Time-resolved dissociative ionization and double photoionization of CO2

Symmetry-breaking dynamics of a photoionized carbon dioxide dimer

Photoionization can initiate structural reorganization of molecular matter and drive formation of new chemical bonds. Here, we used time-resolved extreme ultraviolet (EUV) pump - EUV probe Coulomb explosion imaging of carbon dioxide dimer ionC O 2 2 + dynamics, that combined with ab initio molecular dynamics simulations, revealed unexpected asymmetric structural rearrangement. We show that ionization by the pump pulse induces rearrangement from the slipped-parallel (C2h) geometry of the neutral C O 2 dimer towards a T-shaped (C2v) structure on the ~100 fs timescale, although the most stable slipped-parallel (C2h) structure of the ionic dimer. Moreover, we find that excited states of the ionized C O 2 dimer can exhibit formation of aCO 3 moiety in theC 2 O 4 + complex that can persist even after a suitably time-delayed second photoionization in a metastableC 2 O 4 2 + dication. Our results suggest that charge asymmetry plays an important role in the ionization-induced dynamics in such dimers that are present in C O 2 rich environments.

Molecular photodissociation dynamics revealed by Coulomb explosion imaging

Coulomb explosion imaging (CEI) methods are finding ever-growing use as a means of exploring and distinguishing the static stereo-configurations of small quantum systems (molecules, clusters, etc). CEI experiments initiated by ultrafast (femtosecond-duration) laser pulses also allow opportunities to track the time-evolution of molecular structures, and thereby advance understanding of molecular fragmentation processes. This Perspective illustrates two emerging families of dynamical studies. 'One-colour' studies (employing strong field ionisation driven by intense near infrared or single X-ray or extreme ultraviolet laser pulses) afford routes to preparing multiply charged molecular cations and exploring how their fragmentation progresses from valence-dominated to Coulomb-dominated dynamics with increasing charge and how this evolution varies with molecular size and composition. 'Two-colour' studies use one ultrashort laser pulse to create electronically excited neutral molecules (or monocations), whose structural evolution is then probed as a function of pump-probe delay using an ultrafast ionisation pulse along with time and position-sensitive detection methods. This latter type of experiment has the potential to return new insights into not just molecular fragmentation processes but also charge transfer processes between moieties separating with much better defined stereochemical control than in contemporary ion-atom and ion-molecule charge transfer studies.

Multiparticle Cumulant Mapping for Coulomb Explosion Imaging

We extend covariance velocity map ion imaging to four particles, establishing cumulant mapping and allowing for measurements that provide insights usually associated with coincidence detection, but at much higher count rates. Without correction, a fourfold covariance analysis is contaminated by the pairwise correlations of uncorrelated events, but we have addressed this with the calculation of a full cumulant, which subtracts pairwise correlations. We demonstrate the approach on the four-body breakup of formaldehyde following strong field multiple ionization in few-cycle laser pulses. We compare Coulomb explosion imaging for two different pulse durations (30 and 6 fs), highlighting the dynamics that can take place on ultrafast timescales. These results have important implications for Coulomb explosion imaging as a tool for studying ultrafast structural changes in molecules, a capability that is especially desirable for high-count-rate x-ray free-electron laser experiments.

Sequential mechanism in H3+ formation dynamics on the ethanol dication

Two- and three-body Coulomb explosion dynamics of isolated ethanol dications are studied via single-photon double-ionization with ultrafast extreme-ultraviolet pulses. The measured 3-body momentum correlations obtained via 3D coincidence imaging of the ionic products provide evidence for several concerted and sequential mechanisms: (1) a concerted 3-body breakup mechanism, with dominating channels such as CH₃⁺ + COH⁺ + H₂; (2) sequential dissociation in which the ejection of a low-kinetic-energy neutral OH precedes the Coulomb explosion of C₂H₅²⁺ → CH₃⁺ + CH₂⁺; and (3) a sequential 3-body breakup mechanism that dominates H₃⁺ formation from the ethanol dication via a mechanism that is different from the well-studied H₃⁺ formation in the 2-body Coulomb explosion of the methanol dication. Furthermore, we report surprising branching ratios of the competing C-O bond dissociation channels, resulting in H₃O⁺, H₂O⁺ and OH⁺ formation.

An "inverse" harpoon mechanism

Electron-transfer reactions are ubiquitous in chemistry and biology. The electrons' quantum nature allows their transfer across long distances. For example, in the well-known harpoon mechanism, electron transfer results in Coulombic attraction between initially neutral reactants, leading to a marked increase in the reaction rate. Here, we present a different mechanism in which electron transfer from a neutral reactant to a multiply charged cation results in strong repulsion that encodes the electron-transfer distance in the kinetic energy release. Three-dimensional coincidence imaging allows to identify such "inverse" harpoon products, predicted by nonadiabatic molecular dynamics simulations to occur between H₂ and HCOH²⁺ following double ionization of isolated methanol molecules. These dynamics are experimentally initiated by single-photon double ionization with ultrafast extreme ultraviolet pulses, produced by high-order harmonic generation. A detailed comparison of measured and simulated data indicates that while the relative probability of long-range electron-transfer events is correctly predicted, theory overestimates the electron-transfer distance.

Coulomb Explosion Dynamics of Multiply Charged para-Nitrotoluene Cations

This work explores Coulomb explosion (CE) dissociation pathways in multiply charged cations of para-nitrotoluene (PNT), a model compound for nitroaromatic energetic molecules. Experiments using strong-field ionization and mass spectrometry indicate that metastable cations PNT²⁺ and PNT³⁺ undergo CE to produce NO₂⁺ and NO⁺. The experimentally measured kinetic energy release from CE upon formation of NO₂⁺ and NO⁺ agrees qualitatively with the kinetic energy release predicted by computations of the reaction pathways in PNT²⁺ and PNT³⁺ using density functional theory (DFT). Both DFT computations and mass spectrometry identified additional products from CE of highly charged PNT^q+ cations with q > 3. The dynamical timescales required for direct CE of PNT²⁺ and PNT³⁺ to produce NO₂⁺ were estimated to be 200 and 90 fs, respectively, using ultrafast disruptive probing measurements.

Sequential and concerted C-C and C-O bond dissociation in the Coulomb explosion of 2-propanol

We study the competing mechanisms in the Coulomb explosion of 2-propanol dication, formed by an ultrafast EUV pulse. Over 20 product channels are identified and characterized using 3D coincidence imaging of the ionic fragments. The momentum correlations in the three-body fragmentation channels provide evidence for a dominant sequential mechanism, starting with cleavage of a C-C bond, ejecting and cations, followed by a secondary fragmentation of the hydroxyethyl cation that can be delayed for up to a microsecond after ionization. C-O bond dissociation channels are less frequent, involving proton-transfer and double-proton transfer, forming and products respectively and exhibiting mixed sequential and concerted character. These results can be explained by the high potential barrier for the C-O bond dissociation seen in our ab initio quantum chemical calculations. We also observe coincident COH+ + C2Hn+ ions, suggesting exotic structural rearrangements, starting from the Frank-Condon geometry of the neutral 2-propanol system. Remarkably, the relative yield of the product is suppressed compared with methanol and alkene dications. Ab initio potentials and ground-state molecular dynamics simulations show that a rapid and direct C-C bond cleavage dominates the Coulomb explosion process, leaving no time for roaming which is a necessary precursor to the formation.