Cross Sections For Ionization Of He Research Articles

Single and double over-barrier ionization of He, He+ and Ne system by positron impact

The classical over-barrier ionization model (COBI) method and trajectory calculations were utilized to simulate the ionization of He+ impacted by a positron. The calculated ionization cross sections of He+ agree with other theoretical data. Additionally, we found that the double ionization of He has a definite association with the positron-He+ impact. This result can explain why doubly ionized He seemed to be positron-scattered by the rest of the He+ in our previous study. The COBI model was also extended to study the double ionization caused by positron-Ne impacts. Our theoretical results agree with the experimental data.

Commissioning of a powerful electron gun for electron–ion crossed-beams experiments

The sensitivity of an electron–ion crossed-beams experiment is mainly determined by the densities of both beams in the interaction region. Aiming at the extension of the available range of accessible electron energies and densities, a new high-power electron gun has been developed and fabricated. It delivers a ribbon-shaped beam with currents of up to 1A at variable energies reaching 3500eV at the maximum. The design goals of the gun are being met by using a configuration of electrodes, which allows for a variety of operation modes. Here, we report on the results of the ongoing tests of the new electron gun. In particular, electron-impact ionization cross sections of He+ and Xe5+ ions were measured employing the animated crossed-beams technique. The results are in excellent agreement with literature data.

Nondissociative electron and photon ionization cross sections of molecular hydrogen and deuterium

State-to-state [X 1Σ+g(vi) → X 2Σ+g(vj)] and quasi state-specific [X 1Σ+g(vi, Ji) → X 2Σ+g(vj)] photon and electron-impact ionization cross sections of hydrogen and deuterium molecules have been obtained with a combined analysis of experimental measurement and theoretical calculations. Rotation state ionization transitions are considered using the p-partial wave approximation. Previously calculated photoionization transition moments are refined and extended to ∼300 eV using accurate experimental photoionization cross sections. The transition moments are used to obtain the photoionization cross sections and oscillator strengths for the H2 and D2 X 1Σ+g(vi, Ji) → H+2 and D+2 X 2Σ+g(vj, Jj) transitions. The calculated ionization oscillator strengths, together with excitation functions derived from electron-impact ionization measurements of H2, enable accurate determination of electron-impact ionization cross sections for both H2 and D2 transitions to (vj, Jj) levels of the X 2Σ+g state at energies ranging from threshold to the non-relativistic limit. The autoionizing singlet-ungerade states contribute at least 0.6–1% of the total e+H2 →H+2 cross section. The e+D2 →D+2 and e+H2 →H+2 cross sections are very similar above the threshold energy region. The D2 cross section is ∼1.6% larger than the H2 cross section. Thermally averaged nondissociative ionization cross sections of H2 and D2 at temperatures appropriate for plasma and planetary applications have been tabulated.

Determination of the absolute two-photon ionization cross section of He by an XUV free electron laser

The resonant and non-resonant two-photon single ionization processes of He were investigated using intense free electron laser light in the extreme ultraviolet (XUV) region (53.4–61.4 nm) covering the 1s–2p and 1s–3p resonant transitions of He. On the basis of the dependences of the yield of He+ on the XUV light-field intensity at 53.4, 58.4, 56.0 and 61.4 nm, the absolute values of the two-photon ionization cross sections of He at the four different wavelengths and their dependence on the light-field intensity were determined for the first time.

Electron impact double ionization cross sections of light elements

A simple user-friendly semiempirical model is proposed to calculate electron impact double ionization cross sections of He, Li, Li+, B+, C+, C3+, O, O2+, O3+, Ne, Ne+, Ne2+, Na, Mg, Al3+, S, and Arq+ (q=0–7) targets for the incident electron energies from threshold to 106 eV. The contributions in the total double ionization cross sections from the direct double ionization and inner-shell ionization processes are taken into account on the basis of experimental data considered. The results of the present analysis are compared with the available experimental data and theoretical calculations. The model is found successful for the description of experimental cross sections. Since, this model may be a prudent selection to meet the demand level in plasma modeling due to its simple inherent structure.

Total electron-impact ionization cross sections of helium

By accurate measurement of total cross section ratios for electron-impact ionization and photoionization, total electron-impact ionization cross sections of He were determined. Relative standard uncertainties as low as 1.6 to 2% in the electron energy range from 100 to 4000 eV were achieved using published cross section data for photoionization. Results are discussed in terms of theoretical predictions and by comparison with former experimental results. The data are in excellent agreement with the results by Rejoub et al (2002 Phys. Rev. A 65 042713) and represent an accurate and reliable database for testing theoretical models.

Calculations of integral cross-sections for electron and positron impact of rare gas atoms

In this paper, calculations of integral cross-sections for positron impact ionisation of rare gas atoms, at incident projectile energies from the ionisation threshold up to about 1 keV, by a distorted-wave method with close-coupled target states [D.L. Moores, Nucl. Instr. and Meth. B 143 (1998) 105] are extended to include He and to electron impact on all five targets. Results are compared with experimental measurements carried out at University College London with positrons and by E. Krishnakumar and S.K. Srivastava [J. Phys. B 21 (1988) 1055] and other authors with electrons. Cross-sections for ionisation of He by positron impact with simultaneous excitation of the He + ion are reported for the first time. Results of similar calculations for electron impact show discrepancies with previous experimental and theoretical work.

