Rydberg series of¹Σ_u⁺and¹Δ_ustates of the Li₂molecule studied by the promotion model

Abstract

We studied the Rydberg series for the 12 1Σu+, 7 1Δu, and 1 1Γu states. This set consists of the seventeen states derivable from the Li(2s) + Li(nl) (n = 2, 3, 4, 5) configuration, one 1Σu+ state and one 1Δu state derivable from Li(2s) + Li(6p) and Li(2s) + Li(6d), respectively, plus one ionic state. We used the multireference configuration interaction method, combined with the Stuttgart/Köln group’s effective core potential/core polarization potential method, to calculate their potential energy curves (PECs). Four Rydberg series, σuns (n = 3, 4, 5, 6), σunp (n = 2, 3, 4, 5), δund (n = 3, 4, 5, 6), and δunf (n = 4, 5), are identified near the potential energy minima (referred to as the ‘Rydberg region’). The promotion model is used to examine the behaviour of PECs and quantum defect curves (QDCs) by constructing diabatic PECs and QDCs. Besides QDCs, effective n and promotion curves are also used to directly examine the promotion. This reveals that the promotion model can be successfully applied to the singlet ungerade states of Li2, indicating that the Rydberg region has the same characteristics as the united atom (UA) limit. Thus, the Rydberg region in Li2 is much closer to the UA limit than the separated atoms (SA) limit. Correlation diagrams based on the promotion model show a big difference in the number of avoided crossings for the 1Σu+ and 1Δu states. The larger number of avoided crossings for 1Σu+ states produces features in the PECs of the 1Σu+ states such as shoulders, flattening, and grouping. The promotion model not only helps to identify Rydberg series, but also to explain all the major and minor aspects and subtle phenomena observed in the PECs and spectroscopic constants of the singlet ungerade Rydberg states of Li2.

A Study of 15 Singlet Rydberg Series of HeH and Their Correlation with the Rydberg Series of Li

“…In order to obtain reliable MRCI results for such a large number of excited states, we need good basis sets, suitable active space, and additional universal Rydberg basis functions (devised by Kaufmann) for convergence . For the valence basis functions, we used Dunning's augmented correlation‐consistent quintuple (AV5Z) basis set for He, and the sextuple (AV6Z) basis set for H .…”

“…The involvement of higher l states also adds to the complexity. Our recent study on the Rydberg series of the Li 2 molecule showed that the assignment requires correlation information between the Rydberg orbitals of the united‐atom (UA) and the separated‐atoms (SA) limits …”

Analytical Correlation Rules between States in the United Atom and Separated Atom Limits for Diatomic Screened Orbitals

“…Correlations between the united‐atom (UA) and separated‐atom (SA) states play important roles in various fields . Correlation rules were obtained by Gershtein and Krivchenkov for heterogeneous one‐electron diatomic molecular systems .…”