Charge-Transfer-Induced Predissociation in Rydberg States of Molecular Cations: MgAr(+)

[Image: see text] Very little is known about the Rydberg states of molecular cations, i.e., Rydberg states having a doubly charged ion core. With the example of MgAr(+), we present general features of the structure and dynamics of the Rydberg states of molecular cations, which we find are subject to the process of charge-transfer-induced predissociation. Our study focuses on the spectrum of low-n Rydberg states with potential-energy functions associated with the Mg(+)(3d and 4s) + Ar((1)S(0)) dissociation asymptotes. In particular, we have recorded spectra of the 3dπ(Ω′) (Ω′ = (1)/(2), (3)/(2)) Rydberg states, extending from the lowest (v′ = 0) vibrational levels to their dissociation limits. This spectral range encompasses the region where the onset of predissociation by interaction with the mostly repulsive (2)Σ and (2)Π charge-transfer states associated with the Mg(3s(2)) + Ar(+)((2)P(1/2,3/2)) dissociation asymptotes is observed. This interaction leads to very strong perturbations of the 3dπ Rydberg states of MgAr(+), revealed by vibrational progressions exhibiting large and rapid variations of the vibrational intervals, line widths, and spin–orbit splittings. We attribute the anomalous sign and magnitude of the spin–orbit coupling constant of the 3dπ state to the interaction with a (2)Π Rydberg state correlating to the Mg(+)(4p) + Ar((1)S(0)) dissociation limit. To analyze our spectra and elucidate the underlying process of charge-transfer-induced predissociation, we implemented a model that allowed us to derive the potential-energy functions of the charge-transfer states and to quantitatively reproduce the experimental results. This analysis characterizes the main features of the dynamics of the Rydberg series converging to the ground state of MgAr(2+). We expect that the results and analysis reported here are qualitatively valid for a broader range of singly charged molecular cations, which are inherently prone to charge-transfer interactions.