Total Photoionization Cross Sections Research Articles

Mirroring doubly excited resonances in argon.

New features are revealed in the low-energy photoionization spectrum of Ar by critically combining high photon resolution and differential photoelectron spectroscopic techniques. Two LS-forbidden doubly excited resonances are seen in the 3p(-1)(3/2, 1/2) partial cross sections which exhibit mirroring profiles, resulting in complete cancellation in the total photoionization cross section, as was predicted by Liu and Starace [Phys. Rev. A 59, R1731 (1999)]. These results demonstrate that a new class of weakly spin-orbit induced, mirroring resonances should be observable in partial, but not in total, collisional cross sections involving atoms, molecules, and solids in general.

The photoabsorption of Eu, Eu + and Eu 2+ in the vicinity of 4d thresholds

The results of calculations of the total and partial photoionisation cross-sections of neutral Europium and its ions in the photon energy range from the threshold up to 270 eV are presented. The calculations were performed within the multi-channel Spin-Polarized Random Phase Approximation with Exchange. The main attention is given to the giant autoionizing resonance associated with the 4d → 4f discrete transition to vacant states in the 4f 7 subshell. The comparisons with experimental data obtained recently and the results of previous calculations are made.

Near threshold photoionization of excited alkali atoms Ak(np) (Ak = Na, K, Rb, Cs; n = 3-6)

The effect of the polarization of the atomic core by the outer electron on near threshold photoionization of excited alkali atoms Ak(np) (Ak = Na-Cs; n=3-6) is investigated. Partial and total cross-sections for photo-ionization of the np-electron were computed utilizing the configuration interaction technique with Pauli-Fock atomic orbitals (CIPF) and including the long range core polarization potential (CP). To calculate the core polarization potential the variational principle is applied. Comparison with previous theoretical results and with available experimental data is made for the total cross-section , for the electron angular distribution parameter , for the ratio of the reduced electric dipole matrix elements and for the phase shift difference , associated with the d-wave and s-wave continua, respectively. In the comparison, new experimental results for , , and , measured for laser-excited, polarized 39K(4p 3/2) atoms, have been included.

Generalization of the atomic random-phase-approximation method for diatomic molecules:N2photoionization cross-section calculations

Partial and total photoionization cross sections of ${\mathrm{N}}_{2}$ molecule are calculated using the generalization of the random-phase approximation (RPA) which earlier has been successfully applied to the description of the atomic photoionization processes. According to this method, at first the Hartree-Fock (HF) ground-state wave functions are calculated in prolate spheroidal coordinates using the fixed-nuclei approximation. With their help the zero order basis set of single particle Hartree-Fock wave functions containing both discrete excited states and continuous spectrum is calculated in the field of a frozen core of a singly charged ion. The calculations are performed for all four valence shells of ${\mathrm{N}}_{2}$ molecule, $3{\ensuremath{\sigma}}_{g},$ $1{\ensuremath{\pi}}_{u},$ $2{\ensuremath{\sigma}}_{u},$ and $2{\ensuremath{\sigma}}_{g},$ with the intershell correlations fully taken into account within the RPA method. It is demonstrated that different intershell correlations, especially between three outer shells, play an important role in photoionization process. Examples of the influence of intershell correlations on several transitions are presented. Partial and total photoionization cross sections of ${\mathrm{N}}_{2}$ molecule obtained by this method in the photon energy range from ionization threshold up to 70 eV are in a good agreement with the existing experimental data and with the recent RPA calculations [Cacelli et al., Phys. Rev. A 57, 1895 (1998)].

