Electron-impact Excitation Cross Sections Research Articles

Cascades effects on the polarization properties of x-ray radiation emitted by electron-impact excitation of highly charged He-like Fe24+ ions

SynopsisDetailed calculations using a fully relativistic distorted-wave method are carried out for the electron-impact excitation cross sections from the ground state 1s2 (J= 0) to the individual magnetic sublevels of the excited state 1s2s1/2 (J= 1) of highly charged He-like Fe24+ ions. The cascades effects to the cross sections and the linear polarizations of the 1s2s1/2(J= 1) → 1s2 (J=0) line are investigated systematically. Our results show that the 1s2p, 1s3l, and 1s4l (l = s, p, d) excited states have significant effects and make the cross sections increase. These influences also lead to a remarkable decrease in the linear polarization of subsequent x-ray radiation. The present results agree with the previous theoretical results and the polarization measurements very well.

Electron impact excitation of helium atom**Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921501 and 2013CB922200), the National Natural Science Foundation of China (Grant Nos. 11274035, 11275029, 11328401, 11371218, 11474031, 11474032, and 11474034), and the Foundation of Development of Science and Technology of Chinese Academy of Engineering Physics (Grant Nos. 2013A0102005 and 2014A0102005).

A method to deal with the electron impact excitation cross sections of an atom from low to high incident energies are presented. This method combines the partial wave method and the first Born approximation (FBA), i.e., replacing the several lowest partial wave cross sections of the total cross sections within FBA by the corresponding exact partial wave cross sections. A new set of codes are developed to calculate the FBA partial wave cross sections. Using this method, the convergent e–He collision cross sections of optical-forbidden and optical-allowed transitions at low to high incident energies are obtained. The calculation results demonstrate the validity and efficiency of the method.

Circular polarization of the Lyman-α 1 line radiation emitted by longitudinally-polarized electron impact excitation: effects of Breit interaction and radiation multipoles

Detailed calculations are carried out for the longitudinally-polarized electron impact excitation cross sections from the ground state 1s 1/2 to the individual magnetic sublevels of the excited state 2p 3/2 of highly charged H-like ions using a fully relativistic distorted-wave method. The influence of the Breit interaction and radiation multipoles on the circular polarization of X-ray radiation are investigated systematically. Our results show that the Breit interaction, as the dominant correction to the Coulomb interaction, may lead to a considerable change in the magnetic sublevels cross sections and circular polarizations of Lyman-α 1 (2p 3/2 → 1s 1/2) line radiation with a more pronounced effect at higher incident energies and/or atomic numbers. Moreover, the interference between the electric dipole (E1) decay and the magnetic quadrupole (M2) decay may significantly alter the circular polarizations of the emitted X-rays. The circular polarizations of the Lyman-α 1 line are exhibit a different behavior than the linear polarizations for the same transition line regarding the radiative electron capture [Weber et al., Phys. Rev. Lett. 105, 243002 (2010)], radiative recombination [Bettadj et al., J. Phys. B 47, 105205 (2014)], and electron-impact excitation processes [Chen et al., Phys. Rev. A. 90, 012703 (2014)].

Influence of Debye plasma on the cross section and linear polarization of x-ray radiation following inner-shell electron-impact excitation of highly charged Be-like Sc17+ ions

Detailed calculations using a fully relativistic distorted-wave method incorporating the Debye–Hückel potential are carried out for the electron-impact excitation cross sections from the ground state 1s22s2 (J = 0) to the individual magnetic sublevels of 1s2s22p and states of highly charged Be-like ions in the presence of a plasma environment. The effects of plasma screening on the cross section and linear polarization of the corresponding x-ray emission are investigated. For the line, it is found that the plasma screening has a pronounced effect and causes the cross section to decrease over the whole energy range considered. This dramatic influence also leads to a remarkable decrease in the linear polarization of subsequent x-ray radiation. However, for the line, the plasma screening effect on the cross sections is relatively larger than for the excitations, while its influence on the linear polarization of the radiation is very small.

