Partial Photoionization Cross Sections Research Articles

Photoionization of Ar XVI and Ar XVII

Photoionization of Li-like argon, (hν+ArXVI→ArXVII+e), and He-like argon, (hν+ArXVII→ArXVIII+e), are studied using the relativistic Breit–Pauli R-matrix (BPRM) method. Results include both the partial and total photoionization cross-sections of fine structure levels with 1/2≤J≤17/2, n≤10 and 0≤l≤9 of Ar XVI and with 0≤J≤10, n≤10 and 0≤l≤9 of Ar XVII. They correspond to 98 bound levels of Ar XVI and 191 of Ar XVII. The close coupling wave function expansion for Ar XVI included 17 core levels up to n=3 thresholds and that of Ar XVII 16 core levels up to n=4 thresholds. Because of high-lying core excitations of these ions, the photoionization cross-section, σPI(nSLJ), decreases monotonically without features until at high energies where narrow high-peak resonances of Rydberg series of autoionizing states belonging to excited n=2 thresholds begin to appear. The dominant photoexcitation-of-core (PEC) or Seaton resonances also appear in the excited level cross-sections. The background of the total cross-section shows enhancement at n=2 thresholds due to resonant 1s–2p core excitations. However, the resonances become much weaker beyond n=2 thresholds. Radiation damping of resonances, important for these highly charged He- and Li-like ions, is included. These cross-sections are much improved over the existing TOPbase cross-sections where resonances, and background enhancements for Ar XVII were not included. With consideration of these effects, the present results should be accurate up to 10%–30% for various applications in modeling, recombination rates, opacities and diagnostic spectral analysis in the ultraviolet and X-ray regions. Cross-sections for H-like Ar are also included for completeness.

Experimental and theoretical investigations on photoabsorption and photoionization of trimethylphosphate in the vacuum-ultraviolet energy range

In this work, we report a joint experimental-theoretical investigation on interaction of vacuum-ultraviolet radiation with trimethylphosphate (TMP) molecule (C(3)H(9)O(4)P) in gas phase. This species together with tetrahydrofuran (THF) are model compounds of deoxyribose nucleic acids (DNA)/ribose nucleic acids (RNA) backbone. Absolute photoabsorption cross sections (σ(a)) and ionization yields (η) are measured using the double-ion-chamber technique in the 11.0-21.45 eV energy range. Photoionization (σ(i)) and neutral-decay (σ(n)) cross sections in absolute scale are also derived. Moreover, theoretical photoabsorption cross sections are calculated using the time-dependent density functional theory from the excitation threshold up to 16 eV. Good agreement between the present calculated and experimental photoabsorption cross sections in the 11.0-14.5 eV range is encouraging. Also, the present measured data of σ(a) and σ(i) for TMP are about 1.3 and 1.5 times of those of THF, respectively. Thus, the experimental evidences that the majority of strand breaks being located at sugar rings in the irradiated DNA/RNA backbone moiety may be induced by a possible migration of the hole, initially created at phosphate group, to the linked sugar groups. Finally, absolute partial photoionization cross sections are derived from the experimental time-of-flight mass spectra.

Near-threshold photoionization from the excited mp5 (m+l)p J = 0 − 3 levels of Ar, Kr, and Xe (m = 3 − 5)

Using the configuration interaction Pauli-Fock including core polarization (CIPFCP) approach we have calculated absolute total and partial cross sections for photoionization of the excited mp5(m+1)p J = 0 − 3 levels in Ar, Kr and Xe (m = 3 − 5) near threshold, including the region of the odd nl' (l' = 0, 2, 4) autoionizing resonances. For several intermediate levels of Ar and Kr, the computed cross sections are compared with laser spec-troscopic measurements; the theoretical and experimental lineshapes and branching ratios for different resonances show good mutual agreement.

