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

What is it about?

We theoretically study 37 Rydberg states of HeH derivable from the He(1s2) + H(nl) configuration with n ≤ 5 along with three states with n = 6. The multireference configuration interaction method is used to calculate their electronic energy curves (EECs) for the internuclear distance R from 0 to 28a0, which are regarded as the united-atom (UA) and separated-atoms (SA) limits, respectively. The 15 Rydberg series (nsσ, npσ, ndσ, nfσ, ngσ, ndδ, nfδ, ngδ, ngγ, npπ, ndπ, nfπ, ngπ, nfϕ, and ngϕ) are identified. Smith’s theory of diabatization is directly solved to obtain the diabatic quantum defect curves (QDCs). From the correlations in diabatic QDCs between the UA and SA limits, the following order of orbital energies in the equilibrium region is found: npσ < nsσ < npπ < ndσ < ndπ < nfσ < nfπ < nfδ < nfϕ < ndδ, except 5dδ < 5fϕ. This order is identical to that in one-electron molecular systems at small R, except for the nsσ and npπ series. The correlation rules are nUA = nSA + 1 for the npσ series and nUA = nSA for other series; additionally, lUA = lSA for all cases except for the 2pσ − σ1s correlation. A reversal of the diabatic dipole moment at avoided crossing points of the nsσ and npσ series is observed, and explained by the behaviors of ns and np wavefunctions of H at small R.

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Why is it important?

This work provides the best quality calculations for 37 Rydberg states basically for entire range of nuclear separation. Almost no work has beend done with such a quality and such a wide (entire) range of calculations. This paper may be the first one in which the Smith's diabatic equations are solved for 8 states and expressed in terms of 28 rotational angles for transformation matrix from adiabatic to diabatic states by making use of maple, the code of which is available in suplementary material. Also this is the molecule for which Smith's definition of diabatic states works quite well. Moreover, dipole moment curves are calculated and compared with the curves obtained by using the Buldakov et al's formula at small R. The agreement is remarkable. In summary, HeH is a rare molecule for which currently known theories work quite well. Such agreements are not obtained in general. Therefore, this work may served as a standard work for Smith's diabatization, Buldakov's theory for dipole moment curves and so on.

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