In atomic physics, processes involving negative ions are usually very difficult to treat theoretically due to the large amount of electronic correlation present compared to the correlation in atoms or positive ions. Thus these systems are interesting play grounds for observing new effects and testing theoretical techniques.
One of the processes that we have investigated in detail is the scattering of an electron from a negative ion. The original motivation arose from the experimental and theoretical observation of resonance states in H-- (i.e. p+e+e+e). In our first treatment, we [F. Robicheaux, R.P. Wood, and C.H. Greene, Phys. Rev. A 49, 1866-1874 (1994)] predicted that the resonances were spurious. Later experiments of electron/H- collisions could not reproduce the original resonances, validating this prediction. Later calculations suggest that resonances in H-- exist but for symmetries not observed in scattering experiments (however, these calculations were later contested and confirmed by different theoretical groups). Resonances in electron scattering from small molecular (negative) ions have been observed but their classifications are uncertain. Recent theoretical work [F. Robicheaux, Phys. Rev. Lett. 82, 707-10 (1999)] used an important observation to quantitatively calculate the total detachment cross section for electron-negative ion scattering. The idea is that the strong correlation between the two outgoing electrons in the final state is an artifact of treating them as independent particles; in center of mass coordinates the correlation is almost non-existent.
Figure 1 of Robicheaux. Total electron impact detachment cross section of - versus the energy of the incident electron. The experimental points are plotted as the dotted line. The various calculations have differing levels of accuracy for which the T-matrix integrals are calculate: solid line (2^16 points, l_max=10); short-dashed line (2^15 points, l_max=10); dot-dashed line (2^16 points, l_max=15); and dot-dot-dot-dashed line (2^15 points, L_max=15).
In a more extensive paper [F. Robicheaux, Phys. Rev. A 60 (1999)], the threshold law for electron impact detachment was investigated. A form for the threshold law was proposed and compared to a large numerical calculation.
Figure 2 of Robicheaux. Total detachment cross section for H- using the full calculation (crosses) and the threshold laws from Robicheaux (dotted line) and previous work (solid and dashed lines). All threshold laws have the same dominant behaviour as E->threshold the only differences are in the extension to higher energies. An experimental measurement should be able to distinguish between the different extensions of the threshold law.
A last interesting investigation of negative ions [F. Robicheaux, Phys. Rev. A 60 (1999)] focussed on the existence of peculiar negative molecular ions. Very simple calculations strongly suggest the existence of very weakly bound systems like HHe- and H_3- (i.e. HHH-). These systems are strange due to the possible combinations of 2 body bound states. For example, there are no bound states of HHe and there are no bound states of He-. However, it is possible to stick a H and an electron onto He and form a true bound state. None of the systems discussed in the paper have been experimentally observed.
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