Electron-ion recombination and photoionization of Fe XXI

Results for electron-ion recombination and photoionization of , with emphasis in high-temperature region, are presented from ab initio unified method. The unified method, based on close coupling (CC) approximation and R-matrix method, (i) subsumes both the radiative recombination (RR) and dielectronic recombination (DR), (ii) enables self-consistent sets of photoionization and recombination cross sections from using an identical wavefunction for both the processes, and (iii) provides state-specific recombination rates of a large number of bound states. A large CC wavefunction expansion, which includes the ground and 28 core excitations of n=2 and 3 complexes and span a wide energy range, has been used. Compared to Δn=2-2, Δn=2-3 core excitations are found to introduce strong resonant structures and enhance the background photoionization cross sections (σ_PI) in the high-energy region. These features along with prominent photoexcitation-of-core (PEC) resonances at n=3 core thresholds have increased the unified total recombination rate coefficients (α_R(T)) at temperatures , region of maximum abundance of the ion in collisional equilibrium, by a factor of 1.6 over previous calculations. State-specific recombination rate coefficients α_R(nLS), which include both the RR and DR, are presented for the first time for 685 bound states with n?10 and l?9. The unified total recombination rate with photoelectron energy α_R(E) is presented and the role of low-energy near-threshold fine structure resonances is illustrated. The present results should provide a reasonably complete self-consistent set of recombination rates and photoionization cross sections for astrophysical modelings of high-temperature plasmas from optical to far-ultraviolet wavelength regions.     

Importance of Relativistic Effects for Intermediate-Z Elements： Photoionization Process of Excited Na

Using a modified R-matrix code, the fine-structure-resolved partial photoionization cross sections of excited Na （Z = 11） are calculated within the Breit-Pauli approximation. Our calculated energy levels of Na＋ and Na are in good agreement with the experimental values within 1% and the branching ratios of the J-resolved partial cross sections are consistent with the recent measurements within the experimental uncertainties. The agreements are impossible to be obtained without adequately taking into account the relativistic effects and the electron correlations together. Therefore, even for the intermediate-Z elements （e.g. Na with Z = 11）, the relativistic effects （mainly the spin-orbit interactions） should not be neglected.     

Comment on: “Photoionization of helium-like ions in asymptotic non-relativistic region” [Phys. Lett. A 358 (2006) 211]

The results of Mikhailov et al. [A.I. Mikhailov, A.V. Nefiodov, G. Plunien, Phys. Lett. A 358 (2006) 211] on single and double ionization of He at high energy can be obtained quite simply by combining two previous results long well known in the literature. The results of Mikhailov et al. can also be understood in a larger context, using an asymptotic Fourier transform approach, which also allows a justification of various statements assumed but not demonstrated by Mikhailov et al.     

Photoionization cross section and oscillator strength distribution in the near-threshold region of strontium

We present experimentally measured absolute values of the photoionization cross sections from the 5s5p ¹P₁ and 5s5p ³P₁ excited states of strontium at the first ionization threshold as 11.4±1.8 Mb and 10.7±1.7 Mb respectively using saturated absorption technique along with a thermionic diode ion detector in conjunction with a Nd:YAG pumped dye laser system. These threshold photoionization cross sections values have been utilized to determine the oscillator strengths of the 5s5p ¹P₁↦5snd ¹D₂ and 5s5p ³P₁↦5snd ³D₂ Rydberg transitions. The oscillator strength densities in the continuum corresponding to the 5s5p ³P₁ excited state have also been determined by measuring the photoionization cross sections at five ionizing wavelengths above the first ionization threshold. Smooth merging of the discrete f-values into the oscillator strength densities has been observed for the 5s5p ³P₁↦5snd ³D₂ series across the ionization threshold.     

Calculations of Photo-Ionization Cross Sections for Lithium Atoms

Photo-ionization cross sections for the ground and the ≤5 excited states of lithium atoms are calculated in the photoelectron energy ranging from threshold to 0.5 Rydberg. The wavefunctions for both the bound and continuum states are obtained by solving the SchrSdinger equation numerically in a symplectic scheme. Our results are in excellent agreement with the recent experimental measurements and in harmony with other theoretical calculations.     

Absence of Charge-Resonance-Enhanced Ionization in Attosecond Pulse Photoionization： Numerical Result on One-Dimensional H2^＋

We present a numerical result of photoionization rate for the one-dimensional molecular hydrogen ion model exposed to intense light of 1 × 10^16-2×10^16 W/cm^2, 55-as pulse duration, and 800nm wavelength. In contrast to the previous calculation result of charge-resonance-enhanced ionization for lower intensity and much longer pulse, our result exhibits an ionization saturation. The numerical results are interpreted in the field-dressed potential picture as over-the-barrier liberation of electrons. This extremely short pulsewidth and relatively high field phenomenon requests experimental demonstration.     

