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

Abstract

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.

Characterization of the 3dδ Rydberg state of MgAr+ using a quantum-control optical scheme

“…The 3dπ Rydberg states studied in Ref. [17] possess the same electronic symmetry as the charge-transfer states ( 21/2,3/2 ) and homogeneous interactions cause the rapid decay of most of their vibrational levels by predissociation, which can happen on the sub-ps timescale. The 3dδ states, on the other hand, possess different electronic symmetries ( 23/2,5/2 ), allowing one to study whether and, if so, how heterogeneous charge-transfer interactions affect the structure and dynamics of the Rydberg states of molecular ions.…”

“…In addition to their interest for high-resolution photoelectron spectroscopy [7][8][9][10], the Rydberg states of molecular ions act as transient resonances in ion-neutral collisions [11][12][13][14]. Recent efforts have been directed toward the systematic investigation and characterization of the low-and high-lying Rydberg states of the MgAr + molecular ion [10,[15][16][17]. These efforts are motivated by the fact that MgAr 2+ is so far the only thermodynamically stable doubly charged ion that has been characterized by highresolution spectroscopy [10].…”

“…The low-lying 3p Rydberg complex [15,16] and the 3dπ state belonging to the 3d complex [17] of MgAr + were characterized by isolated-core multiphoton Rydberg dissociation (ICMRD) spectroscopy [18]. This method is analogous to resonance-enhanced multiphoton dissociation (REMPD) spectroscopy [19,20] but for the ion core of a Rydberg molecule.…”

“…[17]). The potentialenergy functions of these five groups of states cross and this energy region represents a prototypical example of the major role played by charge-transfer nonadiabatic interactions in the dynamics of the Rydberg states of molecular ions [17]. The 3dπ Rydberg states studied in Ref.…”

“…[15] and the curves of the charge-transfer (CT) states were taken from Ref. [17]. The red horizontal arrow indicates predissociation of 3dπ vibrational levels into the continua associated with the CT states.…”