Measurements of electron-impact ionization cross sections of argon, krypton, and xenon by comparison with photoionization

Ratios of total cross sections for electron-impact ionization and photoionization in Ar, Kr, and Xe in the energy range from 140 to 4000 eV for electrons and from 16 to 1012 eV for photons were measured. Comparatively low relative standard uncertainties of 1.3--1.9 % were achieved using an apparatus combining two recent instrumental developments. The first is associated with a highly accurate device for the determination of soft-x-ray and vacuum-UV photon flux, a cryogenic electrical substitution radiometer. The second is an upgraded ionization chamber for the precise comparison of total-ion yields for electron and photon impact. On the basis of our measured cross-section ratios and well-known total photoionization cross-section data, we deduced absolute total electron-impact ionization cross sections of the rare gases with relative standard uncertainties as low as 2%.

Photoionization of Al IV (neon-type) from its ground 2s 22p 6 1 S e and excited 2s 22p 53p 3,1 P 0 3 S e states using the R-matrix method

The R-matrix method is used to calculate the total photoionization cross-sections from the ground 2s 22p 6 1 S e and the first three excited 2s 22p 53p 3,1 S e states of Al IV, for photon energies ranging from the first ionization threshold to just above the second threshold of the residual ion Al V. The two lowest LS terms of Al V − 2s 22p 5 2 P 0, 2s2p 6 2 S e, 2s2p 6 2 S e, represented by sophisticated configuration interaction wavefunctions, are included in the R-matrix calculation. The resulting cross-sections are affected by Rydberg series of resonances converging to the 2s2p 6 2 S e excited threshold.

Single and double photoionization of lithium

The photoion ${\mathrm{Li}}^{2+}{/\mathrm{L}\mathrm{i}}^{+}$ production cross section ratio of ground-state atomic lithium has been measured for photon energies ranging from 80 to 424 eV. The absolute cross sections for the ${\mathrm{Li}}^{2+}$ and ${\mathrm{Li}}^{+}$ yield are also derived. In this energy region, the ${\mathrm{Li}}^{2+}{/\mathrm{L}\mathrm{i}}^{+}$ ratio reaches a plateau of about 1.0% before reaching a maximum of about 4.5%, then decreases slowly. Good agreement is found between the measured total photoionization cross sections of lithium and theoretical calculations. The ${\mathrm{Li}}^{2+}{/\mathrm{L}\mathrm{i}}^{+}$ ratio is also compared to the ${\mathrm{He}}^{2+}/{\mathrm{He}}^{+}$ ratio from excited $\mathrm{He}(1s2s)$ for photon energies up to 70 eV above threshold. The branching ratio of ${\mathrm{Li}}^{2+}$ to total Li ion production is also compared to the single-ionization cross section of electron impact on ${\mathrm{Li}}^{+}$ ions.

A theoretical investigation of K-shell photoionization of neon

The total photoionization cross section is calculated for neon (Z = 10) near the 1s threshold using the relativistic random-phase approximation, the relativistic random-phase approximation modified to include relaxation effects, and the relativistic random-phase approximation modified to include relaxation effects and Auger decay. Comparisons are made between theoretical calculations and experimental data. The need to include Auger decay along with relaxation is demonstrated.

K-shell photoionizations in strongly coupled plasmas

In strongly coupled plasmas, the photoionizations from the 1s ground state of hydrogenic ions are investigated using the ion-sphere model potential. The initial bound wave function and energy eigenvalue of the target ion are modified in the ion-sphere potential using the Ritz variation method. The final state of the ejected photoelectron is represented by a plane wave solution. The Coulomb correction is considered using the screened Coulomb wave function with the appropriate effective charge. The K-shell photoionization cross section is obtained for the acceleration form of the matrix element because of the convenience of the calculations. The plasma screening effect is obtained as a function of the ion-sphere radius and photon energy. The retardation correction is also considered in obtaining the total K-shell photoionization cross section in strongly coupled plasmas. The plasma screening effects on the total K-shell photoionization cross section for the ion-sphere radius, RZ⩾2aZ, are found to be less than about 25%. It is also found that the plasma screening effect is almost independent of the incident photon energy.

