The mutual neutralization of hydronium and hydroxide ions is a fundamental chemical reaction. Yet, there is very limited direct experimental evidence about its intrinsically non-adiabatic mechanism. Chemistry textbooks describe the products of mutual neutralization in bulk water as two water molecules; however, this reaction has been suggested as a possible mechanism for the recently reported spontaneous formation of OH radicals at the surface of water microdroplets. Here, following three-dimensional-imaging of the coincident neutral products of reactions of isolated D3O+ and OD−, we can reveal the non-adiabatic pathways for OD radical formation. Two competing pathways lead to distinct D2O + OD + D and 2OD + D2 product channels, while the proton-transfer mechanism is substantially suppressed due to a kinetic isotope effect. Analysis of the three-body momentum correlations revealed that the D2O + OD + D channel is formed by electron transfer at a short distance of ~4 Å with the formation of the intermediate unstable neutral D3O ground state, while 2OD + D2 products are obtained following electron transfer at a distance of ~10 Å via an excited state of the neutral D3O. (Figure presented.)

Product distributions and dynamics of low-collision-energy mutual neutralisation reactions involving even simple molecular ions are largely unknown. Reactions which involve oxygen ions, e.g., O2+ with O−, are expected to be important in atmospheric phenomena such as sprites and in high-pressure air or oxygen discharges. Here we show, by combining cryogenically stored-and-merged ion beams with coincident product-imaging techniques, that the O2+ with O− mutual neutralisation reaction results predominantly in dissociation of the O2+ molecule. Three competing reaction pathways yields both O(3P) (84%) and O(1D) (16%) products, but no O(1S) products. Analysis of the momentum-correlated dynamics of the reaction reveals the dominance of two-step mechanisms involving the 3pλu and 3sσg Rydberg states of O2. Furthermore, use of the 16,18O2+ isotopologue shows that the reaction products strongly depend on the vibrational levels of the O2+ ion for the channel leading to two O(1D) products.

This thesis deals with experimental studies of electron transfer reactions between oppositely charged ions (cations and anions), in a process called mutual neutralisation. These investigations were performed at the double electrostatic ion storage ring DESIREE at Stockholm University, which was put into full operation in 2017. This unique apparatus consists of two cryogenic electrostatic rings where oppositely charged ion beams are stored and merged in a common section where the reactions of interest take place. The neutral products arising from the reactions are detected in coincidence using a sensitive 3D imaging detector. This approach allows the kinetic energy of the products to be measured, and the particular product channels to be identified, such that the branching ratio into the different competing sets of products can be determined. 

The reactions studied in this thesis take place in both natural plasmas, such as our own atmosphere, as well as industrial ones, such as ion propulsion engines and fusion reactors. For the first time, the final-state distribution of the products in a number of mutual neutralisation reactions involving molecular ions were determined, and the reaction dynamics were elucidated. 

The work presented in this thesis deals with the development of data analysis methods necessary for the imaging of coincidence products, as well as simulation methods required to interpret the experimental results. The thesis touches briefly the current theoretical models treating these reactions and their current recent development and results. 

We have studied the mutual neutralization reaction of vibronically cold NO+ with O- at a collision energy of approximate to 0.1 eV and under single-collision conditions. The reaction is completely dominated by production of three ground-state atomic fragments. We employ product-momentum analysis in the framework of a simple model, which assumes the anion acts only as an electron donor and the product neutral molecule acts as a free rotor, to conclude that the process occurs in a two-step mechanism via an intermediate Rydberg state of NO which subsequently fragments.

Mutual neutralization of hydronium (H₃O⁺) and hydroxide (OH⁻) ions is a very fundamental chemical reaction. Yet, there is only limited experimental evidence about the underlying reaction mechanisms. Here, we report three-dimensional imaging of coincident neutral products of mutual-neutralization reactions at low collision energies of cold and isolated ions in the cryogenic double electrostatic ion-beam storage ring (DESIREE). We identified predominant H2O + OH + H and 2OH + H₂ product channels and attributed them to an electron-transfer mechanism, whereas a minor contribution of H₂O + H₂O with high internal excitation was attributed to proton transfer. The reported mechanism-resolved internal product excitation, as well as collision-energy and initial ion-temperature dependence, provide a benchmark for modeling charge-transfer mechanisms. 

