Calculation Of Photoionization Cross Sections Research Articles

Photoionisation of deep impurity levels in semiconductors

A new method for the calculation of photoionisation cross sections is presented. The complex band structure of the host semiconductor is calculated using the spherical model of Penn. The wavefunctions are then modified to provide a description of the wavefunction associated with an impurity. Use of the complex band dispersion relation provides a simple explanation of the inadequacy of the Lucovsky model (1965) when applied to very deep states.

Scattered-wave calculations of photoionization cross-sections and asymmetry parameters for CO, H 2O and H 2S

Partial photoionization cross-sections and asymmetry parameters are calculated for the valence orbitals of the molecules CO, H 2O, and H 2S and of the atoms O and S using a recently developed extension of the self-cosistent field— Xα—scattered-wave method to continuum states. The convergence of the partial-wave expansions for both initial and final states is studied for electron kinetic energies in the range 2–1000 eV. Since convergence is very slow at high kinetic energies, the interesting region between 2 and 50 eV is emphasized, and comparisons are made with experimental UV photoemission results where such data are available. Overall the method appears to be far more reliable than previous calculations for polyatomic molecules which have used plane-wave or orthogonalized plane-wave final states.

Photoionization cross section of water by an atomic extrapolation method

The recently developed molecular form of the ground state inversion potential method (GIPM) is applied here to the calculation of the total subshell photoionization cross sections of the water molecule in the energy range from threshold to 50 eV. The results show that the ionization can be accurately represented as a superposition of atomic processes.

Many body perturbation calculations of photoionization

The application of many body perturbation theory to the calculation of atomic photoionization cross sections is reviewed. The choice of appropriate potential for the single-particle states is discussed and results are presented for several atoms including resonance structure. In addition to single photoionization, the process of double photoionization is considered and is found to be significant.

Calculated photoionization cross sections for SF6

We have calculated the photoionization cross sections for the valence levels of S${\mathrm{F}}_{6}$,using multiple-scattering $X\ensuremath{\alpha}$ methods for both the initial and final states. The theoretical curves are compared with measured cross sections in an attempt to complete the identification of observed photoionization peaks with theoretically predicted valence levels. We find general agreement between theory and experiment, although one notable difficulty remains.

Orthogonalized-plane-wave approximation in the calculation of photoionization cross sections of molecules

The physical bases of the OPW approximation in the calculation of molecular photoionization cross sections are considered in terms of the scattering theory, and the ways of improving the calculation within the framework of OPW approximation are discussed. Numerical calculations are done on several molecules by the ordinary OPW method and by its modifications using the bound orbitals obtained by ab initio SCFMO calculations with gaussian basis set.

Calculation of photoionization cross sections by the absorbing boundary method

A new method for the calculation of quantum state decay rates, the absorbing boundary method (ABM), has recently been developed. In this talk the general form of the ABM will be outlined. The pseudo bound state approach to the calculation of continuum rates such as photoionization cross sections and used by Langhoff and coworkers will be briefly reviewed and shown to lack precision in the absence of information about the line widths and shapes of the pseudo bound states. It will then be noted that the ABM can be used to construct a rigorous new approach to the calculation of photoionization cross sections incorporating the necessary lifetime information for the pseudo bound states while still allowing the computation to be carried out by bound state procedures.

Calculation of atomic and molecular photoionization cross sections using the ground state inversion potential method

Calculations of atomic photoionization cross sections to near Hartree-Fock accuracy can be achieved very simply if the non-local Hartree-Fock potential experienced by the photoelectron is replaced by a local potential obtained by inversion of the corresponding ground state orbital. Subshell cross sections for atomic photoionizatim for representative atoms in the range 0–1500 eV calculated in this way are in good general agreement with Hartree-Fock values and with experiment. The extension to molecular systems and the use of an independent atomic centre approach to calculation of molecular photoionization cross sections at low photon energies is outlined.

