Time-resolving the ultrafast H2 roaming chemistry and H3+ formation using extreme-ultraviolet pulses

Ultrafast Imaging of Jahn-Teller Distortion and the Correlated Proton Migration in Photoionized Cyclopropane

The Jahn-Teller (JT) distortion is one of the fundamental processes in molecules and condensed phase matters. For photoionized organic molecules with high symmetry, the JT effect leads to geometric instability in certain electron configurations and thus has a significant effect on the subsequent isomerization and proton migration processes. Utilizing the femtosecond pump-probe Coulomb explosion method, we probe the isomerization dynamics process of a monovalent cyclopropane cation (C3H6+) caused by proton migration and reveal the relationship between proton migration and JT distortion. We found that the C3H6+ cation evolves from the D3h symmetric equilateral triangle geometry either to the acute triangle via two elongated C-C bonds (JT1) or to the obtuse triangle via a single elongated C-C bond (JT2). The JT1 pathway does not involve proton migration, while the JT2 pathway drives proton migration and can be mapped into the indirect dissociation channel of Coulomb explosion. The time-resolved experiment indicates that the delay time between those two JT pathways can be as large as ∼600 fs. After the JT distortion, the cyclopropane cation undergoes a subsequent structural evolution, which brings a greater variety of dissociation channels.

Initial-site characterization of hydrogen migration following strong-field double-ionization of ethanol

An essential problem in photochemistry is understanding the coupling of electronic and nuclear dynamics in molecules, which manifests in processes such as hydrogen migration. Measurements of hydrogen migration in molecules that have more than two equivalent hydrogen sites, however, produce data that is difficult to compare with calculations because the initial hydrogen site is unknown. We demonstrate that coincidence ion-imaging measurements of a few deuterium-tagged isotopologues of ethanol can determine the contribution of each initial-site composition to hydrogen-rich fragments following strong-field double ionization. These site-specific probabilities produce benchmarks for calculations and answer outstanding questions about photofragmentation of ethanol dications; e.g., establishing that the central two hydrogen atoms are 15 times more likely to abstract the hydroxyl proton than a methyl-group proton to form H[Formula: see text] and that hydrogen scrambling, involving the exchange of hydrogen between different sites, is important in H2O+ formation. The technique extends to dynamic variables and could, in principle, be applied to larger non-cyclic hydrocarbons.

Stereodynamical Control of D3+ Formation from the Bimolecular Photoreaction in the D2-D2 Dimer

We report the stereodynamic control of D3+ formation from the laser-induced bimolecular reaction in a weakly bound D2-D2 dimer via impulsive molecular alignment. Using a linearly polarized moderately intense femtosecond pump pulse, the D2 molecules in the dimer were prealigned prior to the bimolecular reaction triggered by a delayed probe pulse. The rotationally excited D2 in the dimer was observed to rotate freely as if it were a monomer. It was demonstrated that the yield of photoreaction product D3+ is increased or decreased when the molecular axis of D2 is parallel or perpendicular to the probe laser polarization, respectively. The underlying physics of this steric effect is the alignment-dependent bond cleavage of D2+ in the dimer induced by a photon-coupled parallel transition.

What is the Mechanism of H3+ Formation from Cyclopropane?

We examine the possibility that three hydrogen atoms in one plane of the cyclopropane dication come together in a concerted "ring-closing" mechanism to form H3+, a crucial cation in interstellar gas-phase chemistry. Ultrafast strong-field ionization followed by disruptive probing measurements indicates that the formation time of H3+ is 249 ± 16 fs. This time scale is not consistent with a concerted mechanism, but rather a process that is preceded by ring opening. Measurements on propene, an isomer of cyclopropane, reveal the H3+ formation time to be 225 ± 13 fs, a time scale similar to the H3+ formation time in cyclopropane. Ab initio molecular dynamics simulations and the fact that both dications share a common potential energy surface support the ring-opening mechanism. The reaction mechanism following double ionization of cyclopropane involves ring opening, then H-migration, and roaming of a neutral H2 molecule, which then abstracts a proton to form H3+. These results further our understanding of complex interstellar chemical reactions and gas-phase reaction dynamics relevant to electron ionization mass spectrometry.