Influence of radiative cascades on the polarization and angular distribution of radiation following electron-impact excitation of He-like Fe24+ ions

Detailed calculations are carried out for the electron-impact excitation cross sections from the ground state 1s2(J = 0) to the individual magnetic sublevels of the excited state 1s2s1/2 (J = 1) of highly charged He-like Fe24+ ions using a fully relativistic distorted-wave method. The cascades contributions from high-lying levels 1s2p, 1snl (n = 3, 4, 5, 6; l = s, p, d) to the cross sections and the polarization properties of the 1s2s(J = 0) line are investigated systematically. It is found that the 1s2p, 1s3l, and 1s4l (l = s, p, d) excited states have significant effects and make the cross sections increase evidently. These dramatic influences also lead to a remarkable decrease in the linear polarization and angular distribution of subsequent x-ray radiation. The present results are in good agreement with the previous theoretical results and the polarization measurements performed at the Livermore electron beam ion trap.

Fully Relativistic Electron Impact Excitation Cross-Section and Polarization for Tungsten Ions

Electron impact excitation of highly charged tungsten ions in the framework of a fully relativistic distorted wave approach is considered in this paper. Calculations of electron impact excitation cross-sections for the M- and L-shell transitions in the tungsten ions Wn+ (n = 44–66) and polarization of the decay of photons from the excited tungsten ions are briefly reviewed and discussed. New calculations in the wide range of incident electron energies are presented for M-shell transitions in the K-like through Ne-like tungsten ions.

Electron collisions with H2CNH using the R-matrix method

The R-matrix approach is used to calculate the elastic differential, integral, and momentum transfer cross sections for electron impact on H2CNH within static-exchange-polarization (SEP) and close-coupling (CC) approximations. The electron-impact excitation cross sections from the ground state to the first three low-lying electronic excited states of H2CNH molecule have been also studied for the first time. A shape resonance of 2 A′′ symmetry located at 1.95 eV with a width of 0.56 eV is detected within CC model. The effective collision frequencies over a wide electron temperature range (200−30 000 K) are calculated using the data of the momentum-transfer cross section.

Precision calculation of low-energy electron-impact excitation cross sections of helium among the ground and excited states

Low-energy electron-impact cross sections of helium among the ground and some low excited states are calculated using the $R$-matrix method. The convergences of the cross sections are checked systematically by using five sets of high-quality target states; i.e., including 5, 11, 19, 29, and 39 physical target states, respectively. Our calculated cross sections are in excellent agreement with the benchmark high-resolution experimental data. Compared with the recommended theoretical data, there is a deviation of about 6%, which suggests the recommended data may need a revision. Based on our calculation results, the influence of the Rydberg target states on the collision cross sections of the excited states is found to be similar to the case of the ground state; i.e., the amplitude of resonance structures will decrease with respect to the principal quantum number $n$ of Rydberg target states. This result should be very useful for providing the cross-section data in the whole energy regions with high quality, which would be of great importance in related scientific fields.

Theoretical study on electron-impact excitation cross section and polarization for 5s2S1/2 → 5p2P3/2 of Cd

