Theoretical Photoionization Cross Sections Research Articles

K and L Shell X-Ray Fluorescence Cross Sections

The K and L shell x-ray fluorescence (XRF) cross sections for elements with 13 ≤ Z ≤ 92 and 35 ≤ Z ≤ 92, respectively, are calculated for an incident photon energy range 1-200 keV. The XRF cross sections for Kα, Kβ, Ll, Lα, Lβ, and Lγ groups of x rays are tabulated. These calculations use the comprehensive sets of physical parameters currently considered most reliable, namely, theoretical partial photoionization cross sections and x-ray emission rates of Scofield, K shell fluorescence yields compiled by Hubbell, and L i subshell fluorescence yields, Coster-Kronig transition probabilities, and K to L i subshell vacancy-transfer probabilities based on the Dirac-Hartree-Slater model.

Wavelength dependence of the nonresonant photoionization cross section of a two-electron atom near the ionization threshold

We present the theoretical near-threshold photoionization cross sections of He atoms from bound excited states, using a B-spline-based configuration-interaction method. Our study has shown that in the absence of a strongly energy-dependent feature such as a doubly excited resonance, the photoionization cross sections in the immediate vicinity of the ionization threshold depend linearly on the wavelengths of the incident light. Qualitatively, following the quantum-defect theory, this linear dependence can be linked directly to the expected nearly energy-independent scattering phase shift corresponding to the wave function of the ionized electron at near-zero photoelectron energy.

L X-ray fluorescence cross sections in the atomic region 46⩽ Z⩽51 excited by 6.47, 7.57 and 8.12 keV photons

Total L X-ray fluorescence cross sections have been measured for the elements Sb, Sn, In, Cd, Ag and Pd excited by 6.47, 7.57 and 8.12 keV photons. The measurements have been performed using an X-ray tube with a modified secondary exciter system as the excitation source. In order to improve the efficiency of the detection system, the excitation source, detector and the target assembly were placed in a vacuum chamber. The measurements were made at a pressure of 10 −2 mbar. The present system considerably reduces the scattering and background effects and improves the monochromaticity. The experimental results have been compared with the theoretical values evaluated using L-subshell fluorescence yields ( ω i ) and Coster-Kronig transition probabilities ( ƒ ij ) based on relativistic Dirac-Hartree-Slater (RDHS) theory. It is observed that the L X-ray cross-section values for the elements in the region are in good agreement with the theoretical values based on RDHS theory. The average L-shell fluorescence yields have also been computed using the experimentally measured cross-section values and the theoretical photoionization cross sections. The average fluorescence yields have been compared with the available results in the literature.

Photoelectron Spectroscopic Studies of the Electronic Structure and Bonding in TiC and TiN

Titanium carbide (TiC) and titanium nitride (TiN) possess remarkable physical properties, such as extremely high hardness and melting point, that promote their use as antiwear materials under harsh tribological conditions. These physical properties must arise from chemical bonding phenomena that result from the inclusion of the non-metal atom within the metallic matrix, and these bonding phenomena should be apparent in measurements of the valence-band electronic structures of TiC and TiN. This paper explores the surface electronic structure and bonding in TiC(100) and TiN(110) with core and valence level photoelectron spectroscopies (PES's) using X-rays (1486.6 eV) and synchrotron radiation in the range 28-180 eV. Intensity changes in the valence-band features are followed as a function of incident photon energy; these changes are then compared to theoretical atomic photoionization cross sections to determine the atomic origins of these features. Resonant PES at the Ti 3p absorption edge is used to determine titanium 3d contributions to the valence band and to show differences in the electronic structures in TiC and TiN. A new resonance phenomenon near the Ti 3s edge in TiC was observed and its possible assignment is discussed. The electronic structure and bonding in these materials is well described by molecularmore » orbital theory, where the Ti and non-metal ions in their formal oxidation states (e.g., Ti[sup 4+] and C[sup 4[minus]] in TiC) undergo covalent bonding interactions. Overall, the PES results indicate greater covalent mixing for TiC as compared to TiN, consistent with the differences in the electronegativities of the atoms. Specifically, stronger covalent interactions between the C 2s, 2p and the Ti 3d, 4s, 4p levels must occur to explain the spectroscopic differences between TiC and TiN.« less

Photoionization from 1sns 1,3Se states of helium.

