Atomic negative ions: structure, dynamics and collisions

We apply Boltzmann equations for modelling the radiation damage in samples irradiated by photons from free electron lasers (FELs). We test this method in a study case of a spherically symmetric xenon cluster irradiated with VUV FEL photons. Qualitative agreement between the model predictions and experimental data is found. The results obtained demonstrate the potential of the Boltzmann method for describing the complex and non-equilibrium dynamics of samples exposed to FEL radiation.

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Modelling dynamics of samples exposed to free-electron-laser radiation with Boltzmann equations

Electron-impact double-ionization processes of light positive ions from He- to Ne-like isoelectronic sequences, as well as of the heavy ions Arq+ (q = 1–7) and Kr q+ (q = 1–4) are considered for incident-electron energies E < 50 Ith where Ith is the threshold energy for double-electron ionization. On the basis of reliable experimental data and quantum-mechanical calculations, simple semiempirical formulae with three fitting parameters, by taking into account the contribution of direct double ionization and of inner-shell ionization processes, are obtained which describe the experimental cross sections within an accuracy of 20–30%. With this accuracy, the formulae suggested can be used for prediction of the double-ionization cross sections of positive ions with the nuclear charge Z ≤ 26 in the electron energy range of E < 50Ith. According to the model suggested, the direct ionization cross section σdir (simultaneous ionization of two outer electrons) is scaled as I−3th against scaled electron energy E/Ith and contains only one fitting parameter. Two additional fitting parameters are obtained using the least-squares method in conjunction with numerically calculated single-electron inner-shell ionization cross sections. All fitting parameters are found to be constant for ions within a given isoelectronic sequence. The present analysis also provides a method for indirect determination of K-shell ionization cross sections for ions from Be-like to Ne-like sequences. The fluorescence yields ωK for a single K-shell vacancy in ions from Li-like to Ne-like sequences with nuclear charges 3 ≤ Z ≤ 26 are calculated as well.

http://iopscience.iop.org/article/10.1088/0953-4075/38/5/006/pdf

Semiempirical formulae for electron-impact double-ionization cross sections of light positive ions

Ionization yields are reported for Ar, Kr and Xe in ultrastrong fields from 1015 W cm−2 to 1018 W cm−2. Non-sequential ionization (NSI) is shown to be a robust and general feature in ultrahigh field ionization. NSI yields measured are consistent with the trends predicted by a rescattering model, but as one proceeds to higher Z atoms more NSI is observed than predicted theoretically. Additional recollision mechanisms that may need to be considered in future theories of ultrastrong field–atom interactions include 'chain' NSI, NSI from excited states of the atom (e.g. Rydberg states or inner-shell holes) and the possibility of ultrastrong field enhanced recollision/impact processes.

Multielectron ultrastrong laser field ionization of Arn+, Krm+ and Xel+ (n ≤ 9, m ≤ 9, l ≤ 12) at intensities from 1015 W cm−2 to 1018 W cm−2

We examine the most important processes leading to the creation of excited states from the ground configurations of Ar8+ to Ar16+ ions in an electron-cyclotron resonance ion source, which lead to the emission of K x-ray lines. Theoretical values for inner-shell excitation and ionization cross sections, including double KL ionization, transition probabilities and energies for the de-excitation processes, are calculated in the framework of the multi-configuration Dirac-Fock method. With reasonable assumptions about the electron energy distribution, a theoretical Kα x-ray spectrum is obtained, which reproduces very closely a recent experimental result.

Interpretation of x-ray spectra emitted by Ar ions in an electron-cyclotron resonance ion source

New experimental data on double-ionization cross sections of neutral atoms, positive and negative ions by electron impact are reported. These and other relevant data are compared with recent multiple-ionization semiempirical formulae extended for the case of two-electron ionization. Theory gives analytical dependence of the double-ionization cross sections on the main parameters of the collision system in a wide energy range. Far the He atom, He-like ions and H- ion high-energy asymptotic behaviour of the double-ionization cross sections are considered on the basis of a quantum mechanical treatment. Suggested cross section scaling for fast collisions, employing a target-effective nuclear charge, is in good agreement with experimental data.

D.B. Uskov

Papers

Double ionization of neutral atoms, positive and negative ions by electron impact