Measurements of the vacuum-ultraviolet (<80-nm) radiation produced by intense ultraviolet (248-nm) irradiation (1015–1016 W/cm2) of rare gases have revealed the copious presence of both harmonic radiation and fluorescence from excited levels. The highest harmonic observed was the seventeenth (14.6 nm) in Ne, the shortest wavelength ever produced by that means. Strong fluorescence was seen from ions of Ar, Kr, and Xe, with the shortest wavelengths observed being below 12 nm. Furthermore, radiation from inner-shell excited configurations in Xe, specifically the 4d95s5p → 4d105s manifold of Xe7+ at ~17.7 nm, was detected. These experimental findings, in alliance with other studies concerning multielectron processes, give evidence for a role of electron correlations in a direct nonlinear process of inner-shell excitation.

Studies of multiphoton production of vacuum-ultraviolet radiation in the rare gases

One of the most significant developments in computational atomic and molecular physics in recent years has been the introduction of B-spline basis sets in calculations of atomic and molecular structure and dynamics. B-splines were introduced in applied mathematics more than 50 years ago, but it has been in the 1990s, with the advent of powerful computers, that the number of applications has grown exponentially. In this review we present the main properties of B-splines and discuss why they are useful to solve different problems in atomic and molecular physics. We provide an extensive reference list of theoretical works that have made use of B-spline basis sets up to 2000. Among these, we have focused on those applications that have led to the discovery of new interesting phenomena and pointed out the reasons behind the success of the approach.

https://iopscience.iop.org/article/10.1088/0034-4885/64/12/205/pdf

Applications of B-splines in atomic and molecular physics

In analogy to electron impact or photon-induced dissociation of molecules, electronic excitation of adsorbate complexes leads to desorption of ions and neutrals. Both processes require a primary excitation to a repulsive potential curve, by which conversion of electronic excitation to nuclear motion can occur. However, in the surface case strong competition with the desorptive process occurs by transfer of the excitation away from its primary location, because of strong coupling to the many modes of surface layer and substrate; this leads to very strong dependence of cross sections on bonding mode, excitation energy, isotope mass, and coverage. Recently, considerable progress has been made in the understanding of primary, secondary, and tertiary processes by detailed investigations of energy and polarization dependencies of cross sections, and of angle and energy distributions of products, for primary valence as well as core excitations, and for neutral and ionic products. The importance of localization of the excitation and its dependence on many-body interactions becomes obvious. These aspects are reviewed, and some applications sketched.

Desorption induced by electronic transitions: Some recent progress

A comprehensive review is given on photoionization of rare gas atoms using monochromatized synchrotron radiation. Emphasis is put upon the general experimental and theoretical background, and illustrative examples are presented in order to show the present status and the progress in the field during the last decade.

https://iopscience.iop.org/article/10.1088/0034-4885/55/9/003/pdf

Photoionization of atoms using synchrotron radiation

http://amo-csd.lbl.gov/downloads/BIB-INNER-SHELL-BY-HITCHCOCK.pdf

Bibliography of atomic and molecular inner-shell excitation studies

Electron spectroscopy, combined with synchrotron radiation in the photon-energy range $h\ensuremath{\nu}\ensuremath{\simeq}60\ensuremath{-}190$ eV, was used to measure the angular distributions of Xe $5p$ and $5s$ photoelectrons and of ${N}_{4,5}\mathrm{OO}$ Auger electrons. The branching ratios and partial cross sections for photoionization and Auger processes were also measured in certain cases. The measured asymmetry parameter $\ensuremath{\beta}$ for $5p$ photoelectrons agrees well with many-electron calculations, which predict a pronounced oscillation in ${\ensuremath{\beta}}_{5p}$ above the $4d$ ionization threshold due to the $4d\ensuremath{-}5p$ intershell interaction. The ${N}_{4,5}\mathrm{OO}$ Auger electrons are produced with photon-energy-dependent anisotropic angular distributions, resulting from alignmentof ${\mathrm{Xe}}^{+}$ by photoionization. The theoretical analysis of Auger-electron angular distributions is described, and theoretical calculations are found to predict the measured asymmetries well. In addition, Auger-electron peaks were observed to broaden and shift at photon energies near the $4d$ ionization threshold because of postcollision interaction. The measured shifts of the ${N}_{5}{O}_{1}{O}_{1}$ $^{1}S_{0}$ line agree with previous measurements and theory. Electron spectra recorded through the energy region of the $4d\ensuremath{\rightarrow}\mathrm{np}$ Rydberg states show that they decay primarily by the Auger process, while the $5p$ and $5s$ partial cross sections are relatively weakly affected by autoionization. However, distinct resonance structure was observed in the $\ensuremath{\beta}$ parameters for $5{p}_{\frac{3}{2}}$ and $5{p}_{\frac{1}{2}}$ photoelectrons. The measured results are compared with a theoretical calculation of resonant photoionization for the $4d\ensuremath{\rightarrow}6p$ excitation.

