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The planned construction of an X-ray free-electron laser (XFEL) will provide new opportunities for research in various areas of physics, chemistry and biology. The proposed design of the XFELs at DESY (Deutsches Elektronen-Synchrotron) and SLAC (Stanford Linear Accelerator Center) is built on the concept of a fourth-generation synchrotron source and will provide an intense pulse (I(0) approximately 10(16) W cm(-2), tau(gamma) approximately 100 fs) for photon wavelengths down to 1 A. Some guidelines for applications of these sources pertaining to atomic physics are presented here. Issues such as the onset of strong photon-field effects, multiple ionization and hollow-atom formation are analyzed. Attention is especially given to studying the interaction with rare-gas atoms, for which some numerical estimates are provided.

Radiometric transfer standards consisting of windowless diodes with cathodes made of anodized aluminum oxide on aluminum are now available from the National Bureau of Standards with calibrations in the 200-600-A wavelength range. This extends the previously existing range of calibration for these diodes (600-1200 A). For wavelengths shorter than 600 A, synchrotron radiation at NBS-SURF is used as the source of radiant energy. A noble gas double ionization chamber is used to calibrate a secondary standard diode that is then intercompared with the transfer standards. Monitors take into account variations in the intensity of synchrotron radiation and in beam position. Methods of accounting for the effects of second-order radiation in the incident flux and secondary ionization in the double ionization chamber are discussed. Calibration uncertainties are about 10%.

Tunable, polarized, microfocused x-ray pulses were used to record x-ray absorption spectra across the K edges of Kr{sup +} and Kr{sup 2+} produced by laser ionization of Kr. Prominent 1s {yields} 4p and 5p excitations are observed below the 1s ionization thresholds in accord with calculated transition energies and probabilities. Due to alignment of 4p hole states in the laser-ionization process, the Kr{sup +} 1s {yields} 4p cross section varies with respect to the angle between the laser and x-ray polarization vectors. This effect is used to determine the Kr{sup +} 4p{sub 3/2} and 4p{sub 1/2} quantum state populations, and these are compared with results of an adiabatic strong-field ionization theory that includes spin-orbit coupling.

Double K-shell photoionization of Ne at 5000 eV was observed by recording the KK-KLL Auger-electron hypersatellite spectrum. The measured Auger spectrum is compared with the results of multiconfiguration Dirac-Fock calculations. Shake calculations are used to identify likely multivacancy states produced by photoexcitation or ionization in addition to double-K vacancies, and their calculated Auger spectra are compared with the measured spectrum. The measured relative intensities of hypersatellite and diagram Auger lines are combined with experimental and theoretical determinations of branching ratios from single- and double-K vacancies into final states to determine the ratio of double-to-single K-shell photoionization cross sections to be 0.32(4)%. This ratio is much larger than the calculated high-energy-limit ratio and indicates a large contribution of dynamic electron correlation.

We have studied double K‐shell photoionization of Ne and Mo (Z = 10 and 42) at the Advanced Photon Source. Double K‐vacancy production in Ne was observed by recording the KK‐KLL Auger hypersatellite spectrum. Comparison is made with calculations using the multiconfiguration Dirac‐Fock method. For Mo, double K‐vacancy production was observed by recording the Kα, β fluorescence hypersatellite and satellite x rays in coincidence. From the intensities of the Auger or x‐ray hypersatellites relative to diagram lines, the probabilities for double K‐vacancy production relative to single K‐vacancies were determined. These results, along with reported measurements on other atoms, are compared with Z‐scaling calculations of the high‐energy limits of the double‐to‐single K‐shell photoionization ratio.

Absolute inclusive and exclusive cross sections for ionization of H and He by Compton scattering are presented and analyzed. The exclusive cross section for ionization of one electron, leaving the second electron in the ground state, is 20% smaller for H than for He. Furthermore, the exclusive cross section for ionization with excitation to the first excited state is seven times larger for H than for He. However, at high energies the inclusive Compton cross sections for both H and He are within 1% of two times the Thomson cross section for elastic scattering of a free electron, even when ground-state correlation is accurately included. A general analysis based on quasielastic scattering is applied to both shake and correlated calculations.

