Cross Sections For Multiple Ionization Research Articles

Theoretical multiple-ionization cross sections of Ne-like molecules by light-ion impact: H2O

We present a simple theoretical model to calculate multiple-ionization cross sections (MICS) of Ne and Ne-like molecules (molecules with ten electrons as ${\mathrm{H}}_{2}\mathrm{O}, \mathrm{C}{\mathrm{H}}_{4}, \mathrm{N}{\mathrm{H}}_{3}$, and HF) for proton and light-ion impact, taking into account both direct and postcollisional emissions. In this paper, we tackle the case of water molecules, relevant to investigate the radiobiological effects of ion impact on living matter. The theory is developed in the framework of the independent electron model (IEM). To keep the model as simple as possible, we describe the impact parameter dependence of the single-particle ionization probabilities required by the IEM through decreasing exponential functions for each target orbital. We obtain the parameters of the exponential functions from the total net-ionization cross sections for each orbital by applying either the continuum distorted wave-eikonal initial-state (CDW-EIS) approximation or Rudd's model. We then calculate the contribution of Auger postcollisional electron emission to MICS by using the Ne postcollisional emission probabilities. This postcollisional treatment offers a very simple alternative to the much more complex molecular evaluation of postcollisional relaxation and provides results in close agreement with experimental data for proton and other light ions. We also demonstrate the relevance of considering postcollisional emission for water molecules after the $2{a}_{1}$ ($2s$ Ne-like) orbital direct ionization.

Multiple fragmentation mechanisms in ammonia: Collisions with protons in the intermediate velocity regime

The ionization and dissociation of ammonia by impact of protons is investigated experimentally in a wide range of energies from 125 to 2700 keV. Coincidence measurements are used to separate single from multiple ionization. The role of the charge sign of the projectile was analyzed for the ionization by comparing the proton with previous electron data. The energy dependence of the single and multiple-ionization cross sections allowed us to evaluate the role of second-order mechanisms. For single ionization, the role of a two-step mechanism was inferred. The double-ionization cross section showed that postcollisional deexcitation takes over the two-step sequential ionization above 800 keV. Triple ionization is prompted mostly by postcollisional Auger decay.

Dissociative multiple ionization of the H2O molecule induced by ion impact: experimental and theoretical results

SynopsisFragment energy spectra of the water molecule impacted by singly charged (SCI) and highly charged (HCI) ions have been measured in crossed beam experiments. Double differential fragmentation cross sections (DDCS) have been determined. Similarities and differences of the measured DDCSs induced by the medium- and low-energy projectiles have been analyzed. Multiple target ionization cross sections have been deduced and they were further analyzed through the results of the CTMC and CDW-EIS models.

Dissociative ionization of the H2O molecule bombarded by swift singly charged and slow highly charged ions

Fragment ion energy spectra of the water molecule have been measured in conventional crossed‐beam experiments by the impact of 46 keV/u energy, singly charged ions (SCIs) and 4.3 keV/u energy, highly charged ions (HCIs). Double differential cross sections have been determined and a comparative analysis has been performed. We found that the fragmentation spectra for SCIs and HCIs are very similar, indicating that both collisions lead to the same fragmentation channels. This suggests that the Coulomb explosion of the water molecule is dominantly determined by the charge state of the transient molecular ions, and it is almost independent from the primary ionization mechanism. Differences were observed not only between the SCI and HCI impact‐induced fragmentation cross sections, but between those obtained by the 60 keV N6+ and 70 keV O7+ projectiles. The differences were attributed to the selectivity of the electron capture process for HCIs. Multiple target ionization cross sections have been deduced from the fragment ion spectra. We found contributions of up to fivefold ionization for SCIs and up to sixfold ionization for HCIs.

Multiple Ionization Cross Sections of Ne and CO Induced by Very High-<i>q</i> Fast Projectiles (<i>q/v</i> > 1)

In this paper, we extend our previous work of classical over barrier ionization (COBI) model to study the multiple ionization cross section of Ne and CO molecule collided by very high-q fast projectiles(q/v > 1). The model gives similar results to the independent-electron-approximation calculation and is in good agreement with experimental data. The very small computational time required makes it a good candidate for studying the multiple ionization of complex molecules under high linear energy transfers.