Double-Exponentially Decayed Photoionization in CREI Effect： Numerical Experiment on 3D H2^＋

On the platform of the 3D H2^＋ system, we perform a numerical simulation of its photoionization rate under excitation of weak to intense laser intensities with varying pulse durations and wavelengths. A novel method is proposed for calculating the photoionization rate： a double exponential decay of ionization probability is best suited for fitting this rate. Confirmation of the well-documented charge-resonance-enhanced ionization （CREI） effect at medium laser intensity and finding of ionization saturation at high light intensity corroborate the robustness of the suggested double-exponential decay process. Surveying the spatial and temporal variations of electron wavefunctions uncovers a mechanism for the double-exponentially decayed photoionization probability as onset of electron ionization along extra degree of freedom. Henceforth, the new method makes clear the origins of peak features in photoionization rate versus internuclear separation. It is believed that this multi-exponentially decayed ionization mechanism is applicable to systems with more degrees of motion.     

Measurement of total M shell photoionization cross sections for Au,Pb, Th and U in the energy region 6–12 keV

The total M shell relative photoionization cross-sections for Au, Pb, Th and U have been measured in the energy region 6–12 keV. External conversion K X-rays of suitable elements has been employed as incident photons to photo ionize the total M shell of elements under investigation. The method provides relative cross-sections therefore does not make use of theoretically calculated average M shell fluorescence yields which involve uncertainties of the order of 20%. No evidence of deviation from calculated values of cross-sections have been observed within experimental errors for all incident photon energies.     

Two-step single-colour photoionization spectroscopy of atomic uranium

The single-colour photoionization spectrum of atomic uranium is recorded in the spectral region from 366 to 371 nm. Most of the 43 observed resonances are found to be associated with the two-step photoionization process. Based on known energy levels seven new transitions have been proposed. Three of these transitions originate either from the ground state or the lowest metastable state at 620 cm^–1.     

Photoionization of 1s Electron and Corresponding Shake-Up Process in Ground and Excited Lithium Atoms

The cross sections of the ls electron photoionization and corresponding shake-up processes for Li atoms in the ground state 1s^22s and excited states 1s^22p, 1s^23s, 1s^23p and 1s^23d are calculated using the multi-configuration Dirac-Fock method. The latest experimental photoelectron spectrum at hv= 100 eV [Cubaynes D et al. Phys. Rev. Lett. 99 （2007） 213004] has been reproduced by the present theoretical investigation excellently. The relative intensity of the shake-up satellites shows that the effects of correlation and relaxation become more important for the higher excited states of the lithium atom, which are explained very well by the spatial overlap of the initial and final state wavefunctions. In addition, strong dependence of the cross section on the atomic orbitals of the valence electrons are found, especially near the threshold.     

Relativistic and correlation effects in the anisotropy of near-threshold Kr 3d and Xe 4d photoionization

Angular distributions of the spin–orbit split components and their branching ratios have been studied experimentally and theoretically for the 3d photoelectrons of Kr and 4d photoelectrons of Xe. The focus was on the electron dynamics near the ionization threshold of each spin–orbit split component and its behaviour as a function of Z   in passing from Kr to Xe. The experimental spectra were measured with high photon and electron energy resolutions with photon energies at about 3–12 eV above the 3d_3/2$3{\text{d}}_{3/2}$ and 3d_5/2$3{\text{d}}_{5/2}$ thresholds for Kr and at about 5–12 eV above the Xe 4d_3/2$4{\text{d}}_{3/2}$ and 4d_5/2$4{\text{d}}_{5/2}$ thresholds. Experimental results for the angular distribution parameters have been compared with theoretical values obtained with relativistic Dirac–Fock method and results from independent particle approximation with a modified Hartree potential [A. Derevianko, W. Johnson, K. Cheng, Atom. Data Nucl. Data Tables 73 (1999) 153]. The branching ratios were compared with theoretical predictions from Dirac–Fock and relativistic random-phase approximation [K. Cheng, W. Johnson, Phys. Rev. A 28 (1983) 2820].     

Laser isotope separation of titanium by two-step photoionization

Titanium isotopes were selectively excited and photoionized using a two-step photoionization method, and the isotope separation was demonstrated, in which a separation factor of around 15 for⁵⁰Ti was obtained. Spectroscopic parameters such as isotope shifts, photo-ionization cross section and excited state lifetimes were also measured. Isotope shifts up to 0.92 GHz were obtained for the transitions between 0 and 19938 cm^–1 or between 170 and 20006 cm^–1 among five isotopes. The cross section is 7.4×10^–17 cm² for the photo-ionization. The excited state lifetimes are 330±20 ns for 19938 cm^–1, 260±15 ns for 20006 cm^–1 and 250±15 ns for 20126 cm^–1.     

