Theoretical Photoionization Cross Sections Research Articles

Theoretical studies on the photoionization cross-sections of solid silver

An alternative expression for photoionization cross-section of atoms or molecules and a dielectric influence function (DIF) in a high-density system proposed recently are used to study the photoionization cross-sections of solid silver. It is suggested that a density turning point (DTP) of a photoionized system may be viewed as the critical point where the photoionization properties of atoms in a real system may have a notable change. The results show that the present theoretical photoionization cross-sections are in good agreement with the experimental results of a silver crystal both in structure and in magnitude.

L X-ray production cross sections, average L shell fluorescence yield and intensity ratios in heavy elements

L X-ray fluorescence cross sections, and intensity ratios were measured for elements in the 70£Z£92 atomic range at the excitation energy 59.5 keV using a Si(Li) detector. Furthermore, L X-ray fluorescence cross sections and intensity ratios were calculated for elements in the same range. The average L shell fluorescence yields were derived using experimental L X-ray fluorescence cross sections and theoretical photoionization cross sections. The obtained results were compared with other experimental and theoretical values.

Absolute Photoionization Cross Section Measurements of O ii Ions from 29.7 to 46.2 eV

Absolute photoionization cross sections have been measured for a mixture of ground-state and metastable O ii (O + ) ions at photon energies ranging from 29.9 to 46.0 eV (414.7 to 269.5 A ˚ ). All measurements were performed by merging an O + beam with synchrotron radiation from an undulator beam line at the Advanced Light Source (ALS). At a resolution of 17 meV, more than 70 spectral features have been resolved, most of them identified and characterized. These measurements are compared with two independent R-matrix calculations and the data in TOPbase. All three calculations agree within 25% on the direct photoionization cross section, and with the absolute measurements within 40%. Some differences are noted in the predicted resonance positions among the three close-coupling R-matrix calculations, the TOPbase data being the least accurate. The estimated total experimental uncertainty varies from 15% to 20%. Such measurements benchmark theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project. Subject headings: atomic data — atomic processes — methods: laboratory

Transferability of the Valence Ionization Intensities of Chemical Functional Groups between Molecules

The transferability of the valence ionization intensities of chemical functional groups is investigated with three common laboratory ionization sources (Ne Iα, 16.7 eV; He Iα, 21.2 eV; and He IIα, 40.8 eV) for a series of molecules. Each molecule contains two different functional groups that are well separated electronically and spatially by an alkane chain linker. The optimum length of the alkane chain between the functional groups is investigated. In this initial study, the functional groups are RCl, RBr, RSH, and RFc, where R is an alkane chain and Fc is ferrocene with an alkane chain bonded to one cyclopentadienyl ring. An additional feature of this study is that each functional group contains a molecular orbital with nearly pure atomic character, allowing comparison to theoretical atomic ionization cross-section ratios. In general, the observed photoelectron intensity changes are more gradual than those predicted by either theoretical atomic photoionization cross sections or by a complete Gelius analysis based on molecular orbital compositions obtained from electronic structure calculations. Experimentally, there is a high degree of transferability of the relative ionization intensities of the chemical functional groups between the molecules of this set. These results indicate that an empirical library of the relative ionization cross sections of chemical functional groups as a function of photon energy will be a useful aid to the study of more complex molecules and will provide an experimental foundation for further theoretical studies of molecular photoelectron cross sections.

Photoionization of Ions of Astrophysical Interest: Theoretical Survey

The need for accurate data for the interpretation of astrophysical plasmas continues apace: transition probabilities, electron scattering rates and photoionization cross sections/radiative rates. Planetary nebulae, infra-red galaxies, extragalactic regions are some of the examples where data are required. Missions such as the Infrared Space Observatory, Chandra and Newton are all providing observational data of the highest quality. In this paper, we look at where theoretical photoionization cross section data may be found. We consider first the large scale calculations and attempt to provide some idea of the accuracy of these calculations. We focus particularly on the data produced by the R-matrix method. We then turn to some specific elements considered by the Belfast group. In particular we will discuss the 1s-shell excitation/ejection in the photoionization of the 1s22s22p 2Po ground states of N III and O IV.

MEASUREMENT OF L-SUBSHELL X-RAY FLUORESCENCE CROSS SECTIONS AND L-SUBSHELL FLUORESCENCE YIELDS FOR THE ELEMENTS IN THE ATOMIC RANGE 73≤Z≤92 AT 60 KeV

Li (i=1,2, and 3) X-ray fluorescence cross sections have been measured for seven elements in the atomic range 55≤Z≤ 62 at 15.73 keV. The values of Li subshell fluorescence yields (ω1,ω2, and ω3) have been measured for the same elements using the presently measured cross section values and the theoretical Li subshell photoionization cross sections, Coster-Kronig transition probabilities, and emission rates. The measured X-ray fluorescence (XRF) cross section values are in general agreement with the theoretical values evaluated using L subshell fluorescence yields, Coster-Kronig transition probabilities, emission rates, and subshell photoionization cross sections. Furthermore, the L subshell fluorescence yield values are good agreement with the theoretical values.