Ultrafast formation dynamics of D3+ from the light-driven bimolecular reaction of the D2-D2 dimer

The light-driven formation of trihydrogen cation has been attracting considerable attention because of its important role as an initiator of chemical reactions in interstellar clouds. To understand the formation dynamics, most previous studies focused on creating H3+ or D3+ from unimolecular reactions of various organic molecules. Here we observe and characterize the ultrafast formation dynamics of D3+ from a bimolecular reaction, using pump-probe experiments that employ ultrashort laser pulses to probe its formation from a D2-D2 dimer. Our molecular dynamics simulations provide an intuitive representation of the reaction dynamics, which agree well with the experimental observation. We also show that the emission direction of D3+ can be controlled using a tailored two-colour femtosecond laser field. The underlying control mechanism is in line with what is known from the light control of electron localization in the bond breaking of single molecules.

D3+ formation through photoionization of the molecular D2-D2 dimer

The H2-H2 molecular dimer is of fundamental importance in the study of chemical interactions because of its unique bonding properties and its ability to model more complex systems. The trihydrogen cation H3+ is also a key intermediate in a range of chemical processes in interstellar environments, such as the formation of various organic molecules and early stars. However, the unexpected high abundance of H3+ in molecular clouds remains challenging to explain. Here using near-infrared, femtosecond laser pulses and coincidence momentum imaging, we find that the dominant channel after photoionization of a deuterium molecular dimer (D2-D2) is the ejection of a deuterium atom within a few hundred femtoseconds, leading to the formation of D3+. The formation mechanism is supported and well-reproduced by ab initio molecular dynamics simulations. This pathway of D3+ formation from ultracold D2-D2 gas may provide insights into the high abundance of H3+ in the interstellar medium.

H2 formation via non-Born-Oppenheimer hydrogen migration in photoionized ethane

Neutral H2 formation via intramolecular hydrogen migration in hydrocarbon molecules plays a vital role in many chemical and biological processes. Here, employing cold target recoil ion momentum spectroscopy (COLTRIMS) and pump-probe technique, we find that the non-adiabatic coupling between the ground and excited ionic states of ethane through conical intersection leads to a significantly high yield of neutral H2 fragment. Based on the analysis of fingerprints that are sensitive to orbital symmetry and electronic state energies in the photoelectron momentum distributions, we tag the initial electronic population of both the ground and excited ionic states and determine the branching ratios of H2 formation channel from those two states. Incorporating theoretical simulation, we established the timescale of the H2 formation to be ~1300 fs. We provide a comprehensive characterization of H2 formation in ionic states of ethane mediated by conical intersection and reveals the significance of non-adiabatic coupling dynamics in the intramolecular hydrogen migration.

Dynamics of Hydrogen Bond Breaking Induced by Outer-Valence Intermolecular Coulombic Decay

The photoexcitation of weakly bound complexes can lead to several decay pathways, depending on the nature of the potential energy surfaces. Upon excitation of a chromophore in a weakly bound complex, ionization of its neighbor upon energy transfer can occur due to a unique relaxation process known as intermolecular Coulombic decay (ICD), a phenomenon of renewed focus owing to its relevance in biological systems. Herein, we report the evidence for outer-valence ICD induced by multiphoton excitation by near-ultraviolet radiation of 4.4 eV photons, hitherto unknown in molecular systems. In the binary complexes of 2,6-difluorophenylacetylene with aliphatic amines, a resonant two-photon excitation localized on the 2,6-difluorophenylacetylene chromophore results in the formation of an amine cation following an outer-valence ICD process. The unique trends in experimentally observed translational energy distribution profiles of the amine cations following hydrogen bond dissociation, analyzed with the help of electronic structure and ab initio molecular dynamics calculations, revealed the presence of a delicate interplay of roaming dynamics, methyl-rotor dynamics, and binding energy.