K-shell photoabsorption spectra ofN2andN2O using synchrotron radiation

The total photoionization cross section of the $1s$ level of nitrogen in ${\mathrm{N}}_{2}$ and ${\mathrm{N}}_{2}$O molecules has been measured using monochromatized synchrotron radiation. High-resolution spectra in the range $390<\ensuremath{\hbar}\ensuremath{\omega}<450$ eV were recorded. A broadband in the continuum at 9.4 eV above the ionization threshold, due to the $f$-type shape resonance, has been measured with high resolution. The data are compared with theoretical calculations of photoionization cross section. Recent Hartree-Fock calculations in the static-exchange approximation agree better than earlier results, which employed the multiple-scattering method.

Radiative recombination of complex ions

view Abstract Citations (78) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiative recombination of complex ions. Gould, R. J. Abstract The rates of radiative capture by atomic systems is computed for the first four ionization stages of the abundant elements C, N, O, Ne, Mg, Si, S, and Ar. A simple prescription is given for calculating the rates for systems in higher ionization stages. Results for capture to atomic helium are also given. In the general case, recent calculations of photoionization cross sections have been used to compute capture rates to levels of the ground-state configuration of the recombined species. Captures to other levels of the valence shell and to the higher shells are evaluated using hydrogenic formulae but with a correction factor which takes into account the nonhydrogenic nature of these states. This correction factor is due to the incomplete shielding by the inner electrons and represents essentially an effective charge for the recombining ion; it is determined semiempirically from an appropriate weighting of values derived from the observed level structure for each species. In some cases, for the capture to neutral atomic systems, its application increases the resulting computed recombination rate by a significant amount (about 50%). Computed recombination rates are tabulated for the four ionization stages at an electron temperature of 10,000 K and for the first stage at 100 K. A convenient procedure is outlined for evaluating the rates at other temperatures in the general neighborhood of these values. Publication: The Astrophysical Journal Pub Date: January 1978 DOI: 10.1086/155772 Bibcode: 1978ApJ...219..250G Keywords: Astrophysics; Cosmic Plasma; Photoionization; Radiative Recombination; Atomic Energy Levels; Electron Energy; Electron Transitions; Hydrogen; Ionization Cross Sections; Recombination Coefficient; Temperature Effects; Atomic and Molecular Physics; Atomic Processes full text sources ADS |

Molecular Photoionization Cross Sections and Their Angular Distributions Calculated by OPW, EOPW and HOPW Methods

The physical bases of the OPW approximation in the calculation of molecular photoionization cross sections are considered in terms of the scattering theory, and the way of improving the calculation within the framework of OPW approximation are discussed. Numerical calculations are done on H2O molecule by the ordinary OPW method and by its modifications.

The ground state inversion potential method: Application to the calculation of photoionization cross sections

We present a simple one-electron central potential method which allows atomic photoionization cross sections to be calculated to an accuracy comparable to that of the best Hartree–Fock calculations available. The method relies upon the frozen core assumption and the fact that a good approximation to the Hartree–Fock potential seen by the photoelectron can be obtained by inversion of the corresponding ground state orbital as tabulated in the literature. The potential is assumed to be local and independent of final state energy in distinction to the true Hartree–Fock potential which is generally nonlocal due to exchange. Subshell photoionization cross sections for photon energies between 0 and 1500 eV have been calculated for helium, neon, argon, and krypton and compared with the Hartree–Fock results of Kennedy and Manson and available experimental values. The agreement is good except for near-threshold cross sections for deep lying orbitals. The method is compared with the Herman–Skillman approximation. The simplicity and computational efficiency of the present method suggest that it will be of use in practical calculations, particularly for heavy atoms. It offers a promising alternative to the widely used Hartree–Slater and Herman–Skillman calculations.

Moment-trace calculations of atomic and molecular photoabsorption cross sections

Regularization procedures employing positive integer spectral power moments are described for calculation of atomic and molecular photoionization cross sections. Use of positive moments avoids matrix diagonalizations, and thus appears to have substantial computational advantages.

