Assignment of the first five electronic states of Ar2+ from the rotational fine structure of pulsed-field-ionization zero-kinetic-energy photoelectron spectra

The $\mathrm{I}\text{(1/2u),}$ $\mathrm{I}\text{(3/2g),}$ $\mathrm{I}\text{(1/2g),}$ $\mathrm{I}\text{(3/2u),}$ and $\mathrm{II}\text{(1/2u)}$ states of ${\mathrm{Ar}}_2^{+}$ have been investigated by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy following ${\text{(1+1}}^{\prime })$ resonance-enhanced two-photon excitation via the $0_u^{+}$ Rydberg state located below the ${\mathrm{Ar}}^{*}$ ${\text{([3p]}}^5{\text{4s}}^{\prime }{\text{[1/2]}}_1\mathrm{)+}\mathrm{Ar}{(}^1{S}_0)$ dissociation limit of ${\mathrm{Ar}}_2.$ By selecting single rotational levels of the intermediate state, the rotational structure of five of the six lowest electronic states of ${\mathrm{Ar}}_2^{+}$ could be observed. Photoionization selection rules and the magnitude of the observed Ω-doubling of the rotational levels were used to derive unambiguous assignments of the electronic symmetry of the ionic states. From the analysis of the rotational structure, the equilibrium internuclear distances, the adiabatic ionization potentials, the dissociation energies and vibrational and anharmonic constants could be determined for the $\mathrm{I}\text{(1/2u),}$ $\mathrm{I}\text{(3/2g),}$ $\mathrm{I}\text{(1/2g),}$ $\mathrm{I}\text{(3/2u),}$ and $\mathrm{II}\text{(1/2u)}$ states of ${\mathrm{Ar}}_2^{+}.$ A map of the rotational energy level structure of these states, which includes complete symmetry labels, has been derived. The classification of the rotational structure of the first electronic states of ${\mathrm{Ar}}_2^{+}$ in Hund’s cases (a), (b), and (c) is discussed on the basis of the dependence of the ${}^2{\mathrm{\Sigma –}}^2\Pi$ interaction on the internuclear separation.