Ultrafast Roaming Mechanisms in Ethanol Probed by Intense Extreme Ultraviolet Free-Electron Laser Radiation: Electron Transfer versus Proton Transfer

Ultrafast H₂⁺ and H₃⁺ formation from ethanol is studied using pump-probe spectroscopy with an extreme ultraviolet (XUV) free-electron laser. The first pulse creates a dication, triggering H₂ roaming that leads to H₂⁺ and H₃⁺ formation, which is disruptively probed by a second pulse. At photon energies of 28 and 32 eV, the ratio of H₂⁺ to H₃⁺ increases with time delay, while it is flat at a photon energy of 70 eV. The delay-dependent effect is ascribed to a competition between electron and proton transfer. High-level quantum chemistry calculations show a flat potential energy surface for H₂ formation, indicating that the intermediate state may have a long lifetime. The ab initio molecular dynamics simulation confirms that, in addition to the direct emission, a small portion of H₂ undergoes a roaming mechanism that leads to two competing pathways: electron transfer from H₂ to C₂H₄O²⁺ and proton transfer from C₂H₄O²⁺ to H₂.

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.

Strong-field ionization of CH3Cl: proton migration and association

Strong-field ionization of CH₃Cl using femtosecond laser pulses, and the subsequent two-body dissociation of CH₃Cl²⁺ along H_n⁺ (n = 1-3) and HCl⁺ forming pathways, have been experimentally studied in a home-built COLTRIMS (cold target recoil ion momentum spectrometer) setup. The single ionization rate of CH₃Cl was obtained experimentally by varying the laser intensity from 1.6 × 10¹³ W cm^-2 to 2.4 × 10¹⁴ W cm^-2 and fitted with the rate obtained using the MO-ADK model. Additionally, the yield of H_n⁺ ions resulting from the dissociation of all charge states of CH₃Cl was determined as a function of intensity and pulse duration (and chirp). Next, we identified four two-body breakup pathways of CH₃Cl²⁺, which are H⁺ + CH₂Cl⁺, H₂⁺ + CHCl⁺, H₃⁺ + CCl⁺, and CH₂⁺ + HCl⁺, using photoion-photoion coincidence. The yields of the four pathways were found to decrease on increasing the intensity from I = 4.2 × 10¹³ W cm^-2 to 2I = 8.5 × 10¹³ W cm^-2, which was attributed to enhanced ionization of the dication before it can dissociate. As a function of pulse duration (and chirp), the H_n⁺ forming pathways were suppressed, while the HCl⁺ forming pathway was enhanced. To understand the excited state dynamics of the CH₃Cl dication, which controls the outcome of dissociation, we obtained the total kinetic energy release distributions of the pathways and the two-dimensional coincidence momentum images and angular distributions of the fragments. We inferred that the H_n⁺ forming pathways originate from the dissociation of CH₃Cl dications from weakly attractive metastable excited states having a long dissociation time, while for the HCl⁺ forming pathway, the dication dissociates from repulsive states and therefore, undergoes rapid dissociation. Finally, quantum chemical calculations have been performed to understand the intramolecular proton migration and dissociation of the CH₃Cl dication along the pathways mentioned above. Our study explains the mechanism of H_n⁺ and HCl⁺ formation and confirms that intensity and pulse duration can serve as parameters to influence the excited state dynamics and hence, the outcome of the two-body dissociation of CH₃Cl²⁺.

Hydrogen migration in triply charged acetylene

We report on the direct experimental evidence of hydrogen migration in triply charged acetylene. The roaming hydrogen atom in a triply charged molecular ion is counter intuitive. The three body breakup channel [Formula: see text] is studied using the technique of recoil ion momentum spectroscopy. The triply charged ion was generated in collisions of the neutral parent with a slow highly charged Xe9+ ion. Three different dissociation pathways have been identified and separated, namely, concerted breakup in an acetylene configuration, concerted breakup in a vinylidene configuration, and sequential breakup via a [Formula: see text] intermediate, and the branching ratio for all three pathways are determined.

Single photon double and triple ionization of allene

Double and triple ionization of allene are investigated using electron-electron, ion-ion, electron-electron-ion and electron-electron-ion-ion (ee, ii, eei, eeii) coincidence spectroscopies at selected photon energies. The results provide supporting evidence for a previously proposed roaming mechanism in H₃⁺ formation by double ionization. The lowest vertical double ionization energy is found to be 27.9 eV, while adiabatic double ionization is not accessed by vertical ionization at the neutral geometry. The triple ionization energy is found to be close to 50 eV in agreement with theoretical predictions. The doubly charged parent ion is stable up to about 2 eV above the threshold, after which dissociations by charge separation and by double charge retention occur with comparable intensities. Fragmentation to H⁺ + C₃H₃⁺ starts immediately above the threshold as a slow (metastable) decay with 130.5 ± 9.9 ns mean lifetime.