The electron-impact excitation (EIE) cross-sections of 5s2S1/2 → 5p2P3/2 of Cd+ have been calculated by using the relativistic distorted-wave (RDW) method which we have developed recently. In order to discuss the electronic correlation effects, four different models are used to describe the target wave functions, namely Model A, 8S, 5SD and 6SD. Model A is a single configuration model, it includes 5s1/2, 5p1/2 and 5p3/2 only. The 8S is a valence-valence correlation model, it considers the impacts of 6s, 7s, 8s and 6p, 7p, 8p orbitals on wave function of 5s1/2, 5p3/2, respectively. The 5SD is a core-valence correlation model, it includes all the virtual single and double excitations from the 4s, 4p, 4d, 5s and 5p shells into the unoccupied 4f, 5p, 5d, and 5f shells. The 6SD is also a core-valence correlation model, it includes all the virtual single and double excitations from the 4s, 4p, 4d, 5s and 5p shells into the unoccupied 4f, 5p, 5d, 5f, 6s, 6p and 6d shells. The oscillator strength calculated by Model A is 0.72, which is in disagreement with the experiment 0.55 measured by Xu et al., while the results of 6SD 0.57 is in agreement with these of the experiment very well. For the EIE cross-sections, the core-valence correlation is very important. The results of Model A and 8S are larger than the experimental results obtained by Gomonai et al., while the results of 5SD and 6SD is obviously smaller than the results of Model A. In low energy range (2P3/2 → 5s2S1/2 emission lines after excitation are very small, while the linear polarization of Model A, 5SD and 6SD are in consistent with each other and also in good agreement with other theories. But for high energies, the theoretical results have big difference from the experimental results obtained by Goto et al.

Electron-indium atom scattering and analysis of electron and optical spectra

Experimental study of indium atom using electron and optical spectroscopy is presented in this paper. Both experimental techniques including experimental setups are described. Differential and integrated cross sections on elastic and inelastic electron scattering by indium atom are measured using electron spectrometer. The measurements are performed at incident electron energies of E0 = 10, 20, 40, 60, 80 and 100 eV within the large scattering angles ranging from 10° to 150° in steps of 10°. The experimental results are presented and comparison with the values predicted by calculated optical potentials method is conducted, showing good agreement. The differential cross sections (DCSs) for electron-impact excitation of the resonant state 6s 2S1/2 of Indium atom are measured at small and large angles. The forward scattering function method has been used for normalizing the generalized oscillator strengths (GOS) to determine optical oscillator strength and obtaining the absolute DCS values. Optical spectrum of In I and In II lines has been acquired by a streak camera. The experimental results regarding indium lines obtained by time resolved laser induced breakdown spectroscopy (LIBS) could be useful for obtaining the important plasma parameters such as temperature, electron density as well as plasma-expansion velocity and plasma starting times.

Atomic Collision Processes for Astrophysical and Laboratory Plasmas

An accurate knowledge of atomic collision processes is important for a better understanding of many astrophysical and laboratory plasmas. Collision databases which contain electron-impact excitation, ionization, and recombination cross sections and temperature dependent rate coefficients have been constructed using perturbative distorted-wave methods and non-perturbative R-matrix pseudo-states and time-dependent close-coupling methods. We present recent atomic collision results.

Absolute Cross Sections for Near-Threshold Electron-Impact Excitation of the 2s 2S → 2p 2P Transition of Li-Like C3+, N4+, and O5+ Ions

Excitation cross sections of 1s22s 2S1/2 → 1s22p 2P1/2,3/2 transition among the fine-structure levels in Li-like C3+, N4+, and O5+ ions are calculated for energies of the near-threshold by using the relativistic distorted-wave program REIE06. The target state wavefunctions are calculated by using the Grasp92 code. The continuum orbitals are studied in the distorted-wave approximation, in which the direct and exchange potentials among all the electrons are included. The results of the Li-like C3+ ion settle the discrepancy between several previous experiments by using the crossed-beams fluorescence method, in good agreement with the measurements of Savin et al. Moreover, the results in Li-like N4+, and O5+ ions are compared with the previous experiments, and a good agreement is obtained.

