Complete characterization of the 3p Rydberg complex of a molecular ion: MgAr. I. Observation of the Mg(3p)Ar B state and determination of its structure and dynamics.

We report on the experimental observation of the BΣ state of MgAr located below the Mg(3p P) + Ar(S) dissociation asymptote. Using the technique of isolated-core multiphoton Rydberg-dissociation spectroscopy, we have recorded rotationally resolved spectra of the BΣ(v') ← XΣ(v″ = 7) transitions, which extend from the vibrational ground state (v' = 0) to the dissociation continuum above the Mg(3p P) + Ar(S) dissociation threshold. The analysis of the rotational structure reveals a transition from Hund's angular-momentum-coupling case (b) at low v' values to case (c) at high v' values caused by the spin-orbit interaction. Measurements of the kinetic-energy release and the angular distribution of the Mg fragments detected in the experiments enabled the characterization of the dissociation mechanisms. The vibrational levels of the B state above v' = 6 are subject to predissociation into the Mg(3p P) + Ar(S) continuum, and the fragment angular distributions exhibit anisotropy β parameters around 0.5, whereas direct dissociation into the continuum above the Mg(3p P) + Ar(S) asymptote is characterized by β parameters approaching 2. Molecular ions excited to the B state with v' = 0-6 efficiently absorb a second photon to the repulsive part of the Σ state associated with the Mg(3d D) + Ar(S) continua. The interpretation of the data is validated by the results of ab initio calculations of the low-lying electronic states of MgAr, which provided initial evidence for the existence of bound vibrational levels of the B state and for the photodissociation mechanisms of its low vibrational levels.