Double and Triple Photoionization of Ground-State Lithium

The double and triple photoionization of ground-state lithium is investigated in the high-energy limit. Relative to the total photoionization cross section, the fraction of two-electron escape processes is 0.0325, while the fraction leading to triple escape is only $5.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. Double ionization via ${\mathrm{Li}}^{+}$ autoionizing states accounts for more than $40%$ of the two-electron escape processes. A comparison of the high-energy-limit calculations with experiments at 424 eV shows close agreement for double photoionization. A factor of 10 discrepancy exists for triple photoionization at 424 eV, however, which presumably indicates that the high-energy limit has not yet been reached.

Photoionization cross sections and oscillator strengths for oxygen ions: O I-O VII

Ab initio close-coupling calculations are reported using the R-matrix method for photoionization cross sections $({\ensuremath{\sigma}}_{\mathrm{PI}}),$ oscillator strengths (f values), and energy levels (E) for oxygen ions: O I, O II, O III, O IV, O V, O VI, and O VII. Total, partial, and state-specific photoionization cross sections are computed and investigated in detail. Important features are found that should affect overall photoionization and recombination, especially at high temperatures and energies. Although these ions have been previously studied individually, present work aims at studying them as an isonuclear sequence and obtaining more accurate and complete results for applications. More extended eigenfunction expansions than in previous works, including $n=3$ states, show correlation effects primarily manifested as additional groups of resonances in photoionization of oxygen ions such as O VI and O VII that are of importance in extreme ultraviolet and x-ray laboratory and astrophysical plasmas. Lifetimes of excited bound states are obtained from the present oscillator strengths and compared with experiments. The computed radiative atomic parameters, energy levels, oscillator strengths, and photoionization cross sections constitute a larger dataset than the Opacity Project data [M. J. Seaton, J. Phys. B 20, 6363 (1987)], with which comparisons are made.

Inner-shell photoionization of ground-state lithium: Theoretical calculation in the photon energy region of hollow atomic states

Photoion and photoelectron data for hollow lithium states were recently reported with increased spectral resolution. In order to reproduce and identify the important resonances due to ${2ln}^{\ensuremath{'}}{l}^{\ensuremath{'}}{n}^{\ensuremath{''}}{l}^{\ensuremath{''}}$ ${}^{2}{P}^{o}$ autoionizing states new photoionization calculations have been performed using the R-matrix code with a 29-term target representation for incident photon energies between 140 and 167 eV. Excellent agreement between theoretical and measured results over partial or total photoionization cross sections confirms the quality of the theoretical model. A determination of the resonance positions for each series converging to a ${2l2l}^{\ensuremath{'}}$ ${}^{1,3}L$ threshold of Li is proposed.

Measurements of electron-impact ionization cross sections of neon by comparison with photoionization

A method and apparatus have been developed for precise measurements of the ratios of total cross sections for electron-impact ionization and photoionization in rare gases based on the comparison of the total ion yields of the two ionization processes. Low uncertainties for the cross-section ratios are achieved above all by using a cryogenic electrical substitution radiometer as a primary detector standard in the soft-x-ray and vacuum ultraviolet spectral ranges in order to determine the impinging photon flux. On the basis of our measured cross-section ratios and well-known total photoionization cross sections we deduce absolute total electron-impact ionization cross sections of rare gases. As a result we present ratios of total electron-impact ionization cross sections to total photoionization cross sections and deduce total electron-impact ionization cross sections of neon in the energy range of electrons from 140 to 4000 eV and of photons from 100 to 1500 eV. Relative uncertainties as low as 1.3% for the cross-section ratios and 2.8% for the total electron-impact ionization cross sections have been achieved. A comparison of our cross-section data with published experimental and theoretical data is presented.

