Single, double, and triple photoionization of Ne + ions by single photons have been investigated at the synchrotron radiation source PETRA III in Hamburg, Germany. Absolute cross-sections were measured by employing the photon-ion merged-beams technique. Photon energies were between about 840 and 930 eV, covering the range from the lowest-energy resonances associated with the excitation of one single K-shell electron up to double excitations involving one K-and one L-shell electron, well beyond the K-shell ionization threshold. Also, photoionization of neutral Ne was investigated just below the K edge. The chosen photon energy bandwidths were between 32 and 500 meV, facilitating the determination of natural line widths. The uncertainty of the energy scale is estimated to be 0.2 eV. For comparison with existing theoretical calculations, astrophysically relevant photoabsorption cross-sections were inferred by summing the measured partial ionization channels. Discussion of the observed resonances in the different final ionization channels reveals the presence of complex Auger-decay mechanisms. The ejection of three electrons from the lowest K-shell-excited Ne + ( s s p 1 2 2 S 2 6 2 1 2 ) level, for example, requires cooperative interaction of at least four electrons.

Abstract.The photon-ion merged-beams technique has been employed at the new PhotonIon spectrometer at PETRA III (PIPE) for measuring multiple photoionization of Xe Absolute cross sections for 3d photoionization of Xe q+ ions (1 ≤ q ≤ 5) 2

Absolute cross sections for m-fold photoionization (m = 1, . . . , 6) of Fe + by a single photon were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany. Photon energies were in the range 680-920 eV which covers the photoionization resonances associated with 2p and 2s excitation to higher atomic shells as well as the thresholds for 2p and 2s ionization. The corresponding resonance positions were measured with an uncertainty of ±0.2 eV. The cross section for Fe + photoabsorption is derived as the sum of the individually measured cross-sections for m-fold ionization. Calculations of the Fe + absorption cross sections have been carried out using two different theoretical approaches, Hartree-Fock including relativistic extensions and fully relativistic Multi-Configuration Dirac Fock. Apart from overall energy shifts of up to about 3 eV, the theoretical cross sections are in good agreement with each other and with the experimental results. In addition, the complex deexcitation cascades after the creation of inner-shell holes in the Fe + ion have been tracked on the atomic fine-structure level. The corresponding theoretical results for the product charge-state distributions are in much better agreement with the experimental data than previously published configuration-average results. The present experimental and theoretical results are valuable for opacity calculations and are expected to pave the way to a more accurate determination of the iron abundance in the interstellar medium.

Single, double, and triple ionization of C(1+) ions by single photons is investigated in the energy range of 286-326 eV, i.e., in the range from the lowest-energy K-vacancy resonances to well beyond the K-shell ionization threshold. Clear signatures of C(1+)(1s2s(2)2p(2) (2)D,(2)P) resonances are found in the triple-ionization channel. The only possible mechanism producing C(4+)(1s(2)) via these resonances is direct triple-Auger decay, i.e., a four-electron process with simultaneous emission of three electrons.

We report on new measurements of m-fold photodetachment (m = 2 − 5) of carbon anions via K-shell excitation and ionization. The experiments were carried out employing the photon-ion merged-beams technique at a synchrotron light source. While previous measurements were restricted to double detachment (m = 2) and to just the lowest-energy K-shell resonance at about 282 eV, our absolute experimental m-fold detachment cross sections at photon energies of up to 1000 eV exhibit a wealth of new thresholds and resonances. We tentatively identify these features with the aid of detailed atomic-structure calculations. In particular, we find unambiguous evidence for fivefold detachment via double K-hole production.Atomic anions are highly correlated systems where the extra electron is weakly bound to an overall neutral charge distribution. Consequently, the number of excited states of atomic anions is quite limited [1] and some atomic species such as nitrogen do not form anions at all. A thorough treatment of the correlation effects in negative atomic ions still poses a formidable challenge to atomic theory, and this becomes even greater for innershell vacancies, since the valence electrons are then subject to strong many-electron relaxation effects following the creation of core holes [2, 3]. On the experimental side, core holes can be readily created by exciting or ionizing an inner-shell electron by a photon. For light ions, the core-hole state will subsequently decay via Auger transitions such that electrons are emitted with the net effect of photoionization. For negative ions, the entire process is termed (multiple) photodetachment.So far, photodetachment via the initial creation of a single K-hole has been experimentally studied only for a limited number of light anions up to F − [3-9]. For C − ions especially, previous measurements were carried out by Gibson et al. [7] and by Walter et al. [8] who studied double photodetachment in a very narrow photon energy range just covering the 1s 2 2s 2 2p 3 4 S → 1s 2s 2 2p 4 4 P resonance at about 282 eV. This scarcity of data is due to the fact that sufficiently high photon fluxes for more comprehensive studies were not available. Here, we present absolute cross sections, σ m , for m-fold photodetachment of C − ions by a single photon,with m = 2, 3, 4, 5 and photon energies from below the K-edge up to ∼1000 eV. These cross sections provide a view of the photodetachment dynamics of a highly correlated atomic system in unprecedented detail. Apart from the resonance at ∼282 eV, we observed a number of additional photodetachment resonances with a variety of line shapes as well as a clear signature for multiple detachment by direct double K-hole production. To better understand these observations, detailed atomic-structure calculations were performed by using the GRASP [10] and RATIP [11] codes as well as the Jena Atomic Calculator (JAC) [12]. To the best of our knowledge, there is only one other theoretical study of highly excited resonances in C − which, however, is focussed exc...

