Collisional charge transfer reactions (CCTRs) are widely found in complex gas-phase environments such as interstellar medium, planetary atmospheres, and plasmas, etc. Exploring the mechanisms of CCTRs at the molecular level is scientifically important to analyze the matter evolution and energy transfer processes in these complex gas-phase environments.Ar+ + N2 → Ar + N2+ is the classical model system for investigating the dynamics of CCTRs, and it has been extensively investigated experimentally and theoretically over the past half century. experimental and theoretical studies in the past half century. However, the knowledge of the charge transfer mechanism at the molecular level of this model system is limited due to the controversy between different experimental studies and the lack of agreement between experimental and theoretical calculations. This is due to the relatively low energy resolution of product probing in previous experiments, which makes it difficult to obtain the quantum state distribution of the reaction products; in previous experiments, the reactant ion beams contained both spin-orbit quantum states of Ar+ ions, i.e., the ground state Ar+ (2P3/2) and the excited state Ar+ (2P1/2), making it difficult to distinguish between different spin -orbital states of Ar+ ions to the relative contributions of the reaction products.
Gao Hong's group, a researcher at the Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, independently designed and built a quantum state-selective ion-molecule cross-beam device. Its energy resolution reaches the leading level of similar instruments in the world. This study is the first to use laser ionization to prepare high-purity pulsed ion beams in specific quantum states. The kinetic energy of the ion beam is continuously adjustable in the range of 1.0-5.0 eV, with a kinetic energy spread of 150-300 meV. Meanwhile, the team designed a three-dimensional ion velocity imaging system, which can simultaneously realize the velocity focusing and time focusing of the product ions, with a velocity resolution of up to 1.5%.
Recently, the team has made important progress in the study of the spin-orbit state-selected charge transfer reaction Ar + (2P3/2) ＋ N2 → Ar ＋ N2 + (v′, J′). In this study, a pulsed ion beam of Ar + (2P3/2) in the spin-orbit ground state with a purity better than 97% was prepared using a resonance-enhanced multiphoton ionization method. The ion beam reaches the reaction center after decelerated focusing, crosses perpendicularly with the collimated N2 ultrasonic molecular beam and undergoes a charge transfer reaction. The three-dimensional velocity distribution of the reaction product N2+ ions was precisely measured by a three-dimensional ion velocity imaging device. The experiment yielded the best-resolved scattering images to date (Fig. a), and for the first time, the vibrational and transdynamic distributions of the product N2+ ions and their correlation with the scattering angle were accurately measured. Professor Hua Guo and Dr. Dandan Lu of the University of New Mexico, USA, carried out full-dimensional trajectory surface hopping calculations on the reaction system. The calculations reached a semi-quantitative agreement with the experimental results, revealing for the first time the strong product vibrational dynamics-dependent charge transfer mechanism of the reaction (Figs. b-d). It is shown that the reaction has two completely different charge transfer mechanisms simultaneously. One is the classical Harpoon charge transfer mechanism determined by long-range interactions, which occurs mainly in the N2+ (v′ = 1) product channel. The N2+ ions produced by this process are concentrated in the forward scattering region and have low rotational excitation (Fig. c). The other mechanism plays a major role in the N2+ (v′= 2) product channel. The channel products are predominantly distributed in the forward region but have high rotational excitation (Fig. d), which is inconsistent with the predictions of the classical hard-sphere collision model. Theoretical calculations show that this is a Hard collision glory scattering (HCLS) process caused by a delicate balance between the long-range attractive and short-range repulsive potentials of the two reactant molecules, which is the first time that scientists have observed a singular scattering mechanism in a charge-transfer reaction.
This work realizes the kinetic study of collisional charge transfer from quantum state to quantum state, and clarifies the long-standing controversy in the study of the classical charge transfer reaction Ar + + N2 → Ar + N2 +. The relevant research results were published in Nature Chemistry (DOI: 10.1038/s41557-023-01278-y). The research work was supported by the Chinese Academy of Sciences, the Natural Science Foundation of Beijing and the National Research Center for Molecular Sciences in Beijing.
ICC and others made progress in the study of charge transfer reactions with spin-orbit state selection
(a) Scattering diagram of the product N2+, (b) theoretically calculated rotational quantum state distributions of different vibrational energy levels of N2+ and correlation plots of rotational excitations versus scattering angles for the v′ = 1 (c) and v′ = 2 (d) vibrational energy levels of N2+.