Calculation of the polarization fraction and electron-impact excitation cross section for theCd+(5p)2P3/2state

Calculation of the polarization fraction and electron-impact excitation cross section for the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi mathvariant="normal">Cd</mml:mi><mml:msup><mml:mrow /><mml:mo>+</mml:mo></mml:msup></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo><mml:mn>5</mml:mn><mml:mi>p</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mrow /><mml:mn>2</mml:mn></mml:msup><mml:msub><mml:mi>P</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>state

Absolute measurements of electron impact excitation cross-sections of atoms using cavity ringdown spectroscopy

A new method of using cavity ringdown spectroscopy (CRDS) technique to measure apparent electron impact excitation cross-sections (EIECS) for different energy levels of an atom is described. The method involves measurements of absolute population densities of the energy levels in concern using the high sensitivity CRDS, leading to an analytical expression of EIECS as a function of the population densities. Determination of EIECS in this method does not require an external calibration. The proposed method is validated by determining EIECS of the metastable levels of Hg (6s6p 3P0) and Ne (2p53s 3P2), and the results are in good agreement with the cross-sections reported in the literature. The population densities in both cases were extracted from the published data. This method exhibits promising potential for absolute measurements of EIECS of metastable energy levels.

Electron impact excitation of xenon from the metastable state to the levels of 5p57p configuration

Electron impact excitation cross sections from the lowest metastable state 5p56s J = 2 to the excited states of the 5p57p configuration of xenon have been investigated. In order to discuss the correlation effects of target states on the cross sections, two models are used to describe target states based on the multi-configuration Dirac-Fock (MCDF) method. It is found that the electron correlation effects have a large influence on the cross sections and make the cross sections smaller in low energy impact. Good agreement between our results and experimental value has been achieved if more configuration interactions are taken into account.

Polarization of the strongest nf→3d (n = 4, 5, 6) radiative lines emitted from tungsten ions following EIE and DR processes

Electron-impact excitation and resonant electron capture cross sections to the specific magnetic sublevels of highly charged Ni-like to Ge-like tungsten ions have been calculated systematically by using a fully relativistic distorted-wave method. And these magnetic cross sections have further been employed to obtain the degrees of linear polarization of the corresponding strongest nf → 3d (n = 4, 5, 6) x-rays. We compare polarizations of the same lines but from the above two different processes for the first time. It has been found the polarizations following both the both processes are totally different. It is expected that the obvious differences between the polarizations can be used to distinguish the formation mechanism of the corresponding lines.

Nonperturbative electron-ion scattering theory incorporating the Møller interaction

We present the first calculations that investigate the effects of both the Møller interaction and close-coupling in the calculation of electron-impact excitation cross sections. Electron scattering from U91+ is used as a test case. The RCCC method is nonperturbative and we emphasise the restrictions and subsequent limitations associated with employing the Møller interaction in the RCCC method.

The calculation of low-energy electron-impact excitation cross sections of Helium from excited states

The low energy electron impact cross sections of helium from some low excited states are calculated in the R-matrix method. The convergences of the cross sections are checked systematically by using 5 sets of high quality target states; i.e., 5, 11, 19, 29 and 39 physical target states.

On spectroscopic classification of autoionizing levels in Rb atoms

Large scale configuration interaction calculations of energy levels, autoionization probabilities and electron-impact excitation cross sections of the autoionizing states 4p5nln'l' (nl = 5s, 4d, 4f,n'l'=4d, 5s, 5p, 5d, 5f, 5g, 6s, 6p, 6d, 7s, 7p) of Rb up to 19.6 eV were performed for the first time. The relativistic effects were taken into account in the Dirac-Fock-Slater approximation. The calculated data were used for the novel classification of the lines in an experimental ejected-electron spectrum.

Theoretical study of inner-shell electron-impact excitation of highly charged ions: Alignment and angular distribution of electron emission

The influence of the Breit interaction, typically appears as a relativistic correction to the Coulomb repulsion acting among the electrons, on the alignment (i.e. the population of the magnetic sublevels) and the angular distribution of electron emission from the excited state have been investigated systematically. Detailed calculations have been carried out for the electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of highly charged beryllium-like ions by using a fully relativistic distorted-wave (RDW) method. A remarkable change in the alignment and the electron angular distribution due to the Breit interaction is found, especially for the cases with high-energetic incident electron and high-Z target ions.