Experimental determination of the ratio of partial photoionization cross sections from Na 3p 2P3/2 by polarization anisotropy quantum beats

We demonstrate that polarization anisotropy quantum beats (QB) in two-photon ionization can be used to measure the ratio of photoionization cross sections to different angular momentum states of the photoelectron. Measurement of the ratio of photoionization cross sections has so far remained a non-trivial task by conventional photoionization techniques, including photoelectron angular distribution analysis. In this work, the amplitude of the polarization anisotropy QB for photoionization from the Na 3p 2P3/2 state was used to determine the ratio of the ionization cross sections near the ionization threshold. The experimental results are compared with theoretical predictions based on the quantum defect theory.

A Lippmann–Schwinger approach for the determination of photoionization and photodetachment cross sections based on a partial wave Green's function expansion and configuration interaction wave functions

A computational approach is presented for determining partial differential cross sections for molecular photodetachment and photoionization. The approach, which extends a recent treatment of negative ion photodetachment, uses multireference configuration interaction wave functions to describe both the target or residual molecule and its interaction with the scattered electron and can treat states of the residual molecule which can be described adiabatically or are non-adiabatically coupled by conical intersections. Antisymmetry is taken into account in an approximate manner and a Lippmann–Schwinger equation is used to treat the interaction of the ionized/detached electron with the residual molecule. A partial wave expansion of the requisite Green's functions enables a unified treatment of photoionization and photodetachment and achieves computational efficiencies. A matrix representation of the quantities appearing in the iterated solution to the Lippmann–Schwinger equation using the radial Green's function permits the iterated solution to be summed eliminating potential convergence issues. Two methods for determining the radial Green's functions are presented. In the standard approach the Green's function is determined using the regular and irregular solutions to the corresponding homogeneous equation. The numerical issues associated with this well-studied form of the Green's function are non-trivial. An alternative approach is introduced which uses the complex scaling technique to construct the relevant matrix elements of the Green's function. The degree of simplification achieved in this way is significant.

Ba 5s photoionization in the region of the second Cooper minimum

We investigate the 5s angular distribution parameter and partial photoionization cross section of atomic Ba in the region of the second Cooper minimum covering a photon energy region from 120 to 260 eV. We observe a strong drop in the Ba 5s β value from 2.0, reaching a minimum of 1.57 ± 0.07 at a photon energy of 150 eV. The β value then slowly rises back towards its nominal value of 2.0 at photon energies beyond the minimum. Our measured 5s partial cross section also shows a pronounced dip around 170 eV due to interchannel coupling with the Ba 4d photoelectrons. After combining our measurements with previous experimental values at lower photon energies, we obtain a consistent data set spanning the photon energy range prior to the onset of the partial cross section maximum and through the cross section minimum. We also calculate the 5s partial cross section under several different levels of approximation. We find that the generalized random-phase approximation with exchange calculation models the shape and position of the combined experimental cross section data set rather well after incorporating experimental ionization energies and a shift in the photon energy scale.

Radiative recombination and photoionization cross sections for impurities in plasmas: III. Ions of elements with [formula omitted

Presented here are total cross sections of radiative recombination for 55 ions of elements from C to Ca as well as partial photoionization cross sections in the electron kinetic energy range of 1 eV to 50 keV. The data are given for bare nuclei, H-, He-, Be-, Ne-, and Ar-like recombining ions. The calculations were performed in the framework of the relativistic Dirac–Fock method. Partial photoionization cross sections for states with n ⩽ 10 and orbital momenta ℓ ⩽ 4 were approximated by an analytical expression with five fit parameters. The results supplement our database of radiative recombination and photoionization cross sections for impurity ions occurring in fusion and astrophysical plasmas. The data are required for modelling and diagnostics of plasmas.

Theoretical calculation of photonization of F3+ and Ne4+ ions

We have calculated the partial and total photoionization cross sections for ground states 2s22p2 (3P) and excited states 2s22p2(1D,1S) ,2s2p3 (5So) of F3+ and Ne4+ ions and analyzed and identified the corresponding transitions of autoionization resonances for the ions in the photoionization processes. The obtained resonance structures of total photoionization cross sections are compared with the recent experimental ones and other theoretical calculations. It is shown that our results are in good agreement with the recent experimental ones.