We present theoretical photoionization cross sections from the 1sns 1,3 S e bound excited states of the helium atom based on a configuration-interaction procedure for the continuum, using a finite L 2 -basis set constructed from a nearly complete set of B-spline-based one-particle hydrogenic orbitals. In addition to the nonresonant spectra near the first ionization threshold, resonant structures associated with selected sp, 2n ± and 2pnd 1,3 P o autoionization series below the He + N=2 threshold are examined in detail. The widths and resonance energies of all 1,3 P autoionization series are also reported and compared with other available theoretical and experimental results

Accuracy of the Tougaard method for quantitative surface analysis. Comparison of the Universal and REELS inelastic cross sections

Three tests have been performed to investigate whether cross sections determined experimentally from reflection electron energy loss spectroscopy (REELS) are more accurate than the Universal cross section for background correction of electron spectra. In the first test, the shape of background corrected spectra from a thin layer and from an infinitely thick layer of the same element were compared for the Ag3d, Au4d, and Yb4d peaks. It was found that the Universal cross section is more accurate than the experimentally determined REELS cross sections to determine consistently the peak shape of background corrected spectra. In the second test, the intensity ratio of the two components in the Au4d, Cu2p, Zn2p, and Yb4d doublets were compared to theoretical photoionization cross sections. For both the Universal and the REELS inelastic cross sections, the obtained intensity ratios agree well with theory. In the third test, we compare with theory the peak intensity ratios to Au4d of major peaks from Si, Cu, Ag, Yb, and Au obtained by using the Universal and the REELS cross sections for background correction. For the metals, the two cross sections give peak intensity ratios that are equally close to theory to within the expected uncertainties in the theoretical peak intensity ratios. However, for Ag with application of the Universal cross section the deviation from theory is slightly larger. For Si the REELS cross section is clearly most accurate.

X-ray transmission calculations for an aluminum plasma

Computer programs developed recently at Los Alamos have been used to calculate the transmission of x rays through an aluminum plasma. Theoretical energy levels, oscillator strengths, and photoionization cross sections were combined with the local thermodynamic equilibrium population model to calculate the plasma absorption coefficient as a function of photon energy. The transmission spectrum is simulated by accounting for the plasma depth and spectrometer characteristics. These results are compared to spectra observed during recent experiments; excellent agreement is obtained.

X-ray photoelectron spectroscopic (XPS) studies of iodine oxocompounds

The core electron binding energies of eighteen compounds containing iodine in different oxidation states ranging from (-I) to (VII) have been measured. The observed chemical shifts differ from the literature data. The anticipated existence of hexavalent iodine in certain compounds has not been demonstrated. The relative subshell photoemission intensities of iodine have been determined. Empirically derived atomic sensitivity factors and theoretical photoionization cross-sections have been used to calculate the surface stoichiometry. The concentration ratios obtained via the two methods are compared. In agreement with generally accepted view the results of this comparison imply that, for quantitative surface analysis, empirical values should be used rather than theoretical data.

Photoionization cross sections of rovibrational levels of the B 1Σ+u state of H2

We report theoretical cross sections for direct photoionization of specific rovibrational levels of the B 1Σ+u electronic state of H2. The calculated cross sections differ considerably from values recently determined by resonant enhanced multiphoton ionization (REMPI) studies. In an attempt to understand the disagreement, we analyze in detail the REMPI dynamics and find that the multiphoton ionization probability is extremely sensitive to the spatial and temporal profiles of the laser pulses. Accurate characterization of laser profiles and their jitter is therefore necessary for a comparison between theory and experiment.

