Resonance-enhanced multiphoton ionization of argon: reactivity as a probe for the 2P12/2P3/2 population

Imaging the Collision Energy-Dependent Charge-Transfer Dynamics between the Spin-Orbit Ground Ar+(2P3/2) Ion and CO

The collisional charge-transfer reaction between Ar+(2P3/2,1/2) and CO represents one of the most studied ion-molecule systems; many controversies persist among different studies, and the detailed quantum state-to-state charge-transfer dynamics remains unknown. Here, differential cross sections of the charge-transfer process between the spin-orbit ground Ar+(2P3/2) ion and CO are reported at three center-of-mass collision energies of 1.02, 0.72, and 0.40 eV using a home-built three-dimensional velocity-map imaging-based ion-molecule crossed beam setup. At all three collision energies, the direct energy resonant charge-transfer mechanism dominates the reaction, featuring predominantly forward scattering with the CO+ product population peaking at the v' = 6 and v' = 7 vibrational levels. Only at the lowest collision energy of 0.40 eV is the significant backward peaked scattering product observed, with CO+ populated from v' = 4 to v' = 8. There is no obvious evidence for the formation of CO+ in excited electronic state A2Π+, in qualitative accord with previous theoretical predictions.

Imaging the state-to-state charge-transfer dynamics between the spin-orbit excited Ar+(2P1/2) ion and N2

Ar++N2 → Ar+N2+ has served as a paradigm for charge-transfer dynamics studies during the last several decades. Despite significant experimental and theoretical efforts on this model system, state-resolved experimental investigations on the microscopic charge-transfer mechanism between the spin-orbit excited Ar+(2P1/2) ion and N2 have been rare. Here, we measure the first quantum state-to-state differential cross sections for Ar++N2 → Ar+N2+ with the Ar+ ion prepared exclusively in the spin-orbit excited state 2P1/2 on a crossed-beam setup with three-dimensional velocity-map imaging. Trajectory surface-hopping calculations qualitatively reproduce the vibrationally dependent rotational and angular distributions of the N2+ product. Both the scattering images and theoretical calculations show that the charge-transfer dynamics of the spin-orbit excited Ar+(2P1/2) ion differs significantly from that of the spin-orbit ground Ar+(2P3/2) when colliding with N2. Such state-to-state information makes quantitative understanding of this benchmark charge-transfer reaction within reach.

Imaging of the charge-transfer reaction of spin-orbit state-selected Ar+(2P3/2) with N2 reveals vibrational-state-specific mechanisms

Charge-transfer reactions are ubiquitous and play important roles in various gaseous environments, but, despite a long history of extensive research, our understanding of their dynamics at the quantum state-to-state level is still lacking. Here we report quantum-state-resolved experiments for the paradigmatic charge-transfer reaction Ar+ + N2 → Ar + N2+ using a three-dimensional velocity-map imaging crossed-beam apparatus with the Ar+ beam prepared exclusively in the spin-orbit state 2P3/2. High-resolution scattering images show strong dependence of rotational and angular distributions on the vibrational quantum number of the N2+ product. Trajectory surface-hopping calculations, which semi-quantitatively reproduce the experimental observations, support the existence of two distinct charge-transfer mechanisms. One of these, in the dominant N2+(v' = 1) channel, is the well-known long-distance harpooning mechanism. However, the highly rotationally excited products in the forward direction are attributed to a hard-collision glory scattering mechanism, which occurs on account of the strong attraction between the collisional partners counterbalanced by the short-range repulsive interaction.

A photoionized pulsed low-energy ion beam source for quantum state-to-state crossed ion-molecule scattering

Here, we report the design and test of a pulsed low-energy ion beam source for crossed ion-molecule scattering studies. The ions are produced by laser photoionization based methods and thus can be prepared in well-defined quantum states. By using the combination of a double Einzel lenses setup and a specially designed shielding tube, a well spatially confined ion bunch with tunable kinetic energies in the range of 1.0-5.0 eV and typical spreads of ∼150 meV (full width at half maximum) can be formed in the center of a velocity-map imaging (VMI) stack. By combining it with a recently constructed three-dimensional VMI system, the present apparatus is readily available for quantum state-to-state crossed ion-molecule scattering studies.

Inverse kinetic isotope effects in the charge transfer reactions of ammonia with rare gas ions

In the absence of experimental data, models of complex chemical environments rely on predicted reaction properties. Astrochemistry models, for example, typically adopt variants of capture theory to estimate the reactivity of ionic species present in interstellar environments. In this work, we examine astrochemically-relevant charge transfer reactions between two isotopologues of ammonia, NH3 and ND3, and two rare gas ions, Kr+ and Ar+. An inverse kinetic isotope effect is observed; ND3 reacts faster than NH3. Combining these results with findings from an earlier study on Xe+ (Petralia et al., Nat. Commun., 2020, 11, 1), we note that the magnitude of the kinetic isotope effect shows a dependence on the identity of the rare gas ion. Capture theory models consistently overestimate the reaction rate coefficients and cannot account for the observed inverse kinetic isotope effects. In all three cases, the reactant and product potential energy surfaces, constructed from high-level ab initio calculations, do not exhibit any energetically-accessible crossing points. Aided by a one-dimensional quantum-mechanical model, we propose a possible explanation for the presence of inverse kinetic isotope effects in these charge transfer reaction systems.

Velocity map imaging of ion–molecule reactive scattering: The Ar++ N2charge transfer reaction

We present a velocity map imaging spectrometer for the study of crossed-beam reactive collisions between ions and neutrals at (sub-)electronvolt collision energies. The charge transfer reaction of Ar(+) with N(2) is studied at 0.6, 0.8 and 2.5 eV relative collision energies. Energy and angular distributions are measured for the reaction product N. The differential cross section, as analyzed with a Monte Carlo reconstruction algorithm, shows significant large angle scattering for lower collision energies in qualitative agreement with previous experiments. Significant vibrational excitation of N(+)(2) is also observed. This theoretically still unexplained feature indicates the presence of a low energy scattering resonance.

Guided-Ion Beam Measurements of Ar+ + Ar Symmetric Charge-transfer Cross Sections at Ion Energies Ranging from 0.2 to 300 eV

Guided-Ion Beam (GIB) measurements of the Ar