Vibrational branching ratios and photoelectron angular distributions for alternative vibrational levels of C2H2+X 2Πu have been measured in the range 13 eV⩽hν⩽25 eV using synchrotron radiation. Below hν∼16 eV, these data exhibit strong non‐Franck–Condon effects, namely, wavelength‐dependent vibrational branching ratios and vibrational‐state‐dependent photoelectron asymmetry parameters. Moreover, enhanced excitation of bending modes of the ion is observed below hν∼16 eV, in addition to the C–C stretch mode, which is the only mode readily observed in photoelectron spectra of C2H2 at shorter wavelengths, e.g., at the He I (21.2 eV) resonance line. The non‐Franck–Condon behavior is attributed to resonant photoionization processes, whose identification is discussed in the framework of several recent theoretical and experimental studies on acetylene and related molecules.

We have begun a program to measure oscillator strengths of autoionizing resonances that result from a transition in the VUV between a laser excited initial state and a final state in which a core electron is promoted. These measurements demonstrate a new technique to combine synchrotron radiation, laser pumping, and photoelectron spectroscopy.Measurements of the energy positions of autoionizing resonances have been honed to a fine art over the past 50 years. Total cross section measurements and the parameters that describe autoionizing resonances have been determined. Most of these studies have been made from the dipole allowed ground state. Recently autoionizing resonances have been observed from excited initial states and from ion initial states. We have heard several talks, at this meeting which described some of this type of research. In the measurements to be described in this paper, laser radiation is combined with synchrotron radiation, as shown schematicaly in Figure 1, to study the photoionization from excited initial states to continuum final states or to autoionizing final states. Continuum radiation from the Aneau de Collisions d’Orsay (ACO), which is installed at the Universite de Paris-Sud, in Orsay France, is monochromatized by a toroidal grating monochromator (TGM) and is focused by a toroidal output mirror on to a weakly collimated sodium beam emanating from a furnace mounted on the axis of a cylinderical mirror analyzer (CMA). This electron spectrometer is used to study the kinetic energy distribution of the ejected photoelectrons produced by the interaction of the photon beam with the focused synchrotron radiation.

The effects of electric fields of intensity up to about 10 kVcm-1 on the autoionising resonances buying between the 2P3/2 and 2P1/2 ionisation limits in Xe have been studied in absorption. Fields of this strength produce pronounced effects on series members for n>11 The major effects are a decrease in transmission in the neighbourhood of the resonances and a lowering of the classical field ionisation potential.

Collisional ionization of sodium and barium vapors resonantly excited with c.w. or pulsed laser sources, and subsequent electron-atom energy transfers, are studied by electron spectroscopy. Cross sections for associative and Penning ionization of sodium in excited states, are obtained.

A sodium atomic beam with density ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ was illuminated by cw dye laser radiation (a few watts per square centimeter) tuned to the ${D}_{2}$ resonance line. In the energy spectrum of the emitted electrons, several lines were observed between 4 and 7 eV. Their positions and intensities indicate that seed electrons are produced either via associative ionization or via collisional ionization from excited states populated by energy-pooling collisions. These electrons are then heated through successive superelastic collisions with excited $3p$ atoms.

The first observation, by electron spectrometry, is reported in laser-excited sodium vapor of the primary low energy electrons produced by associative ionization and by Penning ionization of sodium atoms in highly excited n1 states. The sequential heating of these primary electrons has been observed in 1, 2 or 3 superelastic collisions with Na(3p) atoms. The variation of associative ionization was measured as a function of the excited state density by using inner-shell photoionization produced by synchrotron radiation. Finally, an associative ionization cross section of 3.8 X 10-17 cm2 and a Penning ionization cross section for the 5s state of 1.1 X 10-12 cm2 were found (within 50% uncertainty) for an oven temperature of 520 K.

Photoelectron studies of molecular autoionization resolved into position within the resonance profile, photoelectron ejection angle, and final vibrational state are reported. By using the first members of prominent window and absorption series converging to N/sub 2//sup ts+/ B/sup 2/..sigma../sub u//sup +/ as an example, striking variations of vibrational branching ratios and photoelectron asymmetry parameters within autoionizing profiles are demonstrated. Such triply differential data represent a very detailed characterization of the rovibronic interactions governing molecular autoionization.

