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Register a new userElectron-impact ionization and ionic fragmentation of O$_{2}$ from threshold to 120 eV energy range
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We study the electron-impact induced ionization of O$_{2}$ from threshold to 120 eV using the electron spectroscopy method. Our approach is simple in concept and embodies the ion source with a collision chamber and a mass spectrometer with a quadruple filter as a selector for the product ions. The combination of these two devices makes it possible to unequivocally collect all energetic fragment ions formed in ionization and dissociative processes and to detect them with known efficiency. The ion source allows to vary and tune the electron-impact ionization energy and the target-gas pressure. We demonstrate that for obtaining reliable results of cross sections for inelastic processes and determining mechanisms for the formation of O$^{+}$($^{4}S,^2{D},^2{P}$) ions, it is crucial to control the electron-impact energy for production of ion and the pressure in the ion source. A comparison of our results with other experimental and theoretical data shows good agreement and proves the validity of our approach.
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The convergent close-coupling method is applied to the calculation of fully differential cross sections for ionization of atomic hydrogen by 15.6 eV electrons. We find that even at this low energy the method is able to yield predictive results with small uncertainty. As a consequence we suspect that the experimental normalization at this energy is approximately a factor of two too high.
Application of the convergent close-coupling (CCC) method to electron-impact ionization of the ground state of atomic hydrogen is considered at incident energies of 15.6, 17.6, 20, 25, 27.2, 30, 54.4, 150 and 250 eV. Total through to fully differential cross sections are presented. Following the analysis of Stelbovics [submitted to Phys. Rev. Lett. (physics/9905020)] the equal-energy sharing cross sections are calculated using a solely coherent combination of total-spin-dependent ionization amplitudes, which are found to be simply a factor of two greater than the incoherent combination suggested by Bray and Fursa [1996 Phys. Rev. A {bf 54}, 2991]. As a consequence, the CCC theory is particularly suited to the equal-energy-sharing kinematical region, and is able to obtain convergent absolute scattering amplitudes, fully ab initio. This is consistent with the step-function hypothesis of Bray [1997 Phys. Rev. Lett. {bf 78}, 4721], and indicates that at equal-energy-sharing the CCC amplitudes converge to half the step size. Comparison with experiment is satisfactory in some cases and substantial discrepancies are identified in others. The discrepancies are generally unpredictable and some internal inconsistencies in the experimental data are identified. Accordingly, new (e,2e) measurements are requested.
Electron-impact direct double ionization (DDI) process is studied as a sequence of two and three step processes. Contribution from ionization-ionization, ionization-excitation-ionization, and excitation-ionization-ionization processes is taken into account. The present results help to resolve the long-standing discrepancies; in particular, a good agreement with experimental measurements is obtained for double ionization cross-sections of $O^{1+}$, $O^{2+}$, $O^{3+}$, $C^{1+}$, and $Ar^{2+}$ ions. We show that distribution of the energy of scattered and ejected electrons, which participate in the next step of ionization, strongly affects DDI cross-sections.
The contribution to electron-impact ionization cross sections from excitations to high-nl shells and a consequent autoionization is investigated. We perform relativistic subconfiguration-average and detailed level-to-level calculations for this process. Ionization cross sections for the W27+ ion are presented to illustrate the large influence of the high shells (n >= 9) and orbitals (l >= 4) in the excitation-autoionization process. The obtained results show that the excitations to the high shells (n >= 9) increase cross sections of the indirect ionization process by a factor of 2 compared to the excitations to the lower shells (n <= 8). The excitations to the shells with orbital quantum number l = 4 give the largest contribution comparedwith the other orbital quantum numbers l. Radiative damping reduces the cross sections of the indirect process approximately twofold in the case of the level-to-level calculations. Determined data show that the excitation-autoionization process contributes approximately 40% to the total ionization cross sections.
Electron-impact ionization of lithium is studied using the convergent close-coupling (CCC) method at 25.4 and 54.4 eV. Particular attention is paid to the spin-dependence of the ionization cross sections. Convergence is found to be more rapid for the spin asymmetries, which are in good agreement with experiment, than for the underlying cross sections. Comparison with the recent measured and DS3C-calculated data of Streun et al (1999) is most intriguing. Excellent agreement is found with the measured and calculated spin asymmetries, yet the discrepancy between the CCC and DS3C cross sections is very large.
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