Double Cross Sections Research Articles

Electron-impact double ionization of the carbon atom

Electron-impact single- and double-ionization cross sections and Maxwellian rate coefficients are presented for the carbon atom. Scaling factors are introduced for the electron-impact excitation and ionization cross sections obtained in the distorted wave (DW) approximation. It is shown that the scaled DW cross sections provide good agreement with measurements for the single ionization of the C atom and C1+ion. The direct double-ionization (DDI) process is studied using a multi-step approach. Ionization–ionization, excitation–ionization–ionization, and ionization–excitation–ionization branches are analyzed. It is demonstrated that the three-step processes contribute ≼40% of the total DDI cross sections for the case where one of the electrons takes all of the excess energy after the first ionization process.

Dissociation kinetics of excited ions: PEPICO measurements of Os3(CO)12 - The 7-35 eV single ionization binding energy region.

In this article, we study the photoinduced dissociation pathways of a metallocarbonyl, Os3(CO)12, in particular the consecutive loss of CO groups. To do so, we performed photoelectron-photoion coincidence (PEPICO) measurements in the single ionization binding energy region from 7 to 35 eV using 45-eV photons. Zero-energy ion appearance energies for the dissociation steps were extracted by modeling the PEPICO data using the statistical adiabatic channel model. Upon ionization to the excited ionic states above 13 eV binding energy, non-statistical behavior was observed and assigned to prompt CO loss. Double ionization was found to be dominated by the knockout process with an onset of 20.9 ± 0.4 eV. The oscillator strength is significantly larger for energies above 26.6 ± 0.4 eV, corresponding to one electron being ejected from the Os3 center and one from the CO ligands. The cross section for double ionization was found to increase linearly up to 35 eV ionization energy, at which 40% of the generated ions are doubly charged.

Double ionization of silicon and phosphorus by electron-impact

I have calculated double ionization cross sections of silicon and phosphorus by electron impact using double binary encounter model. Hartree-Fock momentum distribution has been used for both the ejected electrons during collision process. Contributions from inner shell in the double ionization have been included in the calculations. The results obtained have been found in reasonably good agreement with the experimental observations.

Electron-impact single and double ionization of the Fe atom and atomic ions

Electron-impact single and double ionization cross sections for the Fe atom and atomic ions are calculated using a combination of perturbative distorted-wave (DW) and non-perturbative time-dependent close-coupling (TDCC) methods. The single ionization cross sections include contributions from outer subshell direct ionization and excitation-autoionization. The double ionization cross sections include contributions from direct double ionization, inner subshell direct ionization, and excitation-autoionization. The accuracy of the perturbative DW calculations for direct ionization of the outer subshells of Fe and Fe+ are checked by comparison with non-perturbative TDCC calculations. The cross sections for single and double ionization are then compared with crossed-beams measurements.

DWBA double differential ionization cross sections for positron and electron impact on argon

The ionization of the 3p orbital of argon by 100 eV incident electrons and positrons is studied with the DWBA-WM approximation. The variation of our double differential ionization cross sections with the angles of the ejected electrons is found to be in reasonable agreement with the experiment. We found that the inclusion of the post-collision interaction in the theoretical model is necessary to obtain the size of the difference between the experimental double differential cross sections for electron and positron impact ionization.

Single and double K-shell resonant photoionization and Auger decay of 1s → 2p excited states of O+–O4+

In this work, single and double photoionization cross sections in the vicinity of 1s → 2p resonances are investigated theoretically for quantum states belonging to the ground and first excited configurations of O+ –O4+ . R-matrix method has been employed to obtain the single ionization cross section, whereas the double ionization cross sections are obtained by the branching ratios of the direct double Auger decay to the total Auger decay. By analyzing possible double ionization pathways, we conclude that the double photoionization originates predominately from the direct double Auger decay of the K-shell resonant states. Our theoretical work diagnosed the population fraction of the quantum state prepared in a recent experiment and successfully interpreted the experimental observations on both single and double photoionization cross sections.

Orientation effect on the secondary-electron distributions for the HCl double ionization

Multiple differential cross sections of the double ionization of hydrogen chloride molecule impacted by electrons are here provided within the first Born approximation. The incident and scattered electrons are represented by plane wave functions, whereas the target is described by means of a single-center molecular wave function. The two ejected electrons are modeled by Coulomb wave functions and a Gamow factor is introduced to take into account the Coulomb repulsion between the two outgoing particles. In this work, we study the relative contributions of the four outermost orbitals of the HCl molecule to the double ionization process in the particular case of two electrons ejected from different subshells. The multiple differential cross sections are analyzed for specific target orientations and compared in terms of magnitude and shape. We report a strong dependence of the cross sections versus the target orientation by pointing out the signature of the well-known double ionizing processes, namely, the shake-off and the two-step 1 mechanisms.