Measurement of Lℓ, Lα, Lβ and Lγ X-ray Production Cross Sections and Average L Shell Fluorescence Yields for Elements in the Atomic Range 70 ≤ Z ≤ 92 at 59.54 keV

Lℓ, Lα, Lβ, and Lγ x-ray production cross sections have been measured for the elements Yb, Hf, W, Os, Hg, Tl, Pb, Th and U for 59.54 keV γ-rays from a 100 mCi 241Am filtered source. The average L-shell fluorescence yields have been deduced using the experimentally measured cross section values and the theoretical photoionization cross sections. The experimental results for x-ray production cross sections (Lℓ, Lα, Lβ, Lγ) and average L-shell fluorescence yields are compared with experimental values, theoretical prediction and semiempirical fits. The agreements are reasonably good.

L-subshell fluorescence cross sections and L-subshell fluorescence yields in elements 68 ⩽ Z ⩽ 92 by 59.5 keV photons

L-subshell (σ1,σ2 and σ3) x-ray fluorescence cross sections were measured for Er, Ta, W, Au, Hg, Tl, Pb, Bi, Th and U at an excitation energy of 59.5 keV using a Si(Li) detector coupled to a model 4096 computerized multi-channel analyser. The values of Li-subshell fluorescence yields (ω1, ω2 and ω3) have been measured for the same elements using the previously measured cross section values and the theoretical Li-subshell photoionization cross section Coster-Kronig transition probabilities and emission rates. It is observed that the measured Li (i = 1, 2 and 3) subshell x-ray fluorescence cross section and Li-subshell fluorescence yield (ω1, ω2 and ω3) values for the elements are in general agreement with the theoretical ones evaluated using L-subshell fluorescence yields and the L-subshell photoionization cross section.

Transition metal 2p x-ray photoelectron and high-resolution Kα x-ray emission spectra of K2CrO4 and KMnO4

Chromium and manganese 2p x-ray photoelectron spectra (XPS) were measured for K2CrO4 (potassium chromate) and KMnO4 (potassium permanganate) single crystals fractured in situ in an electron spectrometer. High-resolution Cr and Mn Kα x-ray emission spectra were taken for comparison with the XPS spectra. The observed XPS spectra were deconvoluted by O 1s spectra to remove the inelastic scattering energy loss parts from them. Satellite peaks in the deconvoluted Cr 2p and Mn 2p were not observed. The intensity ratios of 2p3/2 to 2p1/2 of the deconvoluted spectra of K2CrO4 and KMnO4 were 5:1 and 3.5:1, respectively. The ratios were significantly different from the theoretical photoionization cross-sections of 1.93:1. The intensity ratio of Kα1 to Kα2 was about 2:1 for both compounds. Copyright © 1999 John Wiley & Sons, Ltd.

Measurement of the Absolute Photoionization Cross Section of C[TSUP]+[/TSUP] near Threshold

The absolute photoionization cross section of the ground-state C+ ion has been measured from the ionization threshold at 24 eV to 31 eV using a merged ion-photon beam setup with synchrotron radiation from an undulator. The experimental results have been compared with nonrelativistic theoretical data from the Iron Project and the agreement is generally good in the near-threshold region, but differences in the magnitude of the continuum cross section of 15%-35% at higher energies and several deviations in the resonance structure are observed. The present measurements are a part of an experimental program to test the extensive theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project.

Double photoionization and ionization excitation of the metastable heliumSstates

Theoretical double photoionization cross sections are predicted for He initially in the $1s2s{}^{1}S$ or ${}^{3}S$ state. The probability for double photoionization is a factor of 6 higher for the ${}^{1}S$ state at 20 eV above the threshold for double ionization and a factor of 3 larger at 80 eV above threshold. We compare these results with each other and with predictions for the high-energy limit to assess the influence of exchange on the double ejection process. Similar comparisons are also made for the process of photoionization-induced excitation of ${\mathrm{He}}^{+}(n)$.

Experimental and theoretical cross sections for photoionization of metastable and atoms near threshold

Using high-resolution laser photoelectron spectrometry we have determined absolute cross sections and the electron angular distribution parameter for one-photon ionization of metastable and atoms to the resolved ion states near threshold -244 nm). For comparison with the experimental data we have carried out many-electron calculations of the relevant partial cross sections and parameters over the photoelectron energy range (0-6 eV). We have used a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF), designed to take into account the important electron correlation effects which are found to be essential for obtaining satisfactory agreement between the theoretical and experimental results. Including previously reported cross sections for metastable and atoms we present a discussion of the trends which are observed in the different energy-dependent cross sections as a function of the atomic number.