Plasma screening effects on photoionizations in weakly coupled plasmas

Plasma screening effects on photoionizations from the 1s state of hydrogenic ions in dense weakly plasmas are investigated. The particle interaction potential in weakly coupled plasmas is obtained by the static Debye-Hückel model. The initial bound wave function and energy eigenvalue of the target ion are modified in the Debye-Hückel potential using the Ritz variation method. The final state of the ejected photoelectron is represented by a plane wave solution. The Coulomb correction is considered using the screened Coulomb wave function. The photoionization cross section is obtained the acceleration form of the matrix element in order to investigate the plasma screening effects on the interaction potential. The screening effect is obtained as a function of the Debye length and photon energy. The retardation correction effect is also considered in obtaining the total 1s photoionization cross section in plasmas. The plasma screening effects on the photoionization cross section for the interesting domain of the Debye length, Λ⩾10aZ, are found to be less than 10%. It is also found that the plasma screening effect is almost independent of the incident photon energy.

Multichannel photoionization spectroscopy of Ar: Total cross section and threshold photoelectrons

Argon photoionization is studied using the $R$-matrix technique. Total photoionization cross sections, as well as partial cross sections for emission of a $3s$ electron, are obtained and compared with experimental results in the photon energy range between 30 and 38 eV. As a by-product of the photoionization calculations, threshold cross sections have been estimated for ${\mathrm{Ar}}^{+}$ states up to 38 eV above the Ar ground state, and are compared with experiment. These comparisons show that the present approach can describe Ar autoionizing states reasonably well, even up to 38-eV photon energy. However, some modifications of previously used techniques have proven to be important in order to stabilize the calculation in this difficult energy range.

Diffraction effects in the photoionization of clusters

In many atomic or molecular clusters an electron cloud exists which is delocalized over a volume of well defined shape. It is shown that the total and partial photoionization cross sections of those clusters oscillate on a scale of energy typically reached by synchrotron radiation. The frequencies of the oscillations are rrlated to geometrical properties of the electron cloud, such as its thickness and the diameter of the cluster. These properties can in principle be extracted from the experimental photo cross section.. As specific examples we discuss an alkali-metal cluster (Na 40) and the fullerene C 60.

Photoionization of ammonia

We report the results of separated-channel, frozen core calculations of partial and total photoionization cross sections and asymmetry parameters for NH 3. The calculations were performed using the complex Kohn variational method. The results for the total and partial cross sections agree well with experiment as well as previous theoretical calculations. The asymmetry parameters are found to be a more sensitive test of the accuracy of the wave function.

Electronic properties of isolated Ga N As M clusters: photoionization-, photodissociation- and photoluminescence quantum yields

Photoionization quantum yields of GaNAsM clusters with N + M = 85 ± 5 atoms in the spectral range of ω = 3.5–6.4 eV have been investigated by measuring their total photoabsorption and photoionization cross sections. It is found that the photoionization quantum yields of these clusters are strongly increased by about two orders of magnitude against the values of bulk GaAs. Photodissociation of the cluster provides another efficient channel whereas photoluminescence plays a minor role for the observed cluster size distributions with N + M typically between 30 and 130 atoms.

An investigation of the photodissociation of molecular oxygen in the 75 to 85 nm region

Abstract The cross section for photodissociation of molecular oxygen in the 75 to 85 nm region has been obtained from data for the total absorption and photoionization cross sections. This spectrum, together with published data, has been used to adjust a theoretical model for photodissociation in which three Rydberg states were each coupled to two newly proposed dissociating states. It was found that the observed photodissociation cross sections can be approximated by a model in which there is no direct transition from the ground state to one of the dissociating states. It is proposed that this dissociating state has the symmetry 3Σu+, while the other has either 3Σu− or 3Πu symmetry.

Calculations of photodissociation and photoionization cross sections by using a recursive expansion of Green's operator with absorbing boundary conditions

An iterative method for calculating total cross sections for molecular photodissociation and atomic photoionization is presented. It is based on the recursive expansion of Green's operator with absorbing boundary conditions, recently developed by Mandelshtam and Taylor. The method is tested by calculating cross sections for photodissociation of the molecule and another model problem, as well as for the photoionization of a hydrogen atom in the absence and presence of a strong magnetic field. In all applications the discrete variable representation is employed.