented polyethylene and fluorinated polyethylenes: (1) elongated polyethylene (PE) film, (2) elongated poly(vinylidene fluoride) (PVDF) film, (3) poly(tetrafluoroethylene) (PTFE) film, (4) oriented evaporated film of hexatriacontane. CH₃(CH₂)₃₄CH₃ as a model compound of PE, (5) oriented evaporated film of perfluoroeicosane, CF₃(CF₂)₁₈CF₃ as a model compound of PTFF.

Pronounced polarization dependence was observed for each compound, which allows unambiguous assignments of the XANES spectrum. Fluorination effects on the XANES spectra was also discussed

In this paper the optical constants of anodic oxides grown electrochemically on the surface of GaSb, GaAs and GaP substrates were studied in the VUV region by means of reflectivity vs. angle of incidence measurements up to 14eV. The degree of polarization of the radiation from a normal incidence monochromator, a requisite parameter for precise evaluation of the optical constants, was determined from the measurements of reflectivity at two perpendicular planes using Abelès relation [1]. The optical constants we have obtained for GaSb, GaAs and GaP oxides are quite consistent with ellipsometric data by Aspnes et al. [2, 3]. The main observed maxima of ε₂(ω) for the three oxides are shifted to higher energies with decreasing anion atomic number. The larger gap for GaP oxide may be related to the + 5 oxidation state of phosphorus. Our data may be useful in verifying the models for amorphous complex systems.

An analytical expression for the X-ray absorption cross section above the edge in a polyatomic system has been proposed. Features have been revealed near the edge in X-ray absorption fine structure connected with the effect of the electron-optical properties of the surroundings on the excited atom.

The formation mechanism of near-edge-structure features in X-ray absorption spectra of first-row polyatomic systems is discussed. It was found that the molecular π and σ shape resonances in spectra of these systems can be treated as a result of the atomic 1s-2p resonance splitting by a molecular field. This conclusion is in good agreement with data of oscillatior strength measurements for the shape resonances in N₂. It was shown that the energy interval ΔE(π - σ) can be used for determination of the interatomic distance in linear and planar first-row anions in crystals.

The electronic structure of CdTe(110) and the substitutionally disordered alloy Cd_0.65Mn_0.35Te(110) has been investigated with angle-resolved photo electron spectroscopy using synchrotron radiation up to 78 eV and 56 eV photon energy respectively. The changes of the CdTe band structure on alloying were analyzed. The contribution from the Mn 3d states was identified and its dispersion was studied.

The electronic structure of poly(tetrafluoroethylene) (PTFE) was studied by UPS, VUV absorption, and ab-initio MO calculations. The UPS spectra give a photoemission threshold energy of 10.6eV, with deeper valence band features consistent with the reported XPS and the oligomer vapour UPS spectra. The UPS spectra are also consistent with the density of states derived from the calculated band structure, which indicates that the uppermost part of the valence band is formed from the C and F 2p orbitals with C-C bonding and C-F antibonding combinations. The VUV absorption spectrum shows an intense peak at 7.7eV, which most probably corresponds to the valence excitation from the top of the valence band to the bottom of the conduction band. With these data, the structure of the occupied and vacant states are deduced.

The change of electronic structure at UV photopolymerization was studied by XANES and UPS for evaporated films of a long-chain diacetylene compound tricosa-10,12-diynoic acid. The UPS spectra showed a large lowering of ionization threshold energy (from 7.0eV to ca. 5.2eV) due to the formation of a conjugated π electron system, as in our previous study of Langmuir-Blodgett films. The XANES spectra showed that the alkyl chains are inclined to the substrate surface, consistent with X-ray diffraction studies. The Cls → π* absorption showed a drastic change on photopolymerization, which could be well explained with CNDO/S molecular orbital calculations taking account of the hole formation using the equivalent-core approximation. From XANES studies, no essential discontinuity was found between the blue and red forms of the polymers formed.

