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.

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.

We measured the product-state distribution and its dependence on the hydrogen isotope for the mutual neutralization between ¹⁶O⁺ and ^1,2H⁻ at the double electrostatic ion-beam storage ring DESIREE for center-of-mass collision energies below 100 meV. We find at least six product channels into ground-state hydrogen and oxygen in different excited states. The majority of oxygen products populate terms corresponding to 2⁢𝑠²2⁢𝑝³⁢(⁴𝑆^∘)⁢4⁢𝑠 with ⁵S^∘ as the main reaction product. We also observe product channels into terms corresponding to 2⁢𝑠²2⁢𝑝³⁢(⁴𝑆)⁢3⁢𝑝. Collisions with the heavier hydrogen isotope yield a branching into these lower excited states smaller than collisions with ¹H⁻. The observed reaction products agree with the theoretical predictions. The detailed branching fractions, however, differ between the theoretical results, and none of them fully agree with the experiment.

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 measured the branching ratios of the He+ + H- and He+ + D- mutual neutralization at a collision energy below 25 meV. Those are correctly reproduced using the Landau-Zener model applied to potential energy curves computed by an anion-centered asymptotic model. The analyticity of both models allows getting a deeper insight into the reaction. It allows defining a low collision energy regime for the mutual neutralization at which the collision energy no longer affects the branching ratios. Using those models, we explain how heavier isotopes favor the production of higher excited states.

The production of pairs of correlated Lyman-alpha photons upon XUV excitation of molecular hydrogen is studied using coincidence measurements to evaluate at which level the entanglement is transferred from atoms to photons. By referencing the timing of fluorescence photon detection to the synchrotron light pulse, we were able to determine branching ratios for the 2p + 2p and 2p + 3 (with = s, d) channels separately. Time-dependent analysis of the spectral signature recorded around 33.6 eV and close examination of the doubly excited states lying in the Franck-Condon window confirm prior assignments of the 2p + 2p being the main dissociation channel of the Q2 1 pi u (1) molecular state. The angular dependence of the two-photon detection probability measured with respect to the polarization axis is found to be in agreement with earlier measurements [Y. Torizuka et al., Phys. Rev. A 99, 063426 (2019)]. A simple model, assuming a transition from Hund's case (a) to Hund's case (c), reproduces the measured angular distributions satisfactorily.