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 the companion article. 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. I. INTRODUCTIONThe complete experimental characterization of molecular Rydberg states necessitates the measurement of spectra of a large number of states of different principal and angular-momentum quantum numbers and, for each of them, of a large number of rotational and vibrational levels, extending from the vibrational ground state to levels close to the dissociation threshold 1-10 . This task is particularly difficult for cations because of the low densities in which they can be produced compared to neutral species and the larger photon energies required for electronic excitation. In addition, diatomic cations produced by conventional ion sources are in most cases in their vibrational ground state, which restricts the accessible vibrational levels of electronically-excited states to those having significant Franck-Condon overlap with the ground vibronic state. For this reason, hardly any systematic studies of the Rydberg states of molecular cations have been reported so far. This situation is in stark contrast to that prevailing for the ground and lowlying valence states of cations, for which many examples of detailed characterization by high-resolution spectroscopy are known, see, e.g., Refs. 11-20 and references therein.We showed recently, with the example of MgAr + , that state-selected cations can be prepared in a wide range of vibrational levels of the electronic ground state by photoexcitation of neutral molecules to high-lying Rydberg states 21 . For sufficiently large values of the principal quantum number (n ≥ 100) or orbital-angularmomentum quantum number of the Rydberg electron, the ionic core of the Rydberg molecule can be considered as isolated from the Rydberg electron, and behaves as the bare ion. With the method of isolated-core multiphoton Rydberg-dissociation (ICMRD) spectroscopy 21 , the ionic core is first prepared in a selected vibrational level of a)

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

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.

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The photodetachment cross section of H~ was measured using the animated-crossed-beam technique in the 700-to 1064-nm range. The laser beam was repeatedly swept across the ion beam by tilting a fused silica plate in order to eliminate the need for beam profile measurements. After integration of the signal, the cross section was expressed in terms of easily measurable quantities, e.g., the laser power and the ion beam current. The present results are in excellent agreement with previous experiments and compelling theoretical works.