On the R-dependence of the spin-orbit coupling constant: Potential energy functions of Xe2+ by high-resolution photoelectron spectroscopy and ab initio quantum chemistry

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe2 has been measured between 97 350 and 108200cm−1, following resonant two-photon excitation via selected vibrational levels of the C0u+ Rydberg state of Xe2. Transitions to three of the six low-lying electronic states of Xe2+ could be observed. Whereas extensive vibrational progressions were observed for the transitions to the I(3∕2g) and I(3∕2u) states, only the lowest vibrational levels of the II(1∕2u) state could be detected. Assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts and from the modeling of the potential energy curves. Adiabatic ionization energies, dissociation energies, and vibrational constants are reported for the I(3∕2g) and the I(3∕2u) states. Multireference configurational interaction and complete active space self-consistent field calculations have been performed to investigate the dependence of the spin-orbit coupling constant on the internuclear distance. The energies of vibrational levels, measured presently and in a previous investigation (Rupper et al., J. Chem. Phys. 121, 8279 (2004)), were used to determine the potential energy functions of the six low-lying electronic states of Xe2+ using a global model that includes the long-range interaction and treats, for the first time, the spin-orbit interaction as dependent on the internuclear separation.