The photon-ion merged-beams technique was used at a synchrotron light source for measuring absolute cross sections of double and triple photodetachment of O − ions. The experimental photon energy range of 524-543 eV comprised the threshold for K-shell ionization. Using resolving powers of up to 13000, the position, strength and width of the below-threshold 1s 2s 2 2p 6 2 S resonance as well as the positions of the 1s 2s 2 2p 5 3 P and 1s 2s 2 2p 5 1 P thresholds for K-shell ionization were determined with high-precision. In addition, systematically enlarged multi-configuration Dirac-Fock calculations have been performed for the resonant detachment cross sections. Results from these abinitio computations agree very well with the measurements for the widths and branching fractions for double and triple detachment, if double shake-up (and -down) of the valence electrons and the rearrangement of the electron density is taken into account. For the absolute cross sections, however, a previously found discrepancy between measurements and theory is confirmed. Negative atomic ions play an important role in lowtemperature plasmas such as the upper atmosphere or the interstellar medium [1] and also in technical applications. For example, in the context of antihydrogen production, it has been proposed to use an ensemble of laser-cooled anions as a coolant for antiprotons [2]. Negative ions are fundamentally different from neutral atoms or positive ions since the extra electron in a negative ion is not only bound by the long-range Coulomb interaction with the atomic nucleus but, more importantly, also by a short-range attractive force due to the polarization of the atomic core. The accurate theoretical description of these ions still challenges the state-of-the-art quantum computations although the numbers of their bound states are generally finite. The low-excitation levels of negative ions are readily accessible by laser spectroscopy (see, e.g., [3][4][5][6]). Therefore, this technique has been a prime source of experimental information about the mutual interactions among the valence electrons.A sensitive tool for studying the interactions between the valence and the core electrons is inner-shell ionization of negative ions [7,8]. Here, we apply the photon-ion merged-beams technique (see [9] for a recent overview) to determine the absolute cross sections for double and triple ionization (detachment) of oxygen anions in the photon energy range 524-543 eV. In this energy range, a K-shell vacancy may be produced either by direct ionization of an initial 1s electron or via the formation of a resonance state by exciting one 1s electron to a higher shell such as 2p. In either case, the K-vacancy decays subsequently by a cascade of radiative and nonradiative processes leading to a distribution of final charge states with O + and O 2+ as the most prominent charged reaction products. To test and better understand the theoretical prediction of such cascades, we performed extremely comprehensive quantum calculations for the formation...

Relative cross sections for m-fold photoionization (m=1,K,5) of Fe 3+ by single-photon absorption were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source operated at DESY in Hamburg, Germany. The photon energies used spanned the range of 680-950 eV, covering both the photoexcitation resonances from the 2p and 2s shells, as well as the direct ionization from both shells. Multiconfiguration Dirac-Hartree-Fock (MCDHF) calculations were performed to simulate the total photoexcitation spectra. Good agreement was found with the experimental results. These computations helped to assign several strong resonance features to specific transitions. We also carried out Hartree-Fock calculations with relativistic extensions taking into account both photoexcitation and photoionization. Furthermore, we performed extensive MCDHF calculations of the Auger cascades that result when an electron is removed from the 2p and 2s shells of Fe 3+. Our theoretically predicted charge-state fractions are in good agreement with the experimental results, representing a substantial improvement over previous theoretical calculations. The main reason for the disagreement with the previous calculations is their lack of inclusion of slow Auger decays of several configurations that can only proceed when accompanied by de-excitation of two electrons. In such cases, this additional shake-down transition of a (sub)valence electron is required to gain the necessary energy for the release of the Auger electron.

Single, double, and triple ionization of the C + ion by a single photon have been investigated in the energy range 286 to 326 eV around the K-shell single-ionization threshold at an unprecedented level of detail. At energy resolutions as low as 12 meV, corresponding to a resolving power of 24 000, natural linewidths of the most prominent resonances could be determined. From the measurement of absolute cross sections, oscillator strengths, Einstein coefficients, multi-electron Auger decay rates and other transition parameters of the main K-shell excitation and decay processes are derived. The cross sections are compared to results of previous theoretical calculations. Mixed levels of agreement are found despite the relatively simple atomic structure of the C + ion with only 5 electrons. This paper is a follow-up of a previous Letter [Müller et al., Phys. Rev. Lett. 114, 013002 (2015)].