The absorption spectrum of gaseous barium was obtained between 120 AA and 140 AA, the region of the NIV,V thresholds. The similarity to lanthanum of some of the features observed in barium suggests that the 4f orbital contracts and overlaps the 4d orbit. This contraction produces two terms, 3P and 3D, of the 4d94f configuration below the 4d ionization limits, while the large electrostatic exchange interaction drives the 1P term of this configuration some 10 eV above the limit. Furthermore, extensive mixing of the 4d9(6s2)6p configuration with 4d9(5d2)6p and 4d9(5d6s)6p produces many weak resonances in addition to the resonances associated with the 4d94f configuration. A suggested classification of these features is given with the aid of known features of the La I spectrum. Finally, from the known X-ray splitting of the NIV,V threshold and the energy interval between the 6s26p 2P and 6s2 1S levels in La I and La II, the ionization thresholds of the 4d electron were determined to be 814800 (1000) cm-1 and 792500 (1000) cm-1.

The National Bureau of Standards (NBS) Synchrotron Ultraviolet Radiation Facility (SURF-II) is used in conjunction with several high throughput monochromators to study the interaction of vacuum ultraviolet photons with solids and gases. Recent work has been concerned with the photon stimulated desorption of atomic and molecular ions from surfaces, with the effect of electric fields on molecular photoabsorption and with the study of molecular photoionization by angle resolved photoelectron spectroscopy. These research programs yield new information about molecular bonding at surfaces, molecular dynamics near ionization thresholds, and the coupling of the electronic and nuclear motion near resonances in molecules. In addition to these programs in basic research SURF-II is used for the calibration of transfer standard detectors over a photon energy range 20-250 eV. Calibration of monochromator systems is achieved over the photon energy range 5-250 eV by using the now calculable spectral intensity radiated by the electrons, which are confined in a nearly circular orbit.

The central position of the 2s2p /sup 1/P/sup o/ two-electron resonance of He at 206.21 A has been remeasured using the background continuum of the National Bureau of Standards storage ring facility. In addition, the line-shape parameter q and width GAMMA of the resonance have also been obtained. We obtained a value of 60.151 +- 0.010 eV for the resonance position, -2.6 +- 0.3 for the line-shape parameter, and 0.038 +- 0.002 eV for the width of the resonance. Our value for the resonance position is in good agreement with the theoretical calculation of 60.145 eV of Bhatia and Temkin and lies within the error budget of the previous experimental measurements of Madden and Codling. Parameter values for the other resonances have also been obtained and are in good agreement with the earlier measurements of Madden and Codling.

Experiment and theory have been used to determine the final-state symmetry for the n = 2 state in photoionized helium. The angular asymmetry parameter ..beta.. has been measured as a function of photon energy by using angle-resolved photoelectron spectroscopy. From these measured ..beta.. values, which are a weighted average of ..beta../sub 2s/ and ..beta../sub 2p/, together with the theoretical values of ..beta../sub 2p/ and ..beta../sub 2s/, the ratio, R, of the partial 2p photoionization cross section to the partial 2s photoionization cross section has been obtained.

The Al L/sub 2,3/ spectrum for dilute alloys of Al in Mg, and the Mg L satellite spectrum produced by transitions in atoms with doubly ionized 2p core levels have been measured. The two spectra contain similar features associated with screening of the extra core hole, but are otherwise dissimilar, which suggests that the equivalent-core approximation is not valid. The threshold peak is broadened in both spectra. The Al-impurity spectrum is found to be about 1 eV wider than that of the Mg host. The L/sub 3/-L/sub 1/ intercore transition in doubly ionized Mg is observed. Finally, a second, weak, high-energy satellite of Mg is observed that may be produced by transitions from the /sup 1/S/sub 0/ state of the doubly ionized core levels. These results provide important new data for comparison with theories of the electronic states of impurity atoms in metals.

We report striking non-Franck-Condon vibrational intensity distributions associated with the shape resonance in the $5\ensuremath{\sigma}$ photoionization channel of CO. This example confirms the recent theoretical prediction that shape resonances will couple significantly with vibrational motion, leading to different resonance energies and profiles, and non-Franck-Condon intensities in alternative vibrational channels. Analogous effects are expected in connection with the widespread occurrence of shape resonances in both innershell and outershell molecular photoionization spectra.

The complete electron spectra caused by photoionisation of Li and Na vapour in the 1s and 2p shells respectively, have been investigated in the region from 5 eV above threshold up to about twice the binding energy of the ionised electron. Even though the contribution of molecules to the vapour pressure was only of the order of a few per cent, molecular features are found in the experimental spectra with intensities comparable with the atomic structures. The interpretation of all features observed in the spectrum is given. In specific energy regions, where the molecular contributions do not disturb the atomic ones, a detailed study of the fine-structure branching ratio and of the satellite structure accompanying these ionisation processes has been made. The results of this investigation are compared with other experimental and theoretical data.

