An overview of the current state of knowledge of atomic photoabsorption and photoionization cross sections is presented. At high photon energies (<10 keV) the cross sections are generally well-understood, except near thresholds. At lower energies the situation becomes dramatically poorer. A prospectus for amelioration of the problems is discussed.

A program to obtain reliable photoabsorption cross sections for a broad range of elements has been initiated at Lawrence Livermore National Laboratory. Using well-characterized samples, absolute measurements have been made with overall uncertainties of ±10% or less over energy ranges from about 50 eV to several keV, at several synchrotron radiation facilities. More than a dozen elements have been investigated and other measurements are being planned for elements beryllium (Z=4) through uranium (Z=92). Among the interesting phenomena observed are the resonances/enhancements which seem to accompany virtually every inner shell threshold. Specific examples discussed in this paper include carbon, nickel and uranium. Comparison with theory and existing data compilations/data bases indicate the need for substantial theoretical and compilation improvements.

The photoabsorption spectra of the carbon ions have been obtained in the XUV spectral range, by using two laser produced plasmas.The spectra due to transitions both of the optical and of the inner electrons have been observed. The photoionization cross-section of the CV ion has been absolutely measured.

We report here new high-resolution vibrationally resolved studies of photoionization in the molecules CO and CO₂ using synchrotron radiation in the photon-energy range 10-30 eV. The detailed decay dynamics of autoionizing resonances in CO and the origin of symmetry-forbidden vibrational transitions in the C state of CO2+ are discussed. These examples illustrate the need for experimental work with both high optical resolution and electron resolution sufficient to resolve the complex resonances in molecules and the vibrational dependence of molecular dynamics.

The photoionization cross section is partitioned into its components by analyzing the emitted photoelectrons according to energy and intensity. The principle of the method is outlined and experimental results on neon, krypton, manganese and beryllium are discussed.

Fluorescence spectra resulting from extreme ultraviolet photon excitation of a number of molecular systems have been obtained. The final states produced have been determined from the dispersed fluorescence so that the species and the vibronic levels populated could be quantitatively determined. From these data, partial cross sections for the production of excited photofragments of molecules have been obtained. In addition, the kinetic energy distribution of excited atomic photofragments following dissociative ionization excitation of molecular nitrogen has been obtained by using the fluorescence photon-photoion coincidence technique. A discussion of the various multichannel processes and their significance, together with appropriate data are presented.

The photoabsorption and fluorescence cross sections of several halogen compounds were measured in the 50-200 nm region using synchrotron radiation as a light source. The data of halogen molecules, HF, HC1, HBr, C12, CF4, SiF,, CH C1, CH2Cl2, CHC1, ffi , CC1,, CF H, CF3C1 CF Br, and C2F3C1 are summarized in this paper. The ffuorescence spectta fro excited phbitofragmencs and excited ions were dispersed to identify the emitting species.

Electron energy loss spectra of the sulfur 2p, 2s and oxygen is inner-shells of SO2 and the nitrogen is and oxygen is inner-shells of NO2 have been obtained at high energy resolution (0.046-0.090 eV fwhm) under conditions of high impact energy (1-3.7 keV) and forward scattering. The measurements are compared with multichannel quantum defect theory calculations of the transition energies and dipole oscillator strengths. The experimental and calculated results are also compared with literature reports of measurements using synchrotron radiation.

The coupling of high-energy-resolution x-ray absorption and emission spectroscopies with a high-intensity and high-resolution synchrotron-radiation beamline has opened up several new avenues of research in inner-shell molecular physics. Of particular importance is the ability to tune the incident photon energy throughout the near-threshold region for the atomic core levels of the molecules. Sub- and supra-threshold excitations have revealed a diversity of phenomena, including screening effects below threshold, satellite-free fluorescence above threshold, and fluorescence polarization in both energy regions. All of these phenomena can be used in a complementary way to uncover some of the underlying molecular dynamics which occur upon inner-shell excitation and ionization. Some of the latest results from the NBS X-Ray Spectroscopy Beamline at NSLS will be presented to highlight these effects.

We report experimental and theoretical near-edge x-ray absorption fine structure (NEXAFS) spectra of clean and arsenic-capped gallium arsenide and the pseudo-binary alloy indium-gallium arsenide. Experimental data were obtained using synchrotron-radiation total-photoelectron-yield spectroscopy from the Ga(M2,3), As(M2,3), In(N2,3), and In(M4,5) edges. In addition, both C(K) and 0(K) NEXAFS spectra, and photon-stimulated ion-desorption mass spectra were obtained to assess and monitor the sample cleanliness. The samples studied were grown by molecular-beam epitaxy at China Lake and capped with arsenic for protection during transit to the Stanford Synchrotron-Radiation Laboratory. We have found by monitoring the As edges that heating the samples to 300 or 350°C completely removes the arsenic cap. Also, we find that after evaporation of the As cap, the NEXAFS spectra are identical for capped and uncapped samples. Theoretical calculations of the arsenic NEXAFS spectra were performed using a full multiple-scattering theory. The inputs to the calculations were ab initio phase shifts, calculated using pseudopotentials and a model geometry. Theoretical calculations of the arsenic NEXAFS spectra of In0.53Ga0.47As crystals are reported also. Here, the calculations are performed by creating many model crystals that have the appropriate stoichiometry and averaging the resulting spectra.