Determination of partial single differential electron ionization cross sections of H2

The partial single differential cross sections (PSDCS) for the ions H2+ and H+ resulting from the direct and dissociative ionization of H2 by electron impact have been calculated employing a semi-empirical approach. The calculations are made as a function of the secondary/ejected electron energy at 500 and 1000 eV incident electron energies. The partial ionization cross sections (PICS) for direct and dissociative ionization have also been calculated with incident electron energies varying from the ionization threshold to 10 keV. The results are compared with the available experimental data.

Projectile charge and velocity scaling for double ionization of helium

A universal scaling law as a function of the projectile charge q and velocity v for the ratio of double to single ionization cross sections of He by fast positive and negative projectiles is presented based on the experimental data.

Dissociative ionization cross sections of D 2, N 2 and O 2 by fast positive-ion impact

Dissociative ionization of D 2, N 2 and O 2 by the impact of carbon and oxygen ions with energies ranging from 4 to 6 MeV/amu is studied with a view to determine the absolute cross sections. A crossed beam geometry is used along with a time of flight mass spectrometer and the normalization to absolute values is carried out using the relative flow technique with the single ionization cross sections of He as the standard.

Single ionization of He, Ne and Ar by positron impact

The single ionization cross sections of He, Ne and Ar by positron impact are re-examined. The new results differ from earlier published data for impact energies from the threshold to about 100 eV with the new cross sections being smaller.

Excitation and ionization cross sections of He, Ne and Ar by positron impact

Excitation and ionization cross sections of He, Ne and Ar atoms by positron impact have been presented in the energy range from the respective thresholds up to 100 eV using a time-of-flight (TOF) method. These data were compared with the theoretical and experimental data. For excitation, the present data for Ar in the whole range and for Ne in the range 50-100 eV coincide roughly with the electron impact data of de Heer and co-workers. However, for He the present data are much smaller than the electron data above 30 eV, but in good agreement with the theoretical data of Varracchio below 35 eV. The present data for the ionization cross section for He coincide well with the experimental data of Fromme et al. in the range less than 100 eV.

Ratio of double-to-single ionization of He by N7+ projectiles.

We apply the continuum-distorted-wave approximation within the framework of the independent-event model to calculate the ratio ({ital R}) of double-to-single ionization cross sections of He by N{sup 7+} projectiles. Following Deb and Crothers (J. Phys. B 23, L7999 (1990)), we calculate the double-ionization probability as {ital P}{sub 1}{ital P2}, whereas the single-ionization probability is taken to be 2{ital P}{sub 1}(1{minus}{ital P}{sub 1}). Here, {ital P}{sub {ital i}} ({ital i}=1,2) is the probability of ionizing the {ital i}th electron from He. The values of {ital R} found in the 10--40-MeV/u impact-energy range are similar to those obtained in the somewhat different forced-impulse theory of Reading and Ford (Phys. Rev. Lett. 58, 543 (1987)) and by the semiempirical fit of Knudsen {ital et} {ital al}. (J. Phys. B 17, 3545 (1984)), but are much smaller in absolute value than the corresponding measurements of Heber {ital et} {ital al}. (Phys. Rev. Lett. 64, 851 (1990); Nucl. Instrum. Methods B 56/57, 15 (1991)).

Total and ionization cross sections for He *(2 3S)-Ar and He *(2 1S)-Ar in the thermal energy range

Integral total and integral ionization cross sections of He *(2 3S)-Ar and He *(2 1S)-Ar have been measured in the c.m. collision energy range 2–230 meV. The data are compared with optical potential calculations giving preliminary fits which are discussed.

Total ionization cross sections of He, N2, H2and O2due to electron impact

The total ionization cross sections of He, N2, H2 and O2 due to electron impact have been calculated by integrating the expressions given by Khare for the ionization cross sections per unit energy range. The results are in satisfactory agreement with the experimental data. Comparison has also been made with other theoretical values.

A classical model for electron and proton impact ionization

Ionization cross sections of He, Li, O and N by electrons and protons are calculated in a classical model, and compared, where possible, with quantal calculations and experiment.

Partial Ionization Cross Sections of He, Ne, H2, and CH4 for Electrons from 20 to 500 eV

First Page

Total Cross Sections for Ionization and Attachment in Gases by Electron Impact. I. Positive Ionization

The total ionization cross sections of He, Ne, Ar, Kr, Xe, H2, D2, N2, O2, CO, NO, CO2, N2O, and CH4 have been measured from threshold to 1000 eV in a total ionization tube. More limited measurements were performed in C2H4 and SF6. Great care was taken to assure complete collection of electron and ion currents, and the absence of spurious instrumental errors. A new method was devised for obtaining absolute cross sections of gases relative to H2, and a McLeod gauge was used to obtain the absolute cross section in H2. The cross sections in NO and O2 could not be obtained by this method, and an approximate correction to direct McLeod-gauge readings was used for these gases. It is believed that the results are as accurate as is possible with the present method. It is difficult to explain the differences found between cross sections measured by various investigators. McLeod-gauge errors appear to account for most of the difference in absolute magnitude.

A Comparison of the Ionization Cross Sections of H2 and D2

A Comparison of the Ionization Cross Sections of H2 and D2

A Comparison of the Ionization Cross Sections of H2 and D2

First Page