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

We report on the experimental observation of the B+ 2 Sigma(+) state of MgAr+ located below the Mg+(3p P-2(3/2)) + Ar(S-1(0)) dissociation asymptote. Using the technique of isolated-core multiphoton Rydberg-dissociation spectroscopy, we have recorded rotationally resolved spectra of the B+ 2 Sigma(+)(v ') <- X+ 2 Sigma(+)(v '' = 7) transitions, which extend from the vibrational ground state (v ' = 0) to the dissociation continuum above the Mg+(3p P-2(3/2)) + Ar(S-1(0)) 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-2(1/2)) + Ar(S-1(0)) continuum, and the fragment angular distributions exhibit anisotropy beta parameters around 0.5, whereas direct dissociation into the continuum above the Mg+(3p P-2(3/2)) + Ar(S-1(0)) asymptote is characterized by beta 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 (2)Sigma(+) state associated with the Mg+(3d D-2(3/2,5/2)) + Ar(S-1(0)) 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.