Mechanisms of two-electron excitation of the (2s 2) 1 S, (2p 2) 1 D and (2s2p) 1 P autoionizing states of helium are studied both experimentally and theoretically. It is shown that an explicit introduction of a kinematic factor, with a process-specific phase leads to a productive parametrization of experimental cross sections of ionization, allowing one to extract cross sections of excitation of autoionizing states. Using a new fitting procedure together with the proposed parametrization made it possible to obtain the excitation cross sections and magnetic sublevel population from electron spectra as well as, for the first time, to resolve the contribution of resonance and interference components to resonance profiles. Interference with direct ionization is shown to contribute significantly into resonance formation even for backward ejection angles. We demonstrate theoretically that the excitation cross sections thus extracted from experimental electron spectra hold information about the interaction of autoionizing states with an adjacent continuum.
Cross sections for ionization with excitation and for double excitation in helium are evaluated in a full second Born calculation. These full second Born calculations are compared to calculations in the independent electron approximation, where spatial correlation between the electrons is removed. Comparison is also made to calculations in the independent time approximation, where time correlation between the electrons is removed. The two-electron transitions considered here are caused by interactions with incident protons and electrons with velocities ranging between 2 and 10 au. Good agreement is found between our full calculations and experiment, except for the lowest velocities, where higher Born terms are expected to be significant. Spatial electron correlation, arising from internal electron-electron interactions, and time correlation, arising from time ordering of the external interactions, can both give rise to observable effects. Our method may be used for photon impact.
For autoionizing levels converging to the N = 3 threshold of He-like ions with Z = 3–26, energies and widths obtained within the generalized diagonalization approach are compared with the results of S-matrix perturbation theory. The combined data for the expansion coefficients can improve the validity of spectral calculations. Autoionization decay amplitudes and widths are obtained using both unscreened and screened hydrogenic orbitals for the free electron, as well as in the first order in continuum Coulomb coupling. Terms of order 1/Z can be extracted to improve the perturbational data for autoionization amplitudes, involving both screening corrections and the first-order continuum coupling. The relative significance of various atomic interactions in evaluating autoionization decay amplitudes is discussed.
The problem of providing accurate recommended analytical Ðts for electron impact ionisation cross sections is discussed, and a number of approaches are considered on the sample case of neon and its ions. The previously known Ðts are being reassessed using complete experimental and theoretical data, with the preference for experiment, to avoid systematic shifts introduced by the present calculation methods. The feasibility of the standard BELI formula is investigated in detail, and a number of other analytical expressions is suggested, approximating single-ionization cross sections in the whole range of energies. The factors inÑuencing the accuracy of the Ðts and the physical meaning of the parameters obtained are discussed.
A physically correct introduction of a cross section for the excitation of autoionization states into the theory of resonant ionization is considered and the relation of this quantity to measurable values is discussed. Excitation cross sections have been shown to differ from resonance yields even for isolated resonances without any post-collision interactions, so that this difference might be used to study the interplay of various atomic interactions in ionization processes. Numerical results for helium ionization by charged-particle impact in a broad range of velocities are presented in support of the conclusions made. It has been found that the resonance yield can provide a lower estimate for an excitation cross section for asymptotic collision velocities.
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