We have studied the mutual neutralization reaction of atomic iodine ions (i.e., I⁺+I⁻→I+I) in a cryogenic double electrostatic ion-beam storage-ring apparatus. Our results show that the reaction forms iodine atoms either in the ground-state configuration (I(5p⁵²P∘), ∼40%) or with one atom in an electronically excited state (I(6s²[2]), ∼60%), with no significant variation over the branching ratios in the studied collision-energy range (0.1–0.8 eV). We estimate the total charge-transfer cross section to be of the order of 10⁻¹³cm² at 0.1 eV collision energy. Ab initio relativistic electronic structure calculations of the potential-energy curves of I₂ suggest that the reaction takes place at short internuclear distances. The results are discussed in view of their importance for applications in electric thrusters.

This thesis deals with experimental studies of electron transfer reactions between oppositely charged ions (mutual neutralisation). These were performed at the unique double electrostatic ion storage ring DESIREE at Stockholm University, which was put into full operation in 2017. In the first two published articles of this thesis, two atmospheric collision systems are treated, namely O+/O−  and N+/O−. The aim was to reproduce previous published results from a single-pass (non-stored) merged ion beams setup in UCLouvain (Belgium) and thus provide a measure of DESIREE’s capacity and resolution. In addition, the effects of metastable ions were investigated with the support of theoretical calculations. The third published paper of this thesis deals with collisions between I+ and I− (iodine ions), a process relevant to electric thrusters for new spacecraft. The results are compared with theoretical calculations in order to provide an understanding of how the reaction takes place. Preliminary results on electron transfer reactions between diatomic molecules and atoms are presented.

The double ion storage ring DESIREE has been used in combination with position- and time-sensitive detectors to study the mutual neutralization of N⁺ with O⁻ at 40 meV collision energy. Several previously unassigned spectral features observed in a recent single-pass merged-beams experiment at 7 meV collision energy [Phys. Rev. Lett. 121, 083401 (2018)], were also observed in the present experiment. It was found that neutralization channels of the first metastable state of the cation [N⁺(¹D),τ≈256s] could explain the majority of these features, while the second metastable state [N⁺(¹S),τ≈0.9s] was not found to contribute significantly. The branching ratios into the different electronically excited states of N were determined and found to be in good agreement between the two experiments. Theoretical calculations using the multichannel Landau-Zener model were found to yield good results for a number of channels, but could not describe some observed contributions, possibly due to the presence of other processes not accounted for in the model.

We investigate gas-phase structures of homo- and heterochiral asparagine proton-bound dimers with infrared multi-photon dissociation (IRMPD) spectroscopy and quantum-chemical calculations. Their IRMPD spectra are recorded at room temperature in the range of 500-1875 and 3000-3600 cm(-1). Both varieties of asparagine dimers are found to be charge-solvated based on their IRMPD spectra. The location of the principal intramolecular H-bond is discussed in light of harmonic frequency analyses using the B3LYP functional with GD3BJ empirical dispersion. Contrary to theoretical analyses, the two spectra are very similar.

The mutual neutralisation of O+ with O− has been studied in a double ion-beam storage ring with combined merged-beams, imaging and timing techniques. Branching ratios were measured at the collision energies of 55, 75 and 170 (± 15) meV, and found to be in good agreement with previous single-pass merged-beams experimental results at 7 meV collision energy. Several previously unidentified spectral features were found to correspond to mutual neutralisation channels of the first metastable state of the cation (O+(2Do), τ ≈ 3.6 hours), while no contributions from the second metastable state (O+(2Po), τ ≈ 5 seconds) were observed. Theoretical calculations were performed using the multi-channel Landau–Zener model combined with the anion centered asymptotic method, and gave good agreement with several experimentally observed channels, but could not describe well observed contributions from the O+(2Do) metastable state as well as channels involving the O(3s 5So) state.