Collisional transitions resulting from the interaction between H -like Li+2 ions and low energy electrons in recombining laser-produced plasma (LPP) of lithium were inve0i-gated experimentally and theoretically. In the experiment relative intensities of Li+4. lines, located in the VUV region,were measured. The lines correspond to the transitions 22 -rid, n412. The excited levels are populated via three body recombination process twoselectrons plus an ion), which is very intensive in expanding' overcooled IJIT having considerable density and average ion charge but low temperature (ne~I0I5) cm-3}, Te~I eV at 2cm from the target). The population distribution for highly excited levels obtained experimentally is compared with a theoretical picture, calculated using a new approach to the three Coulomb centers problem based on consideration of quasistationary and virtual states.

Atomic processes of interest to the magnetic fusion research are outlined. The interplay of atomic physics and plasma physics pertaining to tokamak plasmas is described. In general, the coronal approximation plus diffusive transport are necessary to account for the spectroscopic measurements. The atomic data base used in fusion research is also summarized.

The technique of soft x-ray fluorescence spectroscopy (SXE) is complimentary to that of photoemission spectroscopy (PES). SXE probes the local partial density of states (PDOS), selects dipole allowed symmetries, and is not necessarily surface sensitive. PES on the other hand, averages over the DOS and can be used to measure the dispersion of the energy bands. PES is also very surface sensitive. We present measurements on the high Tc superconductors, the quasicrystalline phase of A1Mn, and the LiAl intermetallic alloy. These measurements provide insight for theoretical modeling. In the case of the high Tc compound and the intermetallic compound the measurements are in good agreement with the theory. However, for the quasicrystals the measurements provide new insights to challenge theory.

Photoionization cross sections for atomic oxygen and nitrogen have been measured from their respective thresholds to 100Å (for 0) and to 500R (for N). Previously unobserved structure in atomic oxygen is reported between 440 and 481R. Data which extends the atomic nitrogen cross sections to 100Å and data on the cross section for 02+ production are currently being analyzed.

Elements of the theory of x-ray scattering relevant to x-ray optics applications are reviewed. Dispersion-theory and sumrule constraints are considered in relation to determining the refractive index from measured attenuation spectra. Current tabulations of x-ray cross sections and scattering factors are summarized and discussed.

The National Bureau of Standards (NBS) has maintained a data base of experimental and theoretical photon absorption cross sections (attenuation coefficients) since 1950. Currently the measured data include more than 20,000 data points abstracted from more than 500 independen.t literature sources including both published and unpublished reports and private communications. We have recently completed a systematic comparison over the energy range 0.1-100 keV of the measured cross sections in the NBS data base with cross sections obtained using the photoionization cross sections calculated by Scofield and the semi-empirical set of recommended photoionization cross section values of Henke et al. Cross sections for coherent and incoherent scattering were added to that of photoionization to obtain a value which could be compared to the experimental results. At energies above 1 keV, agreement between theory and experiment is rather good except for some special situations which prevent the accurate description of the measured samples as free atoms. These include molecular effects near absorption edges and solid state and crystal effects (such as for silicon). Below 1 keV the comparison indicates the range of atomic numbers and energies where the theory becomes inapplicable. The results obtained using Henke et al. agree well with the measured data when such data exist, but there are many elements for which data are not available over a wide range of energies. Comparisons with other theoretical data are in progress. This study also enabled us to show that a suggested renormalization procedure to the Scofield calculation (from dartree-Slater to Hartree-Fock) worsened the agreement between the theory and experiment. We have recently developed a PC-based computer program to generate theoretical cross section values based on Scofield's calculation. We have also completed a related program to enable a user to extract selected data from the measured data base.

Our low energy x-ray photoabsorption and scattering factor tables along with methods for their application were published in 1982 (1). The photoabsorption cross sections were compiled for the 30-10,000 eV region, with which, using the Kramers-Kronig dispersion integrals, the zero-angle atomic scattering factors were determined for the 100-2000 eV region. These tables are now being revised, based upon the 1987 experimental photoabsorption data base as extended down to 10 eV and with the calculated zero-angle atomic scattering factors for the extended 50-10,000 eV region. Refined methods of application of these tables to modern problems of x-ray spectrometry design involving filters, mirrors and crystal-multilayer analyzers will be included in this revision.

Reflectance measurements were used to derive the optical constants of 22 materials for 36 photon wavelengths from 24 Å to 1216 Å. The samples studied are thin films of the transition metals Ti, Zr, Nb, Mo, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, and Pt, the noble metals Ag and Au, and films of C, diamond, Al, Si, CVD-SiC and a-SiC. We describe the experiment and also the data reduction technique used to derive the optical constants from the reflectance versus incidence angle data. A summary of the results is presented.