Theoretical Calculation of Relative Intensities in ESCA

Calculations of the photoionization cross-sections of atomic and molecular levels for the A1Kα and MgKα lines depending on the wave function of the final and the initial states are analysed. The ns, np and nd photoionization cross-sections of atomic shells for Z ⩽ 36 and the HF, NH3, H2O, CH4, CH3F, CH2F2, CHF3, CF4, N2, CO and C2H4 molecular levels for the YMζ line and the photoionization cross-sections of atoms for Z ⩽ 36 and the CH4, N2 and H2O molecular levels for the ZrMζ line are calculated. The theoretical cross-sections of atoms for Z ⩽ 10 for the FKα line are presented too. Photoionization cross-section calculations using Hartree-Fock-Slater potential are in better agreement with the experiment than those using orthogonalized plane waves for photo-electron wave functions.

Photoelectron intensity spectroscopy  analytical and structural information from peis applied to zeolites

X-ray photoelectron spectroscopy (XPS or ESCA) based on calculated photoionization cross sections was used to investigate the surface Si/Al ratio of different zeolites and the cation distribution in Ag- and Ca-exchanged NaA-zeolites. We find the same module at the surface as in the bulk, but a strong de-alumination of the surface after HCl treatment. The Ag + ions are enriched and the Na + ions depleted in the outer cavities of the zeolite crystals, both in strong dependence on the Ca content.

Application of the relativistic random-phase approximation to the photoionisation of atoms

Exploratory calculations of atomic photoionisation cross sections including both relativistic and correlation effects are undertaken using the relativistic random-phase approximation (RRPA). In these preliminary calculations the authors examine the photoionisation of He and Be atoms. The cross sections and asymmetry parameters for these light elements agree precisely with non-relativistic RPA values. Elastic phaseshifts for the e-+He+ and e-+Be+ systems are obtained and agree with non-relativistic calculations in the 1P eigenchannel; the 3P phaseshifts have no counterpart in non-relativistic theory.

Multiconfiguration Hartree-Fock calculation of photoionization cross sections of the rare gases

A multiconfiguration Hartree-Fock calculation is used to test the hypothesis that ground-state correlations involving excitation of a single of electrons from an outer $p$ shell into $d$ orbitals are the dominant ones in the photoionization cross sections of Ne, Ar, Kr, and Xe. Dramatic improvement in the agreement of dipole length and dipole velocity forms of the cross section is obtaining in going from a Hartree-Fock to a multiconfiguration Hartree-Fock calculation. Results are found to be comparable to recent values obtained using the random-phase approximation with exchange and the $R$ matrix.

Relative intensities in X-ray photoelectron spectra part III

Calculations for the photoionization cross-sections of atomic and molecular levels for the Al Kα and Mg Kα lines depending on the wave function of the final and the initial states are analysed. The ns, np, and nd photoionization cross-sections of atomic shells for Z ⩽ 36 and the HF, NH 3, H 2O, CH 4, CH 3F, CH 2F 2, CHF 3, CF 4, N 2, CO, and C 2H 4 molecular levels for the YMξ line and the photoionization cross-sections of atoms for Z ⩽ 36 and the CH 4, N 2, and H 2O molecular levels for the ZrMξ line are calculated. Photoionization cross-section calculations using the Hartree—Fock—Slater potential are better agreement with experiment than those using orthogonalized plane waves for photoelectron wave functions.

Application examples of calculated photoionization cross sections and mean free paths to the quantitative surface analysis by X-ray photoelectron spectroscopy

Photoionization cross sections and mean free paths are calculated and compared with the experimentally obtained values. As an example of application the surface analysis of Ag-Au, Au-Cu and Cu-Ni alloys is discussed.

Photoionization cross-section calculations for 4d 10 subshell of In in the RPAE

Photoionization cross-section and angular distribution of photoelectrons for 4d 10 subshell of In are calculated in the Random Phase Approximation with Exchange (RPAE). The cross section is in satisfactory agreement with the recent experimental data by Stanford synchrotron.