Electron-impact Multiple-ionization Cross Sections for Atoms and Ions of Helium through Zinc

Abstract We compiled a set of electron-impact multiple-ionization (EIMI) cross section for astrophysically relevant ions. EIMIs can have a significant effect on the ionization balance of non-equilibrium plasmas. For example, it can be important if there is a rapid change in the electron temperature or if there is a non-thermal electron energy distribution, such as a kappa distribution. Cross section for EIMI are needed in order to account for these processes in plasma modeling and for spectroscopic interpretation. Here, we describe our comparison of proposed semiempirical formulae to available experimental EIMI cross-section data. Based on this comparison, we interpolated and extrapolated fitting parameters to systems that have not yet been measured. A tabulation of the fit parameters is provided for 3466 EIMI cross sections and the associated Maxwellian plasma rate coefficients. We also highlight some outstanding issues that remain to be resolved.

Dissociative ionization of the H2O molecule induced by medium-energy singly charged projectiles

We report on the fragmentation of the water molecule by $1$ MeV H$^{+}$, He$^{+}$ and 650 keV N$^{+}$ ion impact. The fragment-ion energy spectra were measured by an electrostatic spectrometer at different observation angles. The obtained double-differential fragmentation cross sections for N$^{+}$ is found to be more than an order of magnitude higher, than that for H$^{+}$. The relative ratios of the fragmentation channels are also different for the three projectiles. Additional fragmentation channels were observed in the spectra for He$^{+}$ and for N$^{+}$ impact, which are missing in the case of H$^{+}$. From the analysis of the kinetic energy of the fragments, the maximum observed degree of ionization was found to be $q\rm{_{max}}=3$, $4$, and $5$ for H${^+}$, He${^+}$ and N${^+}$ impact, respectively. Absolute multiple ionization cross sections have been determined. They are compared with the predictions of the classical trajectory Monte Carlo and continuum-distorted-wave eikonal-initial-state theories. At lower degrees of ionization, theories provide reasonable agreement with experiment. The systematic overestimation of the cross section by the theories towards higher degrees of ionization indicates the failure of the independent particle model.

Multiple ionization of argon by helium ions

We apply the continuum distorted-wave eikonal initial state and the independent electron model to describe the multiple ionization of Ar by He2+ and He+ in the energy range 0.1–10 Mev amu–1. Auger-like post collisional processes are included, which enhance the high energy multiple ionization cross sections via ionization of the inner shells. All Ar electrons (K, L and M-shells) have been included in these calculations. The results agree well with the experimental data at high energies, where the post-collisional ionization is the main contribution. At intermediate impact energies the description is also good though it tends to overestimate the triple and quadruple ionization data at intermediate energies. We analyze this by comparing the present results for He+2 in Ar, with previous ones for He+2 in Ne and Kr. It was found that the theoretical description improves from Ne to Ar and Kr, with the latter being nicely described even at intermediate energies. The present formalism is also tested for Ar inner shell and total ionization cross sections. In all the cases the results above 0.1 MeV amu–1 are quite reasonable, as compared with the experimental data available and with the ECPSSR values.

Experimental determination of multiple ionization cross sections in Si by electron impact

Fil: Perez, Pablo Daniel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Cordoba. Instituto de Fisica Enrique Gaviola. Universidad Nacional de Cordoba. Instituto de Fisica Enrique Gaviola; Argentina. Universidad Nacional de Cordoba. Facultad de Matematica, Astronomia y Fisica; Argentina

Positron and electron-impact multiple-ionization

We present positron-impact multiple (single to quintuple) and total ionization cross sections of Ne, Ar, Kr, and Xe, covering an extended energy range from 50 eV to 7 keV. We improve on previous calculations of the ionization thresholds by adding the mean kinetic energy transferred to the target electrons. In this way the thresholds compare rather well with the experimental appearance energies for the different k-fold ionization cross sections. Present results include not only the novel energy threshold at low energies but also the post-collisional contribution at high energies. We performed a quite complete particle–antiparticle comparison by including our positron-impact results, new calculations for electron-impact ionization including the threshold correction, and a detailed compilation of experimental data. Present positron-impact multiple ionization cross sections are the first ones in such an extended energy region.