Electron-induced double ionization of oriented methane molecules

We report here a theoretical study of the target orientation effect on the total cross sections for the double ionization of methane molecules impacted by electrons. The theoretical description is performed within the first Born approximation. The initial state of the collisional system is composed of an electron projectile modeled by a plane wave and a molecular target described by a one-center wave function while the final state is constituted by a scattered electron described by a plane wave and two ejected electrons both represented by a Coulomb wave and coupled with a Gamow factor. Secondary electron energetic distributions and total cross sections are reported for particular target configurations. Strong orientation effects on the double-ionization process are pointed out in particular when scrutinized orbital by orbital.

Electron-impact single and double ionization of W

Electron-impact single and double ionization cross sections for the W atom are calculated using a semi-relativistic distorted-wave method. The cross sections include contributions from single direct ionization, double direct ionization and excitation-autoionization. Branching ratio calculations are made to determine whether an excitation may contribute to single or double ionization. We check the accuracy of the semi-relativistic distorted-wave calculations for direct ionization of various subshells by comparison with fully-relativistic distorted-wave calculations. We also check the accuracy of the perturbative distorted-wave calculations for direct ionization of the outer most subshells by comparison with non-perturbative time-dependent close-coupling calculations.

Electron-impact single and double ionization of the Mn2+, Mn3+ and Mn4+ atomic ions

Electron-impact single and double ionization cross sections for Mn2+, Mn3+ and Mn4+ are calculated using a semi-relativistic configuration-average distorted-wave method. The ionization cross sections include contributions from single direct ionization, double direct ionization and excitation–autoionization. Certain excitations may contribute to both single and double ionization as determined by branching ratio calculations. For Mn2+ the ratio of the double to single ionization cross section peak heights is found to be 0.15, while dropping to 0.03 for Mn3+ and 0.01 for Mn3+.

Study of proton and alpha particle impact double ionization of Fe

Theoretical calculations of H+ and He2+ impact double ionization cross sections for ground state Fe atoms have been performed in the binary encounter approximation(BEA) in the energies region ranging from 80 to 1440 keV/amu for proton impact and from 47 to 360 keV/amu in the case of alpha particle impact. The accurate expression for αδE (cross section for energy transfer ΔE) and Hartree –Fock velocity distributions for the target electrons have been used throughout the calculations. It has been concluded that the calculated results of H+ and He2+ impact double ionization cross sections are in good agreement with the experimental data throughout the given energy range.

Angular distributions in the double ionization of DNA bases by electron impact

Ab initio calculations of the five-fold differential cross sections for electron-impact double ionization of thymine, cytosine, adenine and guanine are performed in the first Born approximation for an incident energy close to 5500 eV. The wavefunctions of the DNA bases are constructed using the multi-center wave functions from the Gaussian 03 program. These multi-center wave functions are converted into single-center expansions of Slater-type functions. For the final state, the two ejected electrons are described by two Coulomb wave functions. The electron–electron repulsion between the two ejected electrons is also taken into account. Mechanisms of the double ionization are discussed for each case and the best choices of the kinematical parameters are determined for next experiments.

Double ionization of the hydrogen sulfide molecule by electron impact: Influence of the target orientation on multiple differential cross sections

Multiple differential cross sections of double ionization of hydrogen sulfide molecule impacted by electrons are here investigated within the first Born approximation. In the initial state, the incident electron is represented by a plane wave function whereas the target is described by means of a single-center molecular wave function. In the final state, the two ejected electrons are described by Coulomb wave functions coupled by the Gamow factor, whereas the scattered electron is described by a plane wave. In this work, we analyze the role played by the molecular target orientation in the double ionization of the four outermost orbitals, namely 2b1, 5a1, 2b2 and 4a1 in considering the particular case of two electrons ejected from the same orbital. The contribution of each final state to the double ionization process is studied in terms of shape and magnitude for specific molecular orientations and for each molecular orbital we identified the mechanisms involved in the double ionization process, namely, the Shake-Off and the Two-Step 1.