The local electronic structure of NO adsorbed on Ni(111) at 120 K has been studied down to low coverages by angle resolved Auger electron spectroscopy after resonant excitation to the bound π-resonance. The local electronic structure is independent of coverage in the range 0.05 ⩽ θ_NO ⩽ 0.5 ML; the orientation of the NO molecules is always perpendicular to the surface. The small value for U_eff = 0-2 eV for the various Auger final states indicates weak Coulomb interaction due to screening by the metal. Comparison of the Auger spectra after resonant excitation and excitation in the sudden limit shows that the initial state for the Auger decay is always the same well-screened state, independent of the primary excitation.

Angle-resolved photoemission spectra were measured using synchrotron radiation for oriented thin films of hexatriacontane, CH₃(CH₂)₃₄CH₃, and pentatriacontan-18-one, CH₃(CH₂)₁₆CO(CH₂)₁₆CH₃, with their long axes perpendicular to the substrate surface. The valence band dispersion was observed along the long molecular axis over the whole Brillouin zone for both compounds. The results demonstrate that the chemical disorder caused by the C=O group in the CH₂ chain does not affect the band structure so much and both compounds show valence band dispersion similar to the dispersion in an ideal polyethylene chain. Further, we observed a band which was not observed in our previous work. It determines the top of the valence band at the Γ point. Comparison with theoretical results indicates that it corresponds to a band of B₁ (B_2g at Γ) symmetry which consists of only C2p atomic orbitals directed along the carbon chain.

We have investigated electron energy loss features for several detection modes from both theoretical and experimental studeis. It is found from the theoretical study that in backward reflection large oscillations due to interference of primary electrons makes it difficult to extract EXAFS-like signals, and that in the forward reflection mode we have no problem about it. We have actually carried out the EELFS measurements for NiO and SiO₂ surfaces in the latter mode, which gives the nearest neighbor distances slightly shorter than those in the corresponding bulks. This reflects the high surface sensitivity of this detection mode.

Carbon K-edge X-ray absorption near edge structure (XANES) spectra of evaporated films of poly-p-phenylenes and polyacenes were measured by use of synchrotron radiation. Only one dominant resonance associated with a Cls-to-π* transition was observed for each poly-p-phenylene, although there should exist several π* unoccupied orbitals which are energetically well separated from each other. These features were characterized by means of semi-empirical molecular orbital (MO) calculations based on the equivalent core approximation, which leads to a satisfactory agreement between the observed and calculated spectra. The calculations show that the π* orbitals, which are delocalized in the ground state, become localized by the creation of a Cls core hole. Spectral features of polyacenes, which are somewhat different from those of poly-p-phenylenes in their π* regions, indicate a lesser degree of π* localization.

A recent investigation [1] indicated generally good agreement with the lifetimes reported in our experiment, with the notable exception of the P III 1 380 Å 3p²²D-3p³²D transition. This prompted us to reexamine our data and revealed that a copying error had indeed occurred in our manuscript, resulting in a wholly spurious value being reported for this one transition. The meanlife extracted from our original (single exponential) decay curve was actually in exact agreement with the value reported in ref. [1]. Thus, the ninth row, fourth column of Table II on page 200 should read 10 ±1 ns (not 1.8 ±0.4 ns). We are very grateful to Drs Livingston, Kernahan, Irwin and Pinnington for pointing out this unfortunate error.

[1] A E Livingston, J A Kernahan, D J G Irwin and E H Pinnington Physica Scripta12, 233 (1975)

The following typographical errors appeared in this paper: the numerator of equation (27) should not contain the factor 2; the denominator of equation (32) must be multiplied by the spin S; the expression for T₂^-1 (Korr) appearing in the text near the ened of Section 4,3 must be multiplied by the quantity k_BT; as labelled, the correct units for the ordinate in Fig. 5 are 0.1 s^-1.