Photoionization of iodine atoms: Angular distributions and relative partial photoionization cross-sections in the energy region 11.0–23.0 eV

Relative partial photoionization cross-sections and angular distribution parameters, beta, have been measured for the first, I(+)((3)P(2))<--I((2)P(3/2)), and fourth, I(+)((1)D(2))<--I((2)P(3/2)), (5p)(-1) photoelectron (PE) bands of atomic iodine, by performing angle-resolved constant-ionic-state (CIS) measurements on these PE bands in the photon energy range 11.0-23.0 eV. Three Rydberg series, two ns and one nd series, which converge to the I(+) (3)P(1) limit at 11.33 eV and four Rydberg series, two ns and two nd series, which converge to the I(+) (1)D(2) limit at 12.15 eV were observed in the first PE band CIS spectra. The fourth band CIS spectrum showed structure in the 12.9-14.1 eV photon energy range, which is also seen in the first band CIS spectra. This structure arises from excitation to ns and nd Rydberg states that are parts of series converging to the I(+) (1)S(0) limit we reported on earlier, as well as 5s-->5p excitations in the photon energy range 17.5-22.5 eV. These atomic iodine CIS spectra show reasonably good agreement with the equivalent spectra obtained for atomic bromine. The beta-plots for the first PE band recorded up to the I(+) (3)P(1) and I(+) (1)D(2) limits only show resonances corresponding to some of the 5p-->nd excitations observed in the first band CIS spectra scanned to the I(+) (1)D(2) limit (12.15 eV). These plots are interpreted in terms of an angular momentum transfer model with the positive values of beta obtained on resonances corresponding to parity allowed j(t)=1 and 3 channels and the off-resonance negative beta values corresponding to parity unfavored channels, where j(t) is the quantum number for angular momentum transfer between the molecule, and the ion and photoelectron. The beta-plots recorded for iodine are significantly different from those obtained for atomic bromine. Comparison of the experimental CIS spectra and beta-plots with available theoretical results highlights the need for higher level calculations which include factors such as configuration interaction in the initial and final states, relativistic effects including spin-orbit interaction, and autoionization via resonant Rydberg states.

ChemInform Abstract: The Electronic Structure of Trifluorophosphine Studied by Photoelectron Spectroscopy with Variable Photon Energy

The photoelectron spectrum of PF3 has been measured using synchrotron radiation. Relative partial photoionization cross-section and branching ratio data are reported for the valence bands (8–24 eV) over the photon-energy range 30–95 eV. Molecular orbitals regarded previously as purely fluorine Ione pair in composition are shown to have significant phosphorus 3p character. Comparison of the results obtained on PF3 with those obtained previously on [Ni(PF3)4] shows that the complex molecular orbitals of the latter derived from the 5e orbitals of PF3 have some nickel 3d character. Metal-to-phosphorus back donation, proposed to occur through the 7e PF3 molecular orbitals, is thus shown also to involve the 5e.

ChemInform Abstract: Variable Photon Energy Photoelectron Spectroscopy of OsO4 and Pseudopotential Calculations of the Valence Ionization Energies of OsO4 and RuO4.

Relative partial photoionization cross sections and photoelectron branching ratios have been obtained for the valence bands of osmium tetraoxide in the ionization energy range 12-18 eV. The photon energies used ranged between 24 and 115 eV. The ionization cross sections of the 2t2, 1e, and 2a1 orbitals show evidence of substantial metal character. By use of an ab initio many-body Green's function formalism that takes into account the effect of electron correlation and relaxation, ionization energies of OsO4 and RuO4 have been calculated. Considerations of the spectral features and the predictions of the calculation lead to an assignment of ion state ordering of 2T1 < 2T2 < 2A1 < 2E < 2T2. The presence of a substantial p-d resonance feature in the cross section of the upper 2T2 and 2E ion states shows that the 2t2 and le orbitals have significant Os 5d character. The lower 2T2 ion state (arising from ionization from the 3t2 orbital) undergoes a spin-orbit splitting of 0.4 eV resulting from an Os 6p contribution to the 3t2 molecular orbital.