Multiple Ionization of Argon Induced by Carbon Ions at 20–500 keV/u Energies

We measure the multiple ionization cross-section ratios Rk1 of Ar impacted by Cq+ (q = 1−3) ions in the energy range of 20–500 keV/u. The measured ratios Rk1 increase with the projectile energy at lower energies, and reach the maximum at energies of 50–150 keV/u, then decrease for higher energies. We also use the classical over barrier ionization model to calculate the ratios Rk1, and the calculation results are in good agreement with the data, which suggest that the multiple ionization process is described by the sequential over-barrier ionization process.

Multiple ionization of atoms by highly charged ions

A method is suggested for quickly and easily estimating multiple ionization (MI) cross sections of heavy atoms colliding with highly charged ions, using the independent-particle model (IPM). One-electron ionization probabilities p(b) are calculated using the geometrical model for values at zero impact parameter b and the relativistic Born approximation for one-electron ionization cross sections. Numerical results of MI cross sections are presented for Ne and Ar atoms colliding with Ar8+, Fe20+, Au24+, Bi67+ and U90+ ions at energies 1 MeV u–10 GeV uand compared with available experimental data and CTMC (classical trajectory Monte Carlo) calculations. The present method of calculation describes experimental dependencies of MI cross sections on the number of ejected electrons m within a factor of two to three. Numerical calculations show that at intermediate ion energies E = 1 − 10 MeV u, the contribution of MI cross sections to the total, i.e. summed over all m values, is quite large ∼35% and decreases with increasing energy.

Multiple ionization of neon atoms in collisions with bare and dressed ions: A mean-field description considering target response

We investigate projectile-charge-state-differential electron removal from neon atoms by impact of ${\mathrm{He}}^{2+}$, ${\mathrm{Li}}^{3+}$, ${\mathrm{B}}^{2+}$, and ${\mathrm{C}}^{3+}$ ions at intermediate projectile energies $(25\phantom{\rule{0.16em}{0ex}}\text{keV}/\text{u}$ to $1\phantom{\rule{0.16em}{0ex}}\text{MeV}/\text{u})$. The many-electron problem is described with an independent electron model in which active electrons at both collision centers are propagated in a common mean-field potential. Response to electron removal is taken into account in terms of a time-dependent screening potential, and a Slater-determinant-based method is used for the final-state analysis. Total cross sections for net recoil ion production, multiple ionization, and capture channels are mostly in good agreement with published experimental data. Results from equicharged bare and dressed ions are compared and the net recoil ion production cross section is broken down into contributions associated with different final projectile charge states in order to shed light on the role of the projectile electrons.

Energy threshold in multiple ionization by electron or positron impact

The energy threshold for the formation of highly charged ions is experimentally well-known and measured. For single ionization, it is the binding energy of the outermost electrons. For multiple ionization, the ionization begins at impact energies much larger than the theoretically expected ones. In this contribution we present a simple expression for the energy threshold for multiple ionization by electron or positron impact. It was obtained as the mean value of the energy transferred to each ionized electron by using Thompson classical approximation. Present results reproduce quite well the experimental thresholds. Moreover, the inclusion of these values in the theoretical multiple ionization cross sections allows describing rather well the experimental data for single up to sextuple ionization of rare gases.