New developments in the theory of anomalous x-ray scattering are reviewed. The second order S-matrix calculations remove several previous discrepancies, in particular it is now known that there is an error in the Cromer-Liberman tables. In high Z elements a window is found in near threshold scattering at back angles. New results obtained for ions indicate that with increasing degrees of ionization the anomalous behavior near threshold weakens.

Dispersion analysis and sum rules have been used for many years to anal1yze experimental x-ray absorption data and to determine scattering factors from this data. It has been noted that there is disagreement between previously tabulated absorption data and well-known sum rules. There is also substantial disagreement between experimentally determined scattering factors and those that are predicted from scattering theory. We will show that the combined tools of sum rules and dispersion analysis can be used to guide one in determining where corrections need to be made in existing absorption data. Experimentally determined scattering factors can be taken into account in this analysis by the use of subtracted dispersion relations. This provides a check on the consistency of the absorption data used in the dispersion analysis.

Sub-keV x-ray photoabsorption cross section measurements in transmission have been made using synchrotron radiation beam lines on the VUV storage ring at the National Synchrotron Light Source (NSLS) and on the SPEAR storage ring at Stanford. The experimental considerations associated with making absolute measurements are reviewed along with techniques for resolving difficulties. Suggestions for future measurements are included.

The method of using two laser produced plasmas, one acting as background continuum radiation source, the other as absorbing medium, for obtaining the absorption spectra and for measuring the photoionization cross-sections of low-Z ionic species is described. The basic principles of the experiment are discussed. The advantages of some technical solutions, like the use of stigmatic technique in VUV and XUV spectral regions, the use of detecting system using photodiode arrays, and the constrained deconvolution procedure adopted in data processing, are emphasized.

Free standing targets, were produced to measure the sub-keV photoabsorption cross section. of numerous elements. The targets, were fabricated with a computer' controlled, magnetron sputter deposition. system. The targets are structured of multiple layers of a given element interspersed with layers of carbon. The purpose of the multi:layering is twofold: to improve the mechanical strength and to protect the given element from exposure to the atmosphere, using the outermost C layers as a seal. The targets are 1.9 cm in diameter' and their thicknesses, range from 160 nm to 4000 nm depending upon the element selected for the given. experiment. The total thickness of the C layers in each target is a maximum of 200 nm. The first and. last C layers (the sealing layers) are 50 nm thick and the remaining C is divided among the interspersed layers. Multilayer targets were successfully produced in which the given element was either Al, Ti, V, C7 Cr, Ni, Cu, Si, or Au. Areal density. and Rutherford backscatter measurements were performed. to accurately establish the total amount of the deposited. elements.

The dispersion of light by multilayer combined microstructure diffraction gratings is affected by the scattering cross-sections of the elements used to synthesize the multilayer. This effect will be analytically evaluated, and the results of model calculations for spectral regions of interest presented. Experimental results for these domains will also be presented and compared with data from the literature and with the results of modelling calculations.

Characterization of both the composition and thickness of a variety of carbon and carbon-metal multilayer foils has been accomplished using simultaneous ion backscattering (RBS or BS) and particle-induced X-ray emission (PIXE). The quantitative, non-destructive measurements were made on foils with total carbon thicknesses of 96 to 460 nm and total metallic thicknesses of 14 to 260 nm. Foils with the following primary metals have been examined: titanium, chromium, nickel, copper, thorium, and uranium. The PIXE results for X-ray energies above 200 eV permitted unambiguous interpretation of the backscattering results. For the free-standing foils, the quantities and depth distributions of carbon, the primary metal, and impurities, including oxygen, were determined with a depth resolution of tens of nanometers. The foils were analyzed with either protons or helium ions. Two foils were analyzed using both protons and helium ions; the quantitative evaluations of the carbon and primary metal were, within seven percent for the carbon and three percent for the metal, independent of the probing ion.

Recent developments with detector systems for use at vacuum ultraviolet (VUV) and soft x-ray wavelengths have emphasized improvements in the detective quantum efficiencies and the capability for high-resolution imaging while retaining a high photometric accuracy. This paper compares and contrasts the performance characteristics of the three principal classes of imaging detector systems currently under active development, namely, the bare Charge-Coupled Device (CCD), the intensified CCD, and the microchannel plate (MCP) coupled to different imaging electronic readout systems.

With the recent compilation of the low-energy x-ray photoabsorption and scattering factors, it has become more practical to determine low-energy x-ray fluorescence yields and to check the scattering cross sections of materials by x-ray photoexcitation. The x-ray fluorescence yield measurements of carbon in polypropylene, oxygen in mylar, and fluorine in polytetrafluorethylene (teflon), by Al x-ray photoexcitation will be presented. Also a comparison of calculated to observed Al Ka x-ray scattering from these plastics will be presented.