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

• Published in: The Journal of chemical physics Vol. 153; no. 7; pp. 074310 - 74326
• Main Authors: Wehrli, Dominik, Génévriez, Matthieu, Knecht, Stefan, Reiher, Markus, Merkt, Frédéric
• Format: Journal Article
• Language: English
• Published: Melville American Institute of Physics 21.08.2020
• Subjects: Angular distribution; Angular momentum; Anisotropy; Coupling (molecular); Electron states; Molecular ions; Parameters; Photodissociation; Spectrum analysis
• ISSN: 0021-9606; 1089-7690; 1520-9032; 1089-7690
• DOI: 10.1063/5.0015603

We report on the experimental observation of the B+ 2Σ+ state of MgAr+ located below the Mg+(3p 2P3/2) + Ar(1S0) dissociation asymptote. Using the technique of isolated-core multiphoton Rydberg-dissociation spectroscopy, we have recorded rotationally resolved spectra of the B+ 2Σ+(v′) ← X+ 2Σ+(v″ = 7) transitions, which extend from the vibrational ground state (v′ = 0) to the dissociation continuum above the Mg+(3p 2P3/2) + Ar(1S0) 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 2P1/2) + Ar(1S0) continuum, and the fragment angular distributions exhibit anisotropy β parameters around 0.5, whereas direct dissociation into the continuum above the Mg+(3p 2P3/2) + Ar(1S0) 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 2Σ+ state associated with the Mg+(3d 2D3/2,5/2) + Ar(1S0) 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.