Partial photoionization cross sections of C60 and C70: A gas versus adsorbed phase comparison

Abstract We have performed high resolution measurements of the photoelectrons emitted from the valence shell of C 60 and C 70 , in the gas phase and adsorbed phase on a metal surface, in order to derive branching ratios and the partial photoionization cross sections of the two highest occupied molecular orbitals, HOMO and HOMO-1. The comparison between the gas phase and the adsorbed phase shows an interesting and unexpected difference that can be attributed to a small orbital shift in solid C 70 . Density Functional Theory calculations within the Local Density Approximation (LDA) show good agreement for both data sets and give a plausible explanation for the observed difference.

Photoionization of iodine atoms: Rydberg series which converge to the I+(S1)←I(P23/2) threshold

Relative partial photoionization cross sections and angular distribution parameters beta have been measured for the first and fourth (5p)(-1) photoelectron (PE) bands of atomic iodine by performing angle-resolved constant-ionic-state (CIS) measurements on these PE bands between the (1)D(2) and (1)S(0) (5p)(-1) ionic thresholds in the photon energy region of 12.9-14.1 eV. Rydberg series arising from the 5p-->ns and 5p-->nd excitations are observed in both the first PE band, I(+)((3)P(2))<--I((2)P(3/2)), and the fourth PE band, I(+)((1)D(2))<--I((2)P(3/2)), CIS spectra. For each Rydberg state, the resonance energy, quantum defect, linewidth, line shape, and photoelectron angular distribution parameter beta have been determined. For the beta-plots for each PE band, only resonances corresponding to 5p-->nd excitations are observed; no resonances were seen at photon energies corresponding to the 5p-->ns resonances in the CIS spectra. The beta-plots are interpreted in terms of the parity unfavored channel with j(t)=4 being the major contributor at the 5p-->nd resonance positions, where j(t) is the quantum number for angular momentum transferred between the molecule, and the ion and photoelectron. Comparison of the results obtained with those published for bromine shows reasonably good agreement for the CIS spectra but poor agreement for the beta-plots. It appears that parity unfavored channels are playing a greater role in the valence (np)(-1) ionization of atomic iodine than in the corresponding ionization of atomic bromine.

Two-color experiments in the gas phase at FLASH

Intense ultra-short XUV-pulses from the Free Electron Laser in Hamburg (FLASH) were used in combination with the synchronized near-infrared (NIR) radiation from a femtosecond laser to perform two-color experiments on rare gas atoms and small molecules. Results of atomic photoionization in the presence of a NIR dressing field are presented and discussed for the low field regime, i.e. for intensities of less than 10 11 W/cm 2. In particular, the analysis of two-color Above Threshold Ionization (ATI) as a function of the relative orientation of the polarization vectors of both photon beams provided detailed information about the partial photoionization crosssections by comparing the experimental results with theoretical values obtained by employing second-order perturbation theory and the “soft-photon” approximation. Furthermore, a first time-resolved study was performed on the photodissociation of molecular hydrogen. In this proof-of-principle experiment, the excited atomic fragments, produced in the primary interaction with the intense XUV pulse, are probed by a time delayed NIR laser pulse that ionizes these fragments.

Satellite lines in the photoionization of ions: The Be isoelectronic sequence

Partial photoionization cross sections from the ${}^{1}S$ ground and the ${}^{3}P$ metastable states of the Be isoelectronic series to different final states, including a number of satellite (ionization plus excitation) channels are studied using the $R$-matrix method. As expected, the ratio of each satellite partial cross section to the dominant partial cross section is found to be a monotonic decreasing function of $Z$ for both the ground and the metastable states. The complicated patterns of these ratios as functions of photon energy are analyzed.