Two-color photoionization spectroscopy of uranium in a U–Ne hollow cathode discharge tube

A U–Ne hollow cathode discharge tube is used as a source of uranium atomic vapors as well as a photoelectron/photoion detector for carrying out two-color three-photon photoionization spectroscopy of uranium. Using the uranium excitation transition 0 cm⁻¹ (⁵L ₆ ⁰ ) → 16 900.38 cm⁻¹ (⁷M₇) at 591.5-nm laser wavelength as a first step transition and scanning the wavelength of a second laser from 558 to 568 nm, high-lying odd-parity atomic levels of uranium are studied in the energy region 34 500–34 813 cm⁻¹. All the expected 21 odd-parity atomic levels identified by various researchers in this region are observed in a single spectrum, demonstrating the high sensitivity achieved therein. In addition to this, we have identified eight autoionization resonances of uranium starting from its odd-parity atomic level at 33 801.06 cm⁻¹ pumped by two-photon excitation. Four out of these eight autoionization resonances are observed for the first time.     

Ionization of fast rydberg atoms in longitudinal and transverse electric fields

The main properties of longitudinal and transverse electric field ionizers for fast Rydberg atoms n=21–40 have been investigated. The dispersion and the background due to collisional processes between fast atoms and residual gas molecules have been measured and calculated. The kinetic energy spread of ions formed by field ionization of Rydberg atoms and their trajectories have been calculated. The potassium beam energy was 3.9 keV.     

Photoionization cross-sections of the first excited states of sodium and lithium in a magneto-optical trap

A two element magneto-optical trap (MOT) for Na and ⁷Li or ⁶Li is used to cool and trap each of them separately. A fraction of the cold atoms is maintained in the first ²P_3/2 excited state by the cooling laser. These excited state atoms are ionized by laser light in the near-UV region, giving rise to a smaller number of trapped atoms and to different loading parameters. Photoionization cross-sections were derived out of these data. They are in reasonable agreement with data previously obtained using thermal samples and with theoretical predictions. Received 21 March 2001 and Received in final form 3 August 2001     

Thermal photoionization in resonance ionization spectroscopy

In Resonance Ionization Spectroscopy, neutral atoms are usually produced by thermal effects, using an oven or a hot surface at high temperature. The radiation from these thermal sources is intense enough to effectively contribute to the photoionization of atoms excited by laser pumping. We show that, for the most common experimental conditions, the ratio of thermal to laser photoionization can be higher than one for excited levels which lie as far as a few thousand cm^–1 below the ionization threshold. This result is obtained with the use of the analytic expression for the photoionization cross-section of a hydrogen-like atom. We suggest two applications of this thermal photoionization. Namely, the study of highly excited states and Quasi Resonance Ionization Spectrometry, using only one laser of low radiance.Commissariat à l'Energie Atomique, Centre d'Etudes Nucléaires de Fontenay-aux-Roses DERDCA/DCAEA/SEA, BP no 6, F-92265 Fontenay-aux-Roses Cedex, France (Visiting scientist at Ecole Polytechnique from Commissariat à l'Energie Atomique)Commissariat à l'Energie Atomique, Centre d'Etudes Nucléaires de Saclay, IRF/DPHG/PAS. Bât 462, F-91191 Gif-sur-Yvette Cedex, France     

Absorption and ionization of molecular nitrogen by UV femtosecond laser pulses

Mechanisms of non-linear absorption and ionization of molecular nitrogen gas by UV femtosecond laser pulses were studied using photogalvanic and photoacoustic techniques. The effect of the intermediate Rydberg resonance, its dynamic Stark perturbation and ponderomotive upshift of the first ionization potential of nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of ion yield from three to four at increasing laser intensity.     

Alternative evaluation of statistical indicators in atoms: The non-relativistic and relativistic cases

In this work, the calculation of a statistical measure of complexity and the Fisher-Shannon information is performed for all the atoms in the periodic table. Non-relativistic and relativistic cases are considered. We follow the method suggested in [C.P. Panos, N.S. Nikolaidis, K.Ch. Chatzisavvas, C.C. Tsouros, arXiv:0812.3963v1] that uses the fractional occupation probabilities of electrons in atomic orbitals, instead of the continuous electronic wave functions. For the order of shell filling in the relativistic case, we take into account the effect due to electronic spin-orbit interaction. The increasing of both magnitudes, the statistical complexity and the Fisher-Shannon information, with the atomic number Z is observed. The shell structure and the irregular shell filling is well displayed by the Fisher-Shannon information in the relativistic case.