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

A review. Ultrafast x-ray techniques using diffraction and absorption are discussed with an emphasis on the absorption techniques. Ultrafast x-ray sources and detectors for use in x-ray absorption spectroscopy are also reviewed. [on SciFinder (R)]

Ultrafast X-ray absorption spectroscopy

Resonant X-ray Raman scattering

A new method is presented for treating the effects of quadruple excitations in coupled-cluster theory. In the approach, quadruple excitation contributions are computed from a formula based on a non-Hermitian perturbation theory analogous to that used previously to justify the usual noniterative triples correction used in the coupled cluster singles and doubles method with a perturbative treatment of the triple excitations (CCSD(T)). The method discussed in this paper plays a parallel role in improving energies obtained with the full coupled-cluster singles, doubles, and triples method (CCSDT) by adding a perturbative treatment of the quadruple excitations (CCSDT(Q)). The method is tested for an extensive set of examples, and is shown to provide total energies that compare favorably with those obtained with the full singles, doubles, triples, and quadruples (CCSDTQ) method.

Coupled-cluster methods including noniterative corrections for quadruple excitations

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

Photoionization of helium and neon to excited satellite states, ${\mathrm{He}}^{+}$ nl and ${\mathrm{Ne}}^{+}$ 1${s}^{2}$2${s}^{2}$2${p}^{4}$nl, was studied with synchrotron radiation and threshold electron analysis. Photoelectron satellites have been directly measured at threshold for the first time to our knowledge. The relative satellite cross sections were determined over the kinetic energy range from 0 to 1 eV. The angular distributions were also evaluated close to threshold. Strong correlation effects were observed in two cases. For He near threshold, the angular-distribution asymmetry parameter \ensuremath{\beta} is near zero for the n=2 satellite and is increasingly negative for the higher-n satellites, in agreement with the theoretical prediction of Greene. In the threshold photoelectron spectrum of Ne, many final states are present, some with quartet spin multiplicity and others with high-L values.

/pdf/helium-and-neon-photoelectron-satellites-at-threshold-2xe6mai4ft.pdf

Helium and neon photoelectron satellites at threshold

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.

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

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

Photoelectron spectra have been taken between 2460 and 2600 eV photon energy across the discrete and continuum resonances in the vicinity of the sulfur K edge in gaseous ${\mathrm{SF}}_{6}$. Results at the below-threshold S 1s\ensuremath{\rightarrow}6${t}_{1u}$ resonance indicate that ``highly excited'' S 2p and S 2s satellites (with two core holes) are the primary autoionization final states of ${\mathrm{SF}}_{6}$${\mathrm{}}^{+}$. An observed asymmetric profile in the S(LVV) Auger angular distribution suggests interference effects in the alignment of these resonantly produced ${\mathrm{SF}}_{6}$${\mathrm{}}^{+}$ ions. Decay of the low-energy S 1s continuum resonances near 2507 eV photon energy into S 2p, S 2s, and/or valence photoemission channels indicates autoionizing character. These features are assigned as doubly excited states, leading to S 1s satellite thresholds observed here for the first time. At higher photon energies, between 2520 and 2570 eV, large oscillations in the S 1s cross section are reproduced well by multiple-scattering X\ensuremath{\alpha} calculations, but are not explained adequately by single-scattering plane-wave extended x-ray absorption fine structure effects. We speculate that improvements in the description of both the electron scattering process and the molecular potential are necessary to model the diffractive and nondiffractive (barrier interaction) effects in this energy region.

/pdf/sulfur-1s-core-level-photoionization-of-sf6-xazdr1f2mt.pdf

Sulfur 1s core-level photoionization of SF6

We present partial-cross-section results for Si 2p and valence photoionization of ${\mathrm{SiF}}_{4}$ for photon energies in the vicinity of the Si 2p and 2s thresholds. The continuum shape resonances decay only to the Si 2p main line, in accordance with one-electron theory predictions. The Si ${L}_{2}$,3VV Auger spectrum was also measured, and was found to agree with earlier work. We propose that variations in hole-interaction energies are important in assigning the Auger final states. For the Si 2p discrete excitations, spectra obtained on the ${\ensuremath{\sigma}}^{\mathrm{*}}$(${a}_{1}$) resonances compare well with previous qualitative results and the above-threshold Si ${L}_{2}$,3VV Auger spectrum. Our results help to quantify the relative enhancement of satellites and main lines, emphasizing the dominance of spectator decay. Spectra obtained at several higher-energy Si 2p and Si 2s excitations also generally support spectator decay as an important mode of relaxation for these discrete states. The overall assignment of the Si 2p and 2s discrete excitations to molecular-orbital and Rydberg transitions is discussed.

/pdf/si-2p-and-2s-resonant-excitation-and-photoionization-in-sif-t2sahotyse.pdf

T. A. Ferrett

Papers

Helium and neon photoelectron satellites at threshold

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

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

Sulfur 1s core-level photoionization of SF6

Si 2p and 2s resonant excitation and photoionization in sif[sub]4