The electronic states of Ar+2, Kr+2, Xe+2. I. Potential curves with and without spin–orbit coupling

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Abstract
The low‐lying states of Ar+2, Kr+2, and Xe+2 have been investigated using the POL CI method. Spin–orbit coupling has been included with a simple atoms‐in‐molecule approach. The calculated dissociation energies for the ground I (1/2)u states of Ar+2 and Kr+2 are in good agreement (10% error) with experiment, while the agreement is slightly worse (20% error) for Xe+2. The well depth decreases from 1.19 eV in Ar+2 to 0.79 eV in Xe+2 mainly because of spin–orbit effects. As expected, the calculated bond distances increase from Ar+2 to Xe+2 as the atoms increase in size. The first excited state, I (3/2)g, possesses a small well (0.12 eV) at larger distances (3–4 Å) in all the rare gas dimer ions. The higher excited states arising from the lowest asymptote are more repulsive. There are three dipole‐allowed transitions from the ground state. The I (1/2)u→I (3/2)g transition, which occurs in the near infrared, is very weak in accordance with propensity rules based on changes in Ω. The I (1/2)u→I (1/2)g transition occurs between 700–800 nm and grows dramatically in intensity from Ar+2 to Xe+2 because of spin–orbit effects. The predicted increase in intensity is in excellent agreement with experiment. Finally, the I (1/2)u→II (1/2)g transition is very strong with the intensity decreasing slightly for the heavier rare gas dimer ions. The absorption maxima are predicted at 319, 339, and 375 nm for Ar+2, Kr+2, and Xe+2, respectively, which are to be compared with the experimental values of 292 and 325 nm for Ar+2 and Kr+2, respectively. The theoretical values for the peak absorption cross sections are in good agreement with experiment for Ar+2 and Kr+2. Thus, although the calculated wavelengths for the peak absorption are too long by 20–30 nm, the size and shape of the calculated absorption bands should be in good agreement with experiment. Finally, a comparison is made between the ab initio SCF potentials for the 1Σ+g state of Ar2, Kr2, and Xe2 with the potentials predicted by the electron gas model.