Probing the Charge-Transfer Potential Energy Surfaces by the Photodissociation of [Ar-N2]. J Phys Chem Lett 2021;12:4012-4017. [PMID: 33877828 DOI: 10.1021/acs.jpclett.1c00798]

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.

Photodissociation Dynamics of the [O2-H2O]+ Ionic Complex

We present an experimental study on the photodissociation dynamics of [O₂-H₂O]⁺ in the 580-266 nm wavelength range using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces mass selected and internally cold [O2-H2O]+ ions for photodissociation. By detecting both the O₂⁺ and H₂O⁺ photofragments using the time-of-flight mass spectrometry and velocity map imaging techniques, branching ratios and total kinetic energy release distributions of the O₂⁺ + H₂O and H₂O⁺ + O₂ product channels are experimentally measured at 16 different excitation energies. State-resolved photodissociation mechanisms of the parent [O₂-H₂O]⁺ are interpreted as (1) the O₂(X³Σ_g^-) + H₂O⁺(X~2B1), O₂(a¹Δ_g) + H₂O⁺(X~2B1), and O₂(X³Σ_g^-) + H₂O⁺(A~2A1) channels are produced from direct dissociation of [O₂-H₂O]⁺ in its excited B~2A″, D~2A″, and F~2A″ states, respectively; (2) the O₂⁺(X²Π_g) + H2O(X~A11) channel is produced from nonadiabatic relaxations of the excited B~2A″, D~2A″, and F~2A″ states to the X~2A″ ground state with subsequent dissociation. The latter nonadiabatic processes involve charge-transfer on the potential energy surfaces, and the charge-transfer probabilities are determined from experimental results. The dissociation energy of the ground state to the lowest dissociation limit is experimentally refined as D₀ = 1.05 ± 0.05 eV. This work provides important information to understand the charge-transfer dynamics in the photochemistry of [O₂-H₂O]⁺ and in the ion-molecule reaction O₂ + H₂O⁺ → O₂⁺ + H₂O.

Fragment imaging in the infrared photodissociation of the Ar-tagged protonated water clusters H3O+-Ar and H+(H2O)2-Ar

Infrared photodissociation of protonated water clusters with an Ar atom, namely H₃O⁺-Ar and H⁺(H₂O)₂-Ar, was investigated by an imaging technique for mass-selected ions, to reveal the intra- and intermolecular vibrational dynamics. The presented system has the advantage of achieving fragment ion images with the cluster size- and mode-selective photoexcitation of each OH stretching vibration. Translational energy distributions of photofragments were obtained from the images upon the excitation of the bound (ν_b) and free (ν_f) OH stretching vibrations. The energy fractions in the translational motion were compared between ν_b^I and ν_f^I in H₃O⁺-Ar or between ν_b^II and ν_f^II in H⁺(H₂O)₂-Ar, where the labels "I" and "II" represent H₃O⁺-Ar and H⁺(H₂O)₂-Ar, respectively. In H₃O⁺-Ar, the ν_f^I excitation exhibited a smaller translational energy than ν_b^I. This result can be explained by the higher vibrational energy of ν_f^I, which enabled it to produce bending (ν₄) excited H₃O⁺ fragments that should be favored according to the energy-gap model. In contrast to H₃O⁺-Ar, the ν_b^II excitation of an Ar-tagged H₂O subunit and the ν_f^II excitation of an untagged H₂O subunit resulted in very similar translational energy distributions in H⁺(H₂O)₂-Ar. The similar energy fractions independent of the excited H₂O subunits suggested that the ν_b^II and ν_f^II excited states relaxed into a common intermediate state, in which the vibrational energy was delocalized within the H₂O-H⁺-H₂O moiety. However, the translational energy distributions for H⁺(H₂O)₂-Ar did not agree with a statistical dissociation model, which implied another aspect of the process, that is, Ar dissociation via incomplete energy randomization in the whole H⁺(H₂O)₂-Ar cluster.