Ion Velocity Map Imaging Study of the Reactive Collisions between Carbon Dioxide and Helium Ion

The reactive collision between He+ and CO2 plays an important role in substance evolutions of the planetary CO2-rich atmosphere. Using a three-dimensional ion velocity map imaging technique, we investigate the low-energy ion-molecule reactions He+ + CO2 → He + CO2+/He + CO+ + O/He + CO + O+. The velocity images of the products CO+ and O+ of dissociative charge-exchange reactions are distinctly different from those of charge-exchange product CO2+. The remarkable features of stereodynamics are observed in the dissociative charge-exchange reaction and are attributed to the spatial alignment of the initially random target CO2 during the He+ approach. Branching ratios of different channels of dissociative charge exchange are further obtained with the Doppler kinematics model, indicating a high preference for the energy-resonant channel.

Probing Anion-Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted.

Autodetachment over Broad Photon Energy Ranges in the Anion Photoelectron Spectra of [O2-]- ( = Glyoxal, Methylglyoxal, or Biacetyl) Complex Anions

Complexes of anion-neutral pairs are prevalent in chemical and physical processes in the interstellar medium, the atmosphere, and biological systems, among others. However, bimolecular anionic species that cannot be described as simple ion-molecule complexes due to their competitive electron affinities have received less attention. In this study, the [O₂-M]^- (M = glyoxal, methylglyoxal, or biacetyl) anion photoelectron spectra obtained with several different photon energies are reported and interpreted in the context of ab initio calculations. The spectra do not resemble the photoelectron spectra of M^- or O₂^- "solvated" by a neutral partner. Rather, all spectra are dominated by near-threshold autodetachment from what are likely transient dipole bound states of the cis conformers of the complex anions. Very low Franck-Condon overlap between the neutral M·O₂ van der Waals clusters and the partial covalently bound complex anions results in low-intensity, broad direct detachment observed in the spectra. The [O₂-glyoxal]^- spectra measured with 2.88 and 3.495 eV photon energies additionally exhibit features at ∼0.5 eV electron kinetic energy, which is more difficult to explain, though there are numerous quasibound states of the anion that may be involved. Overall, these features point to the inadequacy of describing the complex anions as simple ion-molecule complexes.

Collision-Energy Dependent Stereodynamics of Dissociative Charge Exchange Reaction between Ar+ and CO

Long-distance charge-dipole attraction between atomic ion and randomly oriented polar molecule potentially makes the molecular orientation, which profoundly influences the products' kinetics of collisional reaction. Using the three-dimensional ion velocity map imaging technique, here we report a collision-energy dependent stereodynamics of dissociative charge exchange reaction Ar⁺ + CO → Ar + O + C⁺ in a range of 7.46-9.97 eV. At the lowest collision energy, the most C⁺ products are forward-scattered and are along the collision axis and are attributed to three different dissociation channels including the predominant one experiencing the rotating intermediate ArC⁺. At the high collision energies, the remarkably diffusive distribution of C⁺ arises from the prompt dissociation of the rebounded CO⁺. The different dynamic processes arising from the nearly collinear collision are elaborated explicitly on the basis of the data analyses using the Doppler kinetics models.

Superposition-state N2 + produced in the intermolecular charge transfer from low-energy Ar+ to N2

Molecular electronic or vibrational states can be superimposed temporarily in an extremely short laser pulse, and the superposition-state transients formed therein receive much attention, owing to the extensive interest in molecular fundamentals and the potential applications in quantum information processing. Using the crossed-beam ion velocity map imaging technique, we disentangle two distinctly different pathways leading to the forward-scattered N₂ ⁺ yields in the large impact-parameter charge transfer from low-energy Ar⁺ to N₂. Besides the ground-state (X²Σ_g ⁺) N₂ ⁺ produced in the energy-resonant charge transfer, a few slower N₂ ⁺ ions are proposed to be in the superpositions of the X²Σ_g ⁺-A²Π_u and A²Π_u-B²Σ_u ⁺ states on the basis of the accidental degeneracy or energetic closeness of the vibrational states around the X²Σ_g ⁺-A²Π_u and A²Π_u-B²Σ_u ⁺ crossings in the non-Franck-Condon region. This finding potentially shows a brand-new way to prepare the superposition-state molecular ion.

Ion-Molecule Charge Exchange Reactions between Ar+ and trans-/cis-Dichloroethylene

We report an ion velocity imaging study of the charge exchange reactions between Ar⁺ ion and trans-/cis-dichloroethylene in the collision energy range of 2.1-9.5 eV, and we find that the energy-resonant charge transfer plays a dominant role in the large impact-parameter reaction. The parent yields C₂H₂Cl₂⁺ in the high-lying excited states are directly produced in the charge exchange reactions, while they prefer spontaneous fragmentations in photoionization. This significant difference indicates that the present charge exchange reactions are much slower than the photoelectron detachment. The structural relaxations of the target molecule are allowed in multiple dimensions of freedom during the charge transfer, which should be frequently observed for the charge exchange reactions with large molecules.

Stereodynamics Observed in the Reactive Collisions of Low-Energy Ar+ with Randomly Oriented O2

Stereodynamics of the collisional reaction between mutually aligned or oriented reactants has been a striking topic of chemical dynamics for decades. However, the stereodynamic aspects are scarcely revealed for the low-energy collision with a randomly oriented target. Here in the dissociative charge-exchange reaction between randomly oriented O₂ and low-energy Ar⁺, we, using the three-dimensional ion velocity map imaging technique, clearly observe a linear alignment and a nearly isotropic distribution of the O⁺ yields along the collision axis. These observations are rationalized with the Doppler kinetic models in which the O₂ bond is assumed to be parallel or unparallel to the collision axis of the large impact parameter collision. The linearly aligned O⁺, as the predominant yield, is produced in the parallel collision, while a rotating O₂⁺, as the intermediate in the unparallel collision, leads to the isotropic distribution of O⁺.