Aims. The interpretation of astrophysical spectra depends directly on a knowledge of the ionization state of the emitting plasma. This is determined, in part, from collisional ionization rate coefficients. The most recent assessments of these were performed by Arnaud & Rothenflug (1985, A&AS, 60, 425) and Arnaud & Raymond (1992, ApJ, 398, 394). Since their work, new laboratory measurements of ionization cross sections have become available as well as the Flexible Atomic Code (FAC) which enables theoretical calculations of these rates. Our goal is to provide a complete set of ionization rate coefficients for the elements hydrogen through zinc. Methods. A scaling law, which assists the analysis of ionization cross sections and rate coefficients, has been developed following the approach of Burgess & Tully (1992, A&A, 254, 436). Essentially all available measured cross sections along each isoelectronic sequence have been examined and compared to cross sections calculated with the Flexible Atomic Code (FAC) and with other calculations. Two approaches has been taken to provide a complete set of ionization cross sections. In the first, fits to scaled measured ionization cross sections, particularly for neutral and singly ionized species, are performed. In the second, fits to scaled calculated direct ionization and excitation-autoionization cross sections are performed to provide the remainder of the set. The fits to the cross sections are then integrated over a Maxwellian velocity distribution to derive ionization rate coefficients. Results. A complete set of ground level ionization cross sections and rate coefficients has been developed through the combination of these two approaches. A tabulation of parameter fits to ground level ionization rate coefficients for all atoms and ions of the elements of H through Zn is provided.

/pdf/ionization-rate-coefficients-for-the-elements-hydrogen-376eb006r3.pdf

Ionization rate coefficients for the elements hydrogen through zinc

The available atomic data used for interpreting and modeling x-ray observations are reviewed The applications for these data can be divided into several levels of detail, ranging from compilations which can be used with direct inspection of raw data, such as line finding lists, to synthetic spectra which attempt to fit to an entire observed dataset simultaneously This review covers cosmic sources driven by both electron ionization and photoionization and touches briefly on planetary surfaces and atmospheres The applications to x-ray astronomy, the available data, and recommendations for astronomical users are all reviewed, and an attempt to point out the applications where the shortcomings are greatest is also given

http://ned.ipac.caltech.edu/level5/Sept06/Kallman/paper.pdf

Atomic data for x-ray astrophysics

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.

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

On the basis of average experimental data we demonstrate scaling laws of electron-impact multiple ionization cross sections and propose expressions for the cross sections for arbitrary atoms and ions.

A scaling of multiple ionization cross sections

Absolute cross sections for electron impact ionization, dissociative excitation (DE) and dissociative ionization of N-2(+) ions are measured in the energy range from threshold to 2500 eV. The animated crossed electron-ion beam method has been employed. The individual contributions of ionization products (N-2(2+)) and dissociation fragments (N+), which have both identical mass-to-charge ratio and average velocity, are deduced from the analysis of product velocity distributions. Particular attention was paid to determining the transmission efficiency for dissociation fragments, since their collection was incomplete during the measurements. Threshold energies and kinetic energy released to dissociation fragments are measured. The role of states contributing to different reactions is discussed. For DE, the present results are found to be much smaller than the results of Peterson et al (1998). For ionization (single and dissociative), a satisfactory agreement with their result is obtained as well as with the prediction of Kim et al (2000) obtained in the binary-encounter Bethe approximation.

https://iopscience.iop.org/article/10.1088/0953-4075/34/15/303/pdf

Electron impact dissociation and ionization of N-2(+)

The first absolute cross section measurements for double ionization of C+, N+, O+ and Ne+ ions by electron impact are reported. The animated crossed beams method has been employed in the energy range from below ionization thresholds to approximately 2500 eV. The classical binary encounter approximation overestimates measured cross sections by almost two orders of magnitude. Along the sequence, the cross section maximum does not follow classical scaling laws. A simple scaling law based on an electron pair ejection model is proposed for the prediction of direct double ionization. Inner-shell ionization followed by autoionization is seen to play a dominant role for C+ only.

Electron impact double ionization of singly charged ions C + , N + , O + , F + and Ne +

The first absolute cross section measurements for single and double electron impact ionization of sodium-like Ar7+ are reported. The animated crossed beams method has been employed in the energy range from threshold to 3000 eV. The measured cross sections for single ionization are higher than the theoretical and semi-empirical predictions by about 20-50%. This discrepancy has been associated with the contribution of the indirect ionization processes. The double ionization cross section is only 1% of the single one.

Absolute Cross-section Measurements for Electron-impact Ionization of Ar-7+

Absolute cross-sections for single ionisation of helium-like C4+ and O6+ by electron impact have been measured for incident electron energy ranging from below the expected metastable state ionisation threshold to 3 keV. A crossed electron-ion beam setup has been used in this experiment. From analysis of the ion time of flight, it is concluded that the signal observed below the ground state ionisation threshold can only be due to the triplet state 2 3S. The importance of the various ionisation processes is discussed for this particular state. We present also a calculation of the branching ratio for autoionisation following the electron impact excitation from one of the two metastable states to the doubly excited states of the 2l,2l' configurations. The metastable population has been estimated to 16.3% for C4+ and 3.9% for O6+.

M. Zambra

Papers

Electron impact double ionization of singly charged ions C + , N + , O + , F + and Ne +

Absolute Cross-section Measurements for Electron-impact Ionization of Ar-7+

Electron-impact Ionization of C4+ and O6+