Time-dependent density-functional-theory studies of collisions involving He atoms: Extension of an adiabatic correlation-integral model

A recent model to describe electron correlations in time-dependent density-functional-theory (TDDFT) studies of antiproton-helium collisions is extended to deal with positively charged projectiles. The main complication is that a positively charged projectile can capture electrons in addition to ionizing them to the continuum. As a consequence, within the TDDFT framework one needs to consider three, instead of just one, correlation integrals (${I}_{c}$'s) when formally expressing the probabilities for the one- and two-electron processes in terms of the density. We discuss an extension of an adiabatic model for ${I}_{c}$ to a two-centered system. Total cross sections for single ionization, double ionization, single capture, transfer ionization, and double capture are presented for both proton-helium and ${\text{He}}^{2+}$-He collisions for impact energies in the approximate range 10--1000 keV/amu. One- and two-electron removal cross sections are also presented for the $p$-He system, with a comparison to updated antiproton-helium results. Our results, while mixed, demonstrate the relative importance of dynamic and functional correlations in a TDDFT description of collision processes.

Calculation of fully differential cross sections for the near threshold double ionization of helium atoms

Fully differential cross sectional (FDCS) results are reported for the electron-impact double ionization of helium atoms at 5 and 27 eV excess energy. The present attempt to calculate the FDCS in the second Born approximation and treating the postcollision interaction is helpful to analyze the measurements of Ren et al (2008 Phys. Rev. Lett. 101 093201) and Durr et al (2007 Phys. Rev. Lett. 98 193201). The second-order processes and postcollision interaction have been found to be significant in describing the trends of the FDCS. More theoretical effort is required to describe the collision dynamics of electron-impact double ionization of helium atoms at near threshold.

Observation of four-electron Auger processes

Multiple ionization of ions subsequent to absorption of a single photon has been studied employing a photon-ion merged-beam setup at the PETRA III synchrotron radiation facility of DESY in Hamburg. Absolute cross sections for single, double and triple ionization of C+ ions were measured with emphasis on specific well defined terms of K-shell excited C+. In particular, the terms C+ (1s2s22p2 2D,2P) were excited from the ground level of C+. Subsequent autoionization processes resulted in the production of C2+, C3+ and C4+ ions. The associated decay mechanisms are single-Auger, double-Auger and triple-Auger decay. The observation of C4+ products arising from C+(1s2s22p2 2D,2P) unambiguously confirmed the existence of triple-Auger decay, i.e., a process in which 4 electrons interact with one another such that one fills the K-shell vacancy and the others are simultaneously ejected. The experiment yields branching ratios for the Auger decay channels as well as individual decay rates for autoionization and radiative stabilization of the C+(1s2s22p2 2D,2P) terms.

Double ionization of water molecules induced by swift protons

Experimental cross sections for single and double ionization of ${\mathrm{H}}_{2}\mathrm{O}$ by swift ${\mathrm{H}}^{+}$ with energy ranging from 0.3 to 2.0 MeV are reported. In this energy range the ionization is the dominant collision process and charge transfer reactions can be disregarded. A multihit coincidence technique is used to measure the ${\text{H}}^{+}+{\text{OH}}^{+}$ and ${\text{H}}^{+}+{\text{O}}^{+}$ fragmentation channels. Single- and double-hit differential measurements together with a semiempirical calculation allow separating quantitatively the prompt and Auger-like decay contributions to fragmentation following a vacancy in the $2{a}_{1}$ molecular orbital. Concerning the double-ionization channel, it is found that for lower energies the mechanism of a sequential double-electron removal, known as TS2, dominates. For energies above above 750 keV/u ionization resulting from a single vacancy followed by an Auger like deexcitation takes over the TS2, becoming the main contribution to the double-ionization cross section. Our results are compared to the electron-impact data within the same velocity range and also with theoretical calculations available in the literature.

Application of R-Matrix with Time-dependence Theory to Double Ionisation Using a 2-electron Outer Region

SynopsisR-Matrix with Time-dependence (RMT) theory has been extended to cover double-ionisation processes. An application to photoionisation of He is demonstrated, with an emphasis on double-ionisation cross sections.

Double Photoionization of confined atoms in n-walled fullerenes

SynopsisIn this work we study the effect of fullerene confinement on the triple differential cross section (TDCS) for double ionization of atoms. We compare the results corresponding to the free case with single cage C60 and two cage C60@C240 confinement.

Possible Evidence of 3rd and 4th order Interactions Contributions to Double Ioni- zation of Helium by Protons and Antiprotons

SynopsisPublished cross sections for double ionization of helium by protons and antiprotons are analyzed in terms of the traditional interference between 1st and 2nd amplitudes. Between 3 and 10 a.u., the velocity dependence implies a dominant 2nd order interaction. For slower collisions it is shown that the velocity dependences are consistent with 3rd and 4th order interactions becoming important with these interactions having negative coefficients.