Negative Ion Resonances In Electron-magnesium Collision

In the past decades, electron affinities for alkaline-earth-metal atoms have been of considerable interest, due to its fundamental importance in atomic physics, their dynamics form a critical test for our understanding of electron correlations. A number of calculations and measurements have been carried out. A detailed review can be found in an article by Buckman and Clark.However, for magnesium negative ion resonance states have not be extensively studied, and generally limited to low lying resonance states. The resonance states at lower energy have been reported by many authors, while just few features have been observed and calculated at higher energy, and the discrepancies between experimental observations and theoretical calculations are exist. In addition, there are few theoretical studies which provide unambiguous information on the classifications of the observed resonances.In the present work, we represented new results for the negative-ion resonance in electron-magnesium scattering from 2-5.5 eV. These results are obtained by calculating total cross sections employing the CCO method with a model included real part of the optical potential. The formation of negative ions by closed-shell atoms is caused by the correlation interaction of the extra electron with the neutral atom. At large distances this nonlocal energy-dependent interaction turns into a local polarization potential of the form whereαis the electric-dipole polarizability of the atom. The long-range polarization potential and the short range potential of the target induces a variation in scattering cross sections, which indicates the presence of a resonance state of the projectile electron-atom system. It is very important to include the potential. The CCO method has now been applied for more than a decade in studies of electron and positron collision with atoms. A distinct feature of the model is that it can represent the many-body information in a conveniently way that is achieved by an ab initio complex equivalent local potential has been used to describe excitation of the continuum of target. The real part of the potential represents polarization of targets. At large distances, the method allows us to take the long range polarization and short-range interaction of the incident electron and the target accounted in calculations. Now we apply this method to investigate negative-ion resonances in electron-magnesium collision in the energy range 2-5.5 eV, essential techniques of the method have already been described and need not be repeated here.It is sufficient to note that the space of target states has been split two parts. In the present calculation, the P space consists of the 13 discrete channels: 3s~2 ~1S, 3s3p ~3P, 3s3p ~1P, 3s4s ~3S, 3s4s ~1S, 3s3d ~1D, 3s4p ~3P, 3s3d ~3D, 3s4p ~1P, 3s5s ~1S, 3s4d ~1D, 3s4d ~3D and 3s5p ~1P are explicitly coupled in P space. Bound states of magnesium are represented by CI wave functions. The basic orbitals used in the CI representation of those states consists of 8 orbitals, namely, 3, 4, 5s; 3, 4, 5p; 3, 4d. The real part of optical potential are used in the channel coupled, since the impact energy region is below the ionization threshold of magnesium. The real part describes virtual (off-shell) excitation of the Q space. At long-range the potential has asymptotic behavior of the dipole polarization potential:α/2r~4, andαis the dipole polarizability of magnesium. We obtained some resonances structures by calculatingscattering cross sections and elastic channel's phase shifts. It can be found that there is better agreement of our results with the measurements than other theories. All we have found are core-excited shape resonances. At 3s3p2 2D state of Mg-, a d wave resonance is found at 2.716 eV, in excellent agreement with the experimental and other theoretical values. This distinctive structure is caused by the opening of new channels at the 3s3p ~3P excitation threshold. Another d wave resonance appears at 4.203 eV, and it is a 3s3p~2 ~2S resonance, in a similar level of agreement to the above with the structure was gotten experimentally and theoretically. This resonance converges to the 3s3p ~1P threshold of Mg. Luckily, a p wave resonance behavior is found at 5.172 eV, in good general accord with the measurement of Leep and Gallagher, which has not been revealed in other calculations. It is most likely the 3s4s~2 resonance of Mg~-, which is associated with opening of the 3s4s ~3S level. The peak of our resonances gets weaker and weaker as the energy increases, till the 3s4s4p figure can be hardly found near the thresholds of the 3s4s ~1S state, which is observed at 5.25 eV by Sullivan et al. and found calculated near 5.5 eV by Fursa and Bray. Because in Mg where the spin-orbit interaction is weak, we may expect that the different spin-orbit terms will be degenerate and that there will be substantial overlap of the resonance features. The close coupling calculations are less satisfactory and require additional investigation with a closer energy grid, and also the experiments get the high precision result difficultly.Resonances of low-energy electron scattering from magnesium at 2-5.5 eV using the coupled-channel optical (CCO) method are calculated fairly well. Our result is the most accurate to date and in good agreement with experimental work, although several calculations point toward a significantly lower value and the general discrepancy between theory and experiment is presently not understood. Thus we may try to use CCO method to set out resonances of other collisions.