Electron-impact ionization of Se2+

Absolute cross sections for electron-impact single and multiple ionization of Se2+ ions were measured using the dynamic-crossed-beams technique over the energy range from the ionization threshold to 500 eV and compared to direct ionization cross sections predicted by the Lotz semi-empirical formula. The single-ionization cross section appears to be dominated by excitation-autoionization of the 4s and 3d inner subshells. The double-ionization measurements are reasonably well accounted for by the semi-empirical fitting formula of Shevelko et al that includes both direct double ionization of the outer 4p subshell and single ionization of the 3p and 3d subshells followed by autoionization. In addition to direct triple ionization, 3p and 3s ionization followed by double autoionization and excitation of the 3p and 3s subshells followed by triple autoionization likely contribute to the triple ionization cross section.

Electron-impact multiple ionization of Ne, Ar, Kr and Xe

This work describes the multiple ionization cross sections of rare gases by electron-impact. We pay special attention to the high energy region (0.1–10 keV) where the direct ionization is a minor contribution and the post-collisional electron emission dominates the final target charge state. We report here electron-impact single to sextuple ionization cross sections and total ionization cross sections including direct and post-collisional processes, even in the total values. We use the continuum distorted wave and the first Born approximations adapted to describe light-particle impact, i.e. energy, mass and trajectory corrections are incorporated, the latter by considering the electron-target potential and by using the Abel transformation. Auger-type post-collisional contributions are included in the multinomial expansion through experimental branching ratios after single ionization events. Tabulations of these experimental branching ratios for all the orbitals of the four targets are included. Present results are compared with the large amount of electron-impact experimental data available. We have obtained a good description of the multiple-ionization measurements at high energies, where the post-collisional ionization dominates. At intermediate energies, our theoretical results show the correct tendency, with the electron-impact ionization cross sections being far below the proton-impact ones.

Electron-impact ionization of Se3+

Absolute electron-impact single and multiple ionization cross-section measurements for Se3+ ions were performed using the dynamic-crossed-beams technique over the energy range from the ionization threshold to 1 keV. The experimental single ionization cross section for Se3+ shows evidence of significant contributions of 4s–nl (n ≥ 6) and 3d–nl (n ≥ 4) inner-shell excitation-autoionization. The cross sections for double and triple ionization are dominated by ionization-autoionization and ionization-double autoionization of a 3p or 3s electron, respectively. The measurements were compared with the direct ionization cross sections predicted by the Lotz semi-empirical formula for direct ionization. The maximum values of cross sections for single, double, and triple ionization are compared.

Electron impact multiple ionization of C+ and N+ and O+ ions

Absolute cross sections for electron impact single and multiple ionization of C+, N+ and O+ ions leading to the formation of Cq+ (q=2-4), Nq+ (q=2-5) and Oq+ (q=2-5) are reported in the energy range extending from the respective thresholds up to 2.5 keV. The general feature of the measured cross sections is investigated.

Classical calculation of multiple ionization cross sections of noble gases near Bragg peak energies

In this paper, we extend our previous work of COBI model to study the multiple ionization and mean charge state of noble gases collided by heavy ions at energies not far from Bragg peaks of several to hundreds keV/amu. Being in good agreement with various experimental data and taking extremely small computational time make the model helpful to study the charge state distribution and multiple ionization of gases transforming to plasmas.

Absolute cross sections for electron loss, electron capture, and multiple ionization in collisions of Li2+ with argon

Exclusive absolute cross sections for the electron loss and capture processes, accompanied by target multiple ionization and pure target multiple ionization, as well as total electron loss and capture cross sections, in collisions of Li2+ with Ar have been measured in the 0.5–3.5 MeV energy range. The experimental data of the total electron loss cross section are compared with theoretical results based on the plane-wave Born approximation and the free-collision model, and with the available experimental data. Some discrepancies are observed when comparing the experimental data with the theoretical models, which can be attributed to the competitive mechanisms that lead to electron loss. The dependences of the single-capture and transfer-ionization processes on the projectile charge state are similar to those observed for collisions between other low-charged light ions and noble-gas targets. The same behaviour is observed when one compares the present data for the single- and double-ionization cross sections with those for He2+ projectiles on Ar. These facts indicate that the dynamics of the collision does not seem to depend on the projectile species, so that few-electron projectiles may act as structureless point charges in the intermediate- to high-velocity regime.