Abstract
A comparative study of two earlier three-state and fifteen-state [Chaudhuri et al., Pramana- J. Phys., 43, 175 (1994); ibid., Phys. Rev. A, 52, 1137 (1995)] close-coupled treatments of He2+-He single and double charge transfer collisions is made in this paper with a larger, 27-state close-coupled calculation. The calculations have been done using the diabatic molecular basis set used in the earlier work extended by adding excited orbitals leading to higher excitation channels up to 3s and 3p. For such molecular basis functions that go to the correct separated-atom limits used in this work, the present results show that without the inclusion of the electron translation factors (ETFs) the quantitative cross-section calculations up to velocity ~1.1 a.u. (~30 keV/amu) are in good agreement with both experiment and other calculations. This suggests that if ETFs are properly incorporated into the charge transfer collision studies at low energies this diabatic molecular basis can be used for benchmark calculations. With the aid of the Wannier's picture of the ground state correlations, a combined detailed analysis and comparison has been carried out to find a connection with the dynamic two-electron correlation picture in charge transfer collision processes which involve sequential/simultaneous two-electron exchange. If ETFs are included, the model approach of the present work may open up opportunities to investigate dynamic two-electron correlation effect in charge transfer ion-atom collision processes with benchmark accuracy. PACS codes: 34.70.+e, 34.20.-b
Highlights
We have previously [1,2] made use of a diabatic molecular basis expansion constructed in a similar spirit as in an earlier work [3], to carry out a close-coupled study of single and double charge transfer in He2+-He(1s2) collisions in the impact parameter approximation without electron translation factors (ETFs)
For a two-electron system, an additional property that comes into the picture is electron correlation. It was in this context that the usefulness of a configuration interaction to incorporate a certain degree of electron correlation was discussed in our earlier work [2] where we showed that, in our diabatic basis, early truncation of the series involves a sacrifice of configuration interaction and can lead to neglect of electron correlation. (This was, a demonstration of a point already made by Smith [12].)
Comparison of a 27-state close-coupled calculation of He2+-He charge transfer collision with earlier 3-state and 15-state calculations suggests that (i) dynamic electron correlation effects become visible as the basis set is progressively increased, i.e., as the configuration interaction is taken into account more properly, provided the mentioned conditions in section-4 are satisfied, and (ii) present results provide the importance of these correlation effects in the resonant two-electron
Summary
We have previously [1,2] made use of a diabatic molecular basis expansion constructed in a similar spirit as in an earlier work [3], to carry out a close-coupled study of single and double charge transfer in He2+-He(1s2) collisions in the impact parameter approximation without electron translation factors (ETFs). The results of the fifteen-state calculation agreed very well with experiment [4,5] up to around 10 keV/amu, above which the calculated results dropped rather markedly It could not be ascertained whether the discrepancy at higher energies was due to the basis set truncation, or due to the neglect of ETFs, or both. From a detailed comparative study among the 3-state [1], 15-state [2] and 27-state, the present work focuses on the investigation of the dynamic electron correlation in the two-electron ion-atom collision system He2+-He(1s2) with the help of the Wannier's picture of the ground state correlations. This work is carried out mainly with an aim that, whether or not, present approach based on the close-coupled 27-state diabatic molecular basis calculations without ETFs can be applied to investigate complicated and interesting dynamic electron correlation behaviour in a two-electron ion-atom collision system. If one can choose to study the phenomena of dynamic correlated behaviour of electronic motions in ion-atom/molecule collisions with computational ease (i.e., neglecting ETFs) using the model approach of the present work, or not
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