Confinement and electron correlation effects in photoionization of atoms in endohedral anions: Ne@Cz−60

Trends in resonances, termed confinement resonances, in photoionization of atoms A in endohedral fullerene anions A@Cz−60 are theoretically studied and exemplified by the photoionization of Ne in Ne@Cz−60. Remarkably, above a particular nl ionization threshold of Ne in neutral Ne@C60 (Iz=0nl), confinement resonances in corresponding partial photoionization cross sections σnl of Ne in any charged Ne@Cz−60 are not affected by a variation in the charge z of the carbon cage, as a general phenomenon. At lower photon energies, ω < Iz=0nl, the corresponding photoionization cross sections of charged Ne@Cz−60 (i.e., those with z ≠ 0) develop additional, strong, z-dependent resonances, termed Coulomb confinement resonances, as a general occurrence. Furthermore, near the innermost 1s ionization threshold, the 2p photoionization cross section σ2p of the outermost 2p subshell of thus confined Ne is found to inherit the confinement resonance structure of the 1s photoionization spectrum, via interchannel coupling. As a result, new confinement resonances emerge in the 2p photoionization cross section of the confined Ne atom at photoelectron energies which exceed the 2p threshold by about a thousand eV, i.e., far above where conventional wisdom said they would exist. Thus, the general possibility for confinement resonances to resurrect in photoionization spectra of encapsulated atoms far above thresholds is revealed, as an interesting novel general phenomenon.

Two-color photoionization of atoms in low and high radiation fields

Atomic photoionization was studied by means of electron spectroscopy using two photon sources of different wavelengths for the excitation. In one part of the experiments, alkaline atoms were excited by an optical laser of high spectral resolution prior to their photoionization by short wavelength radiation from the synchrotron radiation source BESSY. These low field studies concentrated on measurements of the linear alignment dichroism in the angular distribution (LADAD) and of the linear magnetic dichroism (LMDAD) in the 2p ionization of laser-polarized sodium atoms. In another type of experiments, the high intensity of a femtosecond optical laser was used for the investigation of dressed atoms, i.e. of the photoionization dynamics in strong external fields. In particular, the above threshold ionization (ATI) of rare gases was studied at the free electron laser facility FLASH in Hamburg, Germany. Measurements of the linear dichroism, here in the direct two-photon ionization of the 1s-shell in He, were performed in order to determine the partial photoionization cross sections related to the emission of electrons with s- and d- symmetry, respectively.

Inner-shell photoionization of fullerenes C20 and C60 within the "Dirac bubble" model

The "Dirac bubble" (δ-potential) model is applied to description of the structure of fullerenes C20 and C60 and photoelectron states. Energy spectra for σ valence orbitals for both fullerenes are obtained. Photoionization from two innermost delocalized electrons states (1s and 2p) is considered. The s-,p-,d- scattering phaseshifts are calculated, they demonstrate clear resonance properties which have oscillation in energy. Partial and total photoionization cross sections are obtained, the latter reveal an interesting "serrated" structure. Intriguing high-energy asymptotics is deduced, σ ∼ ω−5/2 sin2 (kR).

Linear dichroism in the innershell photoionization of alkaline atoms

The partial photoionization cross section of the 2p ionization of laser-excited aligned as well as ground-state oriented Na atoms were determined by high-resolution photoelectron spectroscopy and compared to multi-configuration Dirac-Fock and Hartree-Fock calculations. The measurements of the linear alignment dichroism (LAD) and the linear magnetic dichroism (LMDAD) are in very good agreement with the theoretical results. Similar studies were undertaken for the 3p photoionization of atomic K.

Partial photoionization cross sections of NH4 and H3O Rydberg radicals

Photoionization cross sections for various Rydberg series that correspond to ionization channels of ammonium and oxonium Rydberg radicals from the outermost, occupied orbitals of their respective ground states are reported. These properties are known to be relevant in photoelectron dynamics studies. For the present calculations, the molecular-adapted quantum defect orbital method has been employed. A Cooper minimum has been found in the 3sa(1)-kpt(2) Rydberg channel of NH(4) beyond the ionization threshold, which provides the main contribution to the photoionization of this radical. However, no net minimum is found in the partial cross section of H(3)O despite the presence of minima in the 3sa(1)-kpe and 3sa(1)-kpa(1) Rydberg channels. The complete oscillator strength distributions spanning the discrete and continuous regions of both radicals exhibit the expected continuity across the ionization threshold.