Charge-Transfer-Induced Predissociation in Rydberg States of Molecular Cations: MgAr.

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(1S0) dissociation asymptotes. In particular, we have recorded spectra of the 3dπΩ' (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 2Σ and 2Π charge-transfer states associated with the Mg(3s2) + Ar+(2P1/2,3/2) 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 2Π Rydberg state correlating to the Mg+(4p) + Ar(1S0) 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 MgAr2+. 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.

Spectroscopic characterization of a thermodynamically stable doubly charged diatomic molecule: MgAr2.

Although numerous doubly positively charged diatomic molecules (diatomic dications) are known from investigations using mass spectrometry and ab initio quantum chemistry, only three of them, NO2+, N22+ and DCl2+, have been studied using rotationally resolved optical spectroscopy and only about a dozen by vibrationally resolved double-ionization methods. So far, no thermodynamically stable diatomic dication has been characterized spectroscopically, primarily because of experimental difficulties associated with their synthesis in sufficient densities in the gas phase. Indeed, such molecules typically involve, as constituents, rare-gas, halogen, chalcogen, and metal atoms. We report here on a new approach to characterize molecular dications based on high-resolution photoelectron spectroscopy of the singly charged parent molecular cation and present the first spectroscopic characterization of a thermodynamically stable diatomic dication, MgAr2+. From the fully resolved vibrational and partially resolved rotational structures of the photoelectron spectra of 24MgAr+ and 26MgAr+, we determined the potential-energy function of the electronic ground state of MgAr2+, its dissociation (binding) energy (D0 = 10 690(3) cm-1), and its harmonic (ωe(24MgAr2+) = 327.02(11) cm-1) and anharmonic (ωexe(24MgAr2+) = 2.477(15) cm-1) vibrational constants. The analysis enables us to explain quantitatively how the strong bond arises in this dication despite the fact that Ar and Mg2+ both have a full-shell rare-gas electronic configuration.

Complete characterization of the 3p Rydberg complex of a molecular ion: MgAr+. II. Global analysis of the A+ 2Π and B+ 2Σ+ (3pσ,π) states.

We report a global study of the 3p Rydberg complex of the MgAr+ molecular ion. High-resolution spectroscopic data on the two spin-orbit components of the A+ electronic state were obtained by isolated-core multiphoton Rydberg-dissociation spectroscopy up to vibrational levels as high as v' = 29, covering more than 90% of the potential wells. Accurate adiabatic potential-energy functions of the A+ and B+ states, which together form the 3p Rydberg complex, were obtained in a global direct-potential-fit analysis of the present data and the extensive data on the B+ state reported in Paper I [D. Wehrli et al., J. Chem. Phys. 153, 074310 (2020)]. The dissociation energies of the B+ state, the two spin-orbit components of the A+ state, and the X+ state of MgAr+ are obtained with uncertainties (1 cm-1) more than two orders of magnitude smaller than in previous studies.

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+ 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 ß parameters around 0.5, whereas direct dissociation into the continuum above the Mg+(3p 2P3/2) + Ar(1S0) asymptote is characterized by ß 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.

Determination of the Interaction Potential and Rovibrational Structure of the Ground Electronic State of MgAr+ Using PFI-ZEKE Photoelectron Spectroscopy.

The interaction potential characterizing the ground electronic state of MgAr+ has been determined from the photoelectron spectrum recorded from the a 3Π0 metastable state of MgAr at high resolution by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The photoelectron spectrum provides information on the first ten vibrational levels of MgAr+ and leads to the determination of the adiabatic ionization energy of metastable MgAr (38 742.3(20) cm-1), the ground state dissociation energy of MgAr+ (1254(60) cm-1), and to the characterization of the rovibrational photoionization dynamics of MgAr.