/pdf/electron-spectroscopy-study-of-inner-shell-photoexcitation-4ao0uu3525.pdf

Electron-spectroscopy study of inner-shell photoexcitation and ionization of Xe

Cross sections and angular‐distribution asymmetry parameters were measured directly for C(KVV) Auger electrons and C 1s photoelectrons from CO, CO2, CF4, and OCS, O(KVV) Auger electrons and O 1s photoelectrons from CO and CO2, and S(LVV) Auger electrons and S 2p photoelectrons from OCS using synchrotron radiation. The measurements were made in the photon‐energy ranges 270–350, 545–680, and 160–190 eV, respectively. Shape resonances were observed in all of these molecular systems. The cross‐section results are compared with previous experimental data obtained by electron energy‐loss measurements, electron–ion coincidence results, and photoabsorption studies. The asymmetry‐parameter results are the first of their kind for these molecular core levels. The present results are compared with available theoretical predictions obtained from Stieltjes–Tchebycheff imaging techniques, Hartree‐Fock static‐exchange calculations, and the multiple‐scattering method.

/pdf/core-level-photoelectron-and-auger-shape-resonance-phenomena-i3wgy37p52.pdf

Core‐level photoelectron and Auger shape‐resonance phenomena in CO, CO2, CF4, and OCS

The partial cross section, the satellite branching ratio, and the angular-distribution asymmetry parameter for simultaneous photoionization and excitation to the n=2 states of the ${\mathrm{He}}^{+}$ ion have been measured in the 67.5--90-eV photon-energy range. In the nonresonance regions (67.5 eV\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}69.5 eV and 75 eV \ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}90 eV), the asymmetry-parameter values have been used to infer the ratio of the 2p cross section to the 2s cross section. These results indicate that the ${\mathrm{He}}^{+}$(n=2) satellite is predominantly 2p near threshold, in agreement with the experimental and most of the theoretical results reported to date. In the region below the ${\mathrm{He}}^{+}$(n=3) threshold (69.5 eV\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}73.0 eV), the effects of a series of autoionizing Rydberg levels on the n=2 cross section, branching ratio, and asymmetry parameter have been measured, this being the first detailed measurement of the angular distribution of a satellite over an autoionization resonance. In addition, qualitative information concerning the total cross section and the 1s partial cross section has been obtained for the first member of this series (3s3p), disagreeing with previous experimental and theoretical results for the total cross section, but in agreement with recent photoemission measurements of the 1s cross section. The present results suggest that the qualitative shapes of the total and 1s cross sections over the 3s3p resonance are similar to the profile of the n=2 cross section for this resonance. To illustrate quantitative methods for the interpretation of autoionization phenomena, the derivation from the resonance data of several parameters defining the autoionization process is described.

Photoionization of helium above the He+(n=2) threshold: Autoionization and final-state symmetry.

Partial photoionization cross sections and photoelectron asymmetry parameters were measured for the valence orbitals of NO over the photon energy range 16–31 eV by photoelectron spectroscopy, using synchrotron radiation. The results are compared with previous measurements and with theoretical curves based on the multiple‐scattering model. Maxima observed in the partial cross sections of the 2π, 5σ, and 4σ orbitals are attributed to continuum shape resonances. The multiple‐scattering model calculations predict the observed photoelectron asymmetries of the 5σ and 1π orbitals very well, but the calculated asymmetry curves are shifted strongly from the measured asymmetries produced by the 2π and 4σ orbitals. The π molecular orbitals are observed to produce qualitatively different photoelectron asymmetry curves than do the σ orbitals. We discuss the double‐angle‐time‐of‐flight method for the measurement of photoelectron angular distributions. This technique offers the advantages of increased electron collection efficiency and the elimination of certain systematic errors.

https://escholarship.org/content/qt216378nf/qt216378nf.pdf?t=p0fv15

Photoionization cross sections and photoelectron asymmetries of the valence orbitals of NO

The first photoemission measurements of the I 4d and I ''4p'' subshells in methyl iodide are presented. Cross sections and angular-distribution asymmetry parameters were measured from threshold to 300 eV photon energy for the I 4d level (to 440 eV for the asymmetry parameter), and from 175 to 300 eV for the I ''4p'' level. The I 4d results exhibited atomiclike behavior throughout this energy range, mimicking the behavior of the Xe 4d subshell. Theoretical calculations for the Xe 4d subshell agree very well with the I 4d asymmetry-parameter results, indicating that the I 4d subshell is localized on the iodine atom in CH/sub 3/I. Nearer to threshold, the spin-orbit final states, 4d/sub 5/2/ and 4d/sub 3/2/, were resolved and exhibited nonstatistical intensity ratios mainly due to a kinetic energy effect. The I ''4p'' asymmetry-parameter results were found to be essentially identical to the asymmetry-parameter results for the I 4d subshell at the same photon energies, suggesting strong interchannel coupling. This result is discussed with respect to collective effects in the iodine N shell.

/pdf/inner-shell-photoemission-from-the-iodine-atom-in-ch-sub-3-i-4i3htn6qbu.pdf

P. H. Kobrin

Papers

Electron-spectroscopy study of inner-shell photoexcitation and ionization of Xe

Core‐level photoelectron and Auger shape‐resonance phenomena in CO, CO2, CF4, and OCS

Photoionization of helium above the He+(n=2) threshold: Autoionization and final-state symmetry.

Photoionization cross sections and photoelectron asymmetries of the valence orbitals of NO

Inner-shell photoemission from the iodine atom in CH/sub 3/I