Experimental and theoretical cross sections for photoionization of metastable Xe*. (6s 3 P 2, 3 P 0) atoms near threshold

Using high resolution laser photoelectron spectrometry we have determined absolute cross sections σJ0J1 and the electron angular distribution parameter for one photon ionization of metastable Xe*(6s3PJ0, J0 = 2, 0) atoms to the resolved Xe+ (2PJ1, J1 = 3/2, 1/2) ion states at several wavelengths near threshold. For comparison with the present and future experimental data we have calculated partial cross sections and s-parameters for photoionization of Xe*(6s3PJ0, J0 = 2, 0) and of the analogous alkali atom Cs(6s) over the photoelectron energy range (0–5) eV. We have used both a term-dependent Pauli-Fock (PF) approach and a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF). Through the PF method we include relativistic effects on the atomic orbitals; the CIPF method was designed to take into account the important electron correlation effects which are found to be essential for obtaining good agreement between the theoretical and experimental results.

Experimental and theoretical cross sections for photoionization of metastable Xe $^*$ (6s $^3{\rm P}_2$ , $^3{\rm P}_0$ ) atoms near threshold

Experimental and theoretical cross sections for photoionization of metastable Xe $^*$ (6s $^3{\rm P}_2$ , $^3{\rm P}_0$ ) atoms near threshold

Doubly excited resonance structures in He photoionization from1s2s1,3Smetastable states

The resonant structures corresponding to selected He sp, 2n{sup {plus_minus}} and 2pnd{sup 1,3}P{sup o} autoionization series below the He{sup +} N=2 threshold, following photoionization from 1s2s{sup 1,3}S metastable states, are examined in detail. Theoretical photoionization cross sections, calculated using a B-spline-based configuration-interaction approach with a length-velocity agreement better than 1{percent}, are presented. A sign change in Fano profile parameters q in 1s2s{sup 1}S{r_arrow}sp,2n{sup +}{sup 1}P transitions along the 2n{sup +} autoionization series is identified and analyzed. {copyright} {ital 1997} {ital The American Physical Society}

Long-range interactions of sodium atoms

Long-range interaction potentials between two and three sodium atoms, between a sodium atom and a perfectly conducting wall, and for a sodium atom between two perfectly conducting walls, are calculated with an electric-dipole oscillator strength distribution constructed from combinations of experimental and theoretical energy levels, oscillator strengths, and photoionization cross section data, constrained by accurate values of oscillator strength sum rules. The leading dispersion coefficients for Na-Na and Na-Na-Na and the Lennard-Jones coefficient for the Na-wall system are determined. For Na-Na and for the Na-wall systems, the retarded (Casimir) potentials are also calculated.

Vacuum ultraviolet photoionization study of atomic Ir in the region of the 5p and 4f excitation

A vacuum ultraviolet photoionization study of atomic Ir in the region between 40 and 70 eV was performed with the use of monochromatized synchrotron radiation and photoion detection. The atomic beam of Ir was obtained with the technique of evaporating thin Ir wires. The photoion signals are compared with the theoretical photoionization cross section in the region of the 5p and 4f excitation calculated in the framework of overlapping Fano resonances.

Background subtraction from transition metal 2p XPS by deconvolution using ligand atom XPS: study on first transition metal cyanide complexes

The primary excitation spectra of the first transition metal series 2p X-ray photoelectron spectra for the metal cyanide ions are obtained by a deconvolution method. As the CN− ions coordinate to the transition metal, N1s spectra are used as a response function for the deconvolution using a Fourier transform method. The obtained spectra have flat zero level backgrounds, whereas the universal curve of the Tougaard method cannot remove the full contribution of energy loss. Among some background subtraction methods, the intensity ratios of 2p32 to 2p12 calculated by the present method are closest to the theoretical photoionization cross section ratios.

Experimental and theoretical cross sections for photoionization of metastable atoms near threshold

Using high-resolution laser photoelectron spectrometry we have determined for the first time absolute cross sections and the electron angular distribution parameter for one-photon ionization of metastable atoms to the resolved ion states near threshold as well as the -parameter for metastable atoms at , 257.25 nm. For comparison with present and future experimental data we have calculated partial cross sections and -parameters for photoionization of , and of the analogous alkali atom over the photoelectron energy range 0 - 10 eV. We have used both a term-dependent Hartree - Fock approach and a configuration-interaction method involving Pauli - Fock atomic orbitals (CIPF); the latter was designed to take into account the important electron correlation effects which are found to be essential for obtaining good agreement between the theoretical and experimental results. .

L X-ray fluorescence cross sections in the atomic region 59 ≤ Z ≤ 82 excited by 16.58 keV photons

LL, Lα, Lβ, and Lγ X-ray fluorescence cross sections for Pr, Sm, Gd, Dy, Ho, Er, Yb, Pt Au, and Pb were measured at the excitation energy 16.58 keV. An X-ray tube and a secondary excitor system was used instead of radioisotopes for the measurements. Experimental cross sections are compared with the theoretical estimates based on relativistic Dirac–Hartree–Slater theory. Average L-shell fluorescence yields [Formula: see text] are deduced using the present experimental cross sections and the theoretical subshell photoionization cross sections. The derived average fluorescence yields are fitted by least squares to polynomials in Z of the form ΣnanZn and compared with theoretical and earlier fitted values. Good agreement is observed ' between the experimental results and the theoretical estimates based on relativistic Dirac–Hartree–Slater theory.