A cryogenic cylindrical ion trap velocity map imaging spectrometer

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.

Ultraviolet photodissociation of Mg+-NO complex: Ion imaging of a reaction branching in the excited states

Ultraviolet photodissociation processes of gas phase Mg⁺-NO complex were studied by photofragment ion imaging experiments and theoretical calculations for excited electronic states. At 355 nm excitation, both Mg⁺ and NO⁺ photofragment ions were observed with positive anisotropy parameters, and theoretical calculations revealed that the two dissociation channels originate from an electronic transition from a bonding orbital consisting of Mg⁺ 3s and NO π* orbitals to an antibonding counterpart. For the NO⁺ channel, the photofragment image exhibited a high anisotropy (β = 1.53 ± 0.07), and a relatively large fraction (∼40%) of the available energy was partitioned into translational energy. These observations are rationalized by proposing a rapid dissociation process on a repulsive potential energy surface correlated to the Mg(¹S) + NO⁺(¹Σ) dissociation limit. In contrast, for the Mg⁺ channel, the angular distribution was more isotropic (β = 0.48 ± 0.03) and only ∼25% of the available energy was released into translational energy. The differences in the recoil distribution for these competing channels imply a reaction branching on the excited state surface. On the theoretical potential surface of the excited state, we found a deep well facilitating an isomerization from bent geometry in the Franck-Condon region to linear and/or T-shaped isomer. As a result, the Mg⁺ fragment was formed via the structural change followed by further relaxation to lower electronic states correlated to the Mg⁺(²S) + NO(²Π) exit channel.

Photofragment ion imaging in vibrational predissociation of the H2O+Ar complex ion

Vibrational predissociation processes of the H₂O⁺Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H₂O⁺ unit were studied by photofragment ion imaging. The anisotropy parameters, β, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, β > 0 for the P-branch excitations, while β < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H₂O⁺ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H₂O⁺ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H₂O⁺ fragment ions was discussed based on the steric configuration of the H₂O⁺ and Ar units at the moment of dissociation.

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]

Chemical reaction pathways and product state correlations of gas-phase ion-molecule reactions are governed by the involved potential energy surfaces (PESs). Here, we report the photodissociation dynamics of charge-transfer complex [Ar-N₂]⁺, which is the intermediate of the model system of the Ar⁺ + N₂ → Ar + N₂⁺ reaction. High-resolution recoiling velocity images of photofragmented N₂ and N₂⁺ from different dissociation channels exhibit a vibrational state-specific correlation, revealing the nonadiabatic charge-transfer mechanisms upon the photodissociation of [Ar-N₂]⁺. The state-resolved product branching ratios have yielded an accurate determination of the resonant charge-transfer probabilities. This work provides a powerful approach to elucidating the detailed dynamics of chemical events of charge-transfer complex [Ar-N₂]⁺ and to probing the state-to-state charge-transfer PESs.

A new setup for low-temperature gas-phase ion fluorescence spectroscopy

Here, we present a new instrument named LUNA2 (LUminescence iNstrument in Aarhus 2), which is purpose-built to measure dispersed fluorescence spectra of gaseous ions produced by electrospray ionization and cooled to low temperatures (<100 K). LUNA2 is, as an earlier room-temperature setup (LUNA), optimized for a high collection efficiency of photons and includes improvements based on our operational experience with LUNA. The fluorescence cell is a cylindrical Paul trap made of copper with a hole in the ring electrode to permit laser light to interact with the trapped ions, and one end-cap electrode is a mesh grid combined with an aspheric condenser lens. The entrance and exit electrodes are both in physical contact with the liquid-nitrogen cooling unit to reduce cooling times. Mass selection is done in a two-step scheme where, first, high-mass ions are ejected followed by low-mass ions according to the Mathieu stability region. This scheme may provide a higher mass resolution than when only one DC voltage is used. Ions are irradiated by visible light delivered from a nanosecond 20-Hz pulsed laser, and dispersed fluorescence is measured with a spectrometer combined with an iCCD camera that allows intensification of the signal for a short time interval. LUNA2 contains an additional Paul trap that can be used for mass selection before ions enter the fluorescence cell, which potentially is relevant to diminishing RF heating in the cold trap. Successful operation of the setup is demonstrated from experiments with rhodamine dyes and oxazine-4, and spectral changes with temperature are identified.

Dissociation dynamics of carbon dioxide cation (CO2 +) in the C2Σg + state via [1+1] two-photon excitation

The dissociation dynamics of CO₂ ⁺ in the C²Σ_g ⁺ state has been studied in the 8.14-8.68 eV region by [1+1] two-photon excitation via vibronically selected intermediate A²Π_u and B²Σ_u ⁺ states using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces an internally cold mass selected ion sample of CO₂ ⁺. Total translational energy release (TER) and two-dimensional recoiling velocity distributions of fragmented CO⁺ ions are measured by time-sliced velocity map imaging. High resolution TER spectra allow us to identify and assign three dissociation channels of CO₂ ⁺ (C²Σ_g ⁺) in the studied energy region: (1) production of CO⁺(X²Σ⁺) + O(³P) by predissociation via spin-orbit coupling with the repulsive 1⁴Π_u state; (2) production of CO⁺(X²Σ⁺) + O(¹D) by predissociation via bending and/or anti-symmetric stretching mediated conical intersection crossing with A²Π_u or B²Σ_u ⁺, where the C²Σ_g ⁺/A²Π_u crossing is considered to be more likely; (3) direct dissociation to CO⁺(A²Π) + O(³P) on the C²Σ_g ⁺ state surface, which exhibits a competitive intensity above its dissociation limit (8.20 eV). For the first dissociation channel, the fragmented CO⁺(X²Σ⁺) ions are found to have widely spread populations of both rotational and vibrational levels, indicating that bending of the parent CO₂ ⁺ over a broad range is involved upon dissociation, while for the latter two channels, the produced CO⁺(X²Σ⁺) and CO⁺(A²Π) ions have relatively narrow rotational populations. The anisotropy parameters β are also measured for all three channels and are found to be nearly independent of the vibronically selected intermediate states, likely due to complicated intramolecular interactions in the studied energy region.

Photodissociation processes of a water–oxygen complex cation studied by an ion imaging technique

Photodissociation dynamics of O₂⁺–H₂O in the visible and ultraviolet regions was studied by ion imaging experiments and theoretical calculations.

N-loss photodissociation dynamics of N2O+(B2Π) near the NO+(Σ+1) + N(2P) dissociation limit

Photodissociation dynamics of the N₂O⁺ cation in its B²Π state has been experimentally studied in an energy region around the NO⁺(¹Σ⁺) + N(²P) dissociation limit using a cryogenic cylindrical ion trap velocity map imaging spectrometer. The results show that the NO⁺(¹Σ⁺) + N(²D) product channel dominates the dissociation dynamics and requires the NNO angle to change by 30°-50° prior to dissociation. The NO⁺(¹Σ⁺) + N(²P) product channel, which directly correlates with the B²Π state but less competitive, opens immediately when the photon energy reaches the dissociation limits, indicating a flat dissociation pathway without bending on the B²Π state surface.