Comparative He I and He II photoelectron spectroscopic studies of the benzoyl halides

The He I and He II photoelectron spectra of benzoyl fluoride, benzoyl chloride and benzoyl bromide have been recorded and subjected to a relative band-intensity analysis. A comparison of experimental intensity data with the results of theoretical photoionization cross-section calculations is combined with a composite-model approach in interpreting the spectra. This interpretation supports the photoelectron-spectral assignments provided by appropriate molecular-orbital calculations. The low-ionization-energy regions of the benzoyl chloride and benzoyl bromide spectra comprise the localized benzene and non-bonding oxygen and halogen orbitals, while the corresponding fluorine orbitals are embedded in the band structure observed at higher ionization energy.

Photoionization of group-IIBelements

Theoretical photoionization cross sections, angular distributions, and spin-polarization parameters for outer $\mathrm{ns}$ and $(n\ensuremath{-}1)d$ subshells of the group-$\mathrm{II}B$ elements zinc, cadmium, and mercury are determined above their respective $(n\ensuremath{-}1)d$ thresholds. Account is taken of electron-electron correlation and of the spin-orbit interaction by means of the relativistic random-phase approximation. Comparisons are made with previous theoretical work and with available experimental measurements.

Excitation Efficiencies for K and L X-Ray Fluorescence by a Mo Transmission Tube

The excitation efficiencies for K- and L- x-ray fluorescence by a Mo transmission tube operated at 30 kV have been measured. K x-ray excitation efficiencies for 14 elements ranging from A1 to Y and L x-ray excitation efficiencies for 12 elements ranging from Br to U are presented. The K x-ray excitation efficiencies are compared to those predicted from theoretical photoionization cross sections and empirical fluorescence yields, and the L x-ray excitation efficiencies are used in conjunction with theoretical photoionization cross sections to derive average L-shell fluorescence yields.

X-ray photoionization cross sections for quantitative analysis

The relative photoemission intensities from subshells of 51 elements and simple compounds have been measured using X-ray photoelectron spectroscopy. The chemical composition of each specimen surface was monitored by Auger electron spectroscopy and contamination was minimized by Ar + ion bombardment. Experimental photoelectron cross sections derived for MgKα (1254 eV) radiation were compared with theoretical Hartree-Slater subshell photoionization cross sections calculated by Scofield. Good correlation ( r = 0.96) between theory and experiment was observed over two orders of magnitude for most of the elements studied. Relative cross sections for subshells of a given element are also consistent with theoretical values, with several notable exceptions. The results indicate that quantitative analysis of ESCA spectra is possible using the Scofield cross sections.

Dipole Spectrum of Water Vapor and Its Relation to the Energy Loss of Fast-Charged Particles

Experimental and theoretical photoabsorption and photoionization cross sections and quantum mechanical sum rules are used to construct nonrelativistic dipole oscillator strength distributions for the following ''processes'' involving the ground state water molecule: total absorption, total ionization, total excitation to neutral species, and partial ionization to the ions H$sub 2$O$sup +$, OH$sup +$, and H$sup +$. These distributions are used to evaluate a number of dipole oscillator strength sums that are important in the radiation physics and chemistry of the water molecule and in the evaluation of the dipole properties of H$sub 2$O. Various parameters that occur in the Bethe--Born approximation for the total inelastic scattering cross sections that characterize the energy loss spectrum of fast-charged particles in water vapor are also evaluated and, where possible, compared with experimental results. The dipole oscillator strength sums are also used to evaluate the mean excitation energies for the stopping power and straggling of fast-charged particles in water vapor; I$sub 0$ = 70.8 eV and I$sub 1$ = 935 eV. (2 figs, 5 tables, 67 refs)

Theoretical photoionization cross-sections, relative experimental photoionization intensities, and angular distributions of photoelectrons

Theoretical photoionization cross-sections, relative experimental photoionization intensities, and angular distributions of photoelectrons

Effect of Core Polarizability on Photoionization Cross-Section Calculations

Demonstration of the importance of core polarizability in a case where cancellation is only moderate, with suggestion of an improvement to the scaled Thomas-Fermi (STF) wave functions of Stewart and Rotenberg (1965). The inclusion of dipole polarizability of the core for argon is shown to substantially improve the agreement between the theoretical and experimental photoionization cross sections for the ground-state configuration.