Elsevier

Chemical Physics

Volume 246, Issues 1–3, 15 July 1999, Pages 57-64
Chemical Physics

Potential energy curves of 58 states of Li2+

https://doi.org/10.1016/S0301-0104(99)00084-1Get rights and content

Abstract

Adiabatic potential curves for all electronic states correlated to Li++Li[2s] up to Li++Li[6g] have been computed in the framework of a model potential method, over a rather large range of internuclear distances (4 a.u.<R<40 a.u.). A good agreement has been obtained with previous theoretical determinations as well as with available experimental results for the ground and the first excited states. Long-range-type predictions for the lowest states have been considered.

Introduction

The spectroscopy of alkali dimers has received considerable attention during the last twenty years with a recent renewed interest due to the photoassociative spectroscopy of cold alkali atoms. For these species accurate data are available from both experimental and theoretical sources. The situation is by far not so favourable for alkali dimer ions M2+. To the best of our knowledge, only the ground state of Li2+ 1, 2and the ground state [3]and the first excited state 2Πu [4]of Na2+ have yet been observed. For these two ions, all-electron ab initio as well as pseudopotential and model potential calculations have been performed 5, 6, 7, 8, 9for the lowest states. In the framework of the model potential method, adiabatic potential curves have recently been computed [10]over a rather large range of internuclear distances for all the molecular states of Na2+ correlating up to the limit Na++Na[7i]. Such results are involved as a first step in a model potential approach of alkali dimers and they were used to describe with a good accuracy numerous electronic states of Na2 [11].

Here we present similar results for all molecular states of Li2+ correlating up to the limit Li++Li[6g] in view of further calculations for highly excited states of Li2 as well as investigations of multiphotonic dissociation of Li2+. After a recall of our model potential approach for Li2+ in Section 2, results are presented in Section 3. A long-range-type description for the lowest states is also considered in Section 4.

Section snippets

Recall of the model potential method for Li2+

The Li2+ ion is treated as a system with a one-active electron moving in the field of two ionic cores. Then three interactions are to be considered and are described by effective potentials. The interaction between the active electron and each ionic core is represented by a Klapisch [12]model potential:V(r)=−1rZ−1r(e−7.90875r+5.1605re−3.90006r).The interaction due to core polarization effects is represented by an effective potential, first proposed by Bottcher and Dalgarno [13]and afterwards

Potential curves and spectroscopic constants

Molecular potential curves of the 2Σ+g,u, 2Πg,u and 2Δg,u states have been computed for R=4a0 to 40a0. They are displayed in Fig. 1Fig. 2Fig. 3Fig. 4Fig. 5Fig. 6 for electronic states of a given symmetry dissociating into Li++Li[2s] up to Li++Li[6g]. Present curves are compared with those from Schmidt et al. [5]for the ground and the first excited states in Fig. 7. The overall agreement is seen to be good.

As previously observed for Na2+ [10], numerous avoided crossings appear in the higher

Long-range-type predictions for some potential curves

Present model potential results show that some molecular states of Li2+ are slightly bound with rather large equilibrium internuclear distances. Then they may be predicted through a long-range-type model in which molecular energies are computed as the sum of atomic energies E(Li+)+E(Li[nl2L]) plus an interaction energy. The interaction energy has been calculated in a one-active-electron approach, via a perturbative scheme for the Coulombic part [17]and from a surface integral approach for the

Conclusions

We have reported model potential calculations for the ground and highly excited states of Li2+, over a wide range of internuclear distances. Comparison with available experimental data and other theoretical works are very satisfactory. In the case of the first excited states, we have also presented a long range model accurate enough to permit the prediction of potential curves up to internuclear distances close to 200a0.

Informations contained in this paper will be used in the determination of

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