We report the first measurement of the angular distribution of the 304-\AA{} ${\mathrm{He}}^{+}(n=2)$ radiation following photoionization. This distribution reflects the alignment of the ion, which is related to the fraction $\ensuremath{\xi}=\frac{\ensuremath{\sigma}(2p,kd)}{[\ensuremath{\sigma}(2p,ks)+\ensuremath{\sigma}(2p,kd)]}$ of $d$ component in the electron wave. The experimental angular distributions correspond to alignments of -0.62 \ifmmode\pm\else\textpm\fi{} 0.03 and -0.62 \ifmmode\pm\else\textpm\fi{} 0.02 at photon energies of 65.5 and 66.5 eV, respectively. These translate into ratios $\ensuremath{\xi}=0.25\ifmmode\pm\else\textpm\fi{}0.04 \mathrm{and} 0.25\ifmmode\pm\else\textpm\fi{}0.03$, in good agreement with close-coupling calculations.

The excitation and decay of core-excited molecular lithium has been investigated by experimental and theoretical methods. The associated far-UV photoabsorption spectrum, which was observed with partly resolved vibrational structure, was interpreted on the basis of configuration-interaction (CI) calculations. In addition, the autoionisation of these states was investigated by ejected-electron spectroscopy and explained through the use of a stationary Born-Oppenheimer model. A theoretical determination of the related potential curves and the 1s ionisation potential is presented along with a discussion of the electronic structure of the core-excited states.

The absolute $K$-photoionization cross section of helium has been measured in the region extending from 100 to 250 \AA{} to a precision of 5% or better. Above 250 \AA{}, where the present results overlap with earlier work performed in this laboratory, there is satisfactory agreement and the data have been combined so as to extend the range of measurements by about 8 Ry above the ionization threshold. It has been possible to join these results with a previous observation made at a higher energy (20.3 Ry) so that helium is the only gas other than Ne for which the cross section is known over a large energy range with comparable precision. This information, combined with the outcome of theoretical calculations and with estimates of discrete oscillator strengths, has led to the evaluation of four sum rules. The calculations attest to the reliability of the observations.

The paper presents measurements of the absolute photoionization cross section of neon in the soft x-ray region (80-600A) to an accuracy of 5% or better. Increased reliability of measurements was brought about by various technical improvements such as the use of an absorber cell with suitable windows and the use of a Geiger-Mueller tube specially designed for counting photons whose energy was in excess of 40 eV. At the L2,3 threshold (575 A) the cross section sigma, has a value of 6.3 Mb, rises to a peak value of 10.0 Mb at 375A, and then decreases to 7.8 Mb at the L1 edge (about 261 A). The range of measurements has been extended to 10 Ry above the ionization threshold. Consequently, it was possible to join the results with previous observations made at high energies. With the exception of He, neon is the only other gas for which the value of sigma is known over a large energy range with sufficient precision. This information, combined with estimates of discrete oscillator strengths, led to the first successful evaluation of the Thomas-Reiche-Kuhn sum rule. (Author)

In a triple, orthogonal crossed beam experiment, we have studied photoionization of excited Na atoms. A cw ring dye laser (few W/cm2) locked to the D1 or D2 absorption lines excited up to 20% of the 2013 cm−3 ground state atoms to Na(3p 2p3/2). Monochromatized synchrotron radiation from the ACO storge ring provided the photoionizing radiation. A cylindrical mirror electron spectrometer was used to measure photoelectron spectra. First measurements of the 2p63p→2p53pEl photoionization were obtained. Decay of autoionizing resonances to the 2p53p ionic channel were also observed.

Publisher Summary This chapter presents various ionization processes such as photoionization, autoionization, and collisional ionization to be examined in excited atoms. The photon energy range from threshold to several hundred electron volts above threshold provides the most fruitful region to study these many body interactions. The information obtained from these measurements, while having great value, is somewhat restrictive because of the dipole selection rules. Only a limited class of states can be probed and the initial state is often an ensemble of nearly degenerate levels. Synchrotron radiation is a source of radiation that extends over a spectral range extending from x-rays to microwaves. Because of its intensity, synchrotron radiation is ideally suited for the observation of photoionization in gaseous samples. Most of the electrostatic interaction occurs within the core and the d wave function amplitude increases slowly with increasing radial distance. The overlap of the wave function of the excited d electron with the core is small in neon, but much larger for these other rare gases.