Ultrafast molecular dynamics in ionized 1- and 2-propanol: from simple fragmentation to complex isomerization and roaming mechanisms

Upon photoexcitation, molecules can undergo numerous complex processes, such as isomerization and roaming, leading to changes in the molecular and electronic structure. Here, we report on the time-resolved ultrafast nuclear dynamics, initiated by laser ionization, in the two structural isomers, 1- and 2-propanol, using a combination of pump-probe spectroscopy and coincident Coulomb explosion imaging. Our measurements, paired with quantum chemistry calculations, identify the mechanisms for the observed two- and three-body dissociation channels for both isomers. In particular, the fragmentation channel of 2-propanol associated with the loss of CH₃ shows possible evidence of methyl roaming. Moreover, the electronic structure of this roaming methyl fragment could be responsible for the enhanced ionization also observed for this channel. Finally, comparison with similar studies done on ethanol and acetonitrile helps establish a correlation between the length of the alkyl chain and the likelihood of hydrogen migration.

Ellipticity controlled dissociative double ionization of ethane by strong fields

The yields of all dissociation channels of ethane dications produced by strong field double ionization were measured. It was found that the branching ratios can be controlled by varying the ellipticity of laser pulses. The CH₃⁺ formation and H⁺ formation channels show a clear competition, producing the highest and lowest branching ratios at ellipticity of ∼0.6, respectively. With the help of theoretical calculations, such a control was attributed to the ellipticity dependent yields of different sequential ionization pathways.

Femtosecond dynamics and coherence of ionic retro-Diels-Alder reactions

Ultrafast tunnel ionization enables femtosecond time-resolved dynamic measurements of the retro-Diels-Alder reactions of positively charged cyclohexene, norbornene, and dicyclopentadiene. Unlike the reaction times of 500-600 ps that are observed following UV excitation of neutral species, on the ionic potential energy surfaces, these reactions occur on a single picosecond timescale and, in some cases, exhibit vibrational coherence. In the case of norbornene, a 270 cm^-1 vibrational mode is found to modulate the retro-Diels-Alder reaction.

Strong-field ionization of polyatomic molecules: ultrafast H atom migration and bond formation in the photodissociation of CH3OH

Strong-field ionization induces various complex phenomena like bond breaking, intramolecular hydrogen migration, and bond association in polyatomic molecules. The H-atom migration and bond formation in CH3OH induced by intense femtosecond laser pulses are investigated using a Velocity Map Imaging (VMI) spectrometer. Various laser parameters like intensity (1.5 × 1013 W cm-2-12.5 × 1013 W cm-2), pulse duration (29 fs and 195 fs), wavelength (800 nm and 1300 nm), and polarization (linear and circular) can serve as a quantum control for hydrogen migration and the yield of Hn+ (n = 1-3) ions which have been observed in this study. Further, in order to understand the ejection mechanism of the hydrogen molecular ions H2+ and H3+ from singly-ionized CH3OH, quantum chemical calculations were employed. The dissociation processes of CH3OH+ occurring by four dissociative channels to form CHO+ + H3, H3+ + CHO, CH2+ + H2O, and H2O+ + CH2 are studied. Using the combined approach of experiments and theory, we have successfully explained the mechanism of intramolecular hydrogen migration and predicted the dissociative channels of singly-ionized CH3OH.

Role of the Environment in Quenching the Production of H_{3}^{+} from Dicationic Clusters of Methanol

Ionization and subsequent isomerization of organic molecules has been suggested as an important source of trihydrogen H_{3}^{+} cations in outer space. The high interest in such reactions has initiated many experimental and theoretical studies for various molecules. Here, we report measurements as well as ab initio molecular dynamics simulations on the fragmentation of dicationic methanol monomers and clusters ionized by low-energy (90 eV) electrons. Experimentally, for dicationic monomers, a fragmentation channel for the formation of H_{3}^{+} in coincidence with a COH^{+} cation is observed. The simulations show that an intermediate neutral H_{2} is formed in the first step, and its roaming around the dication ends in the formation of H_{3}^{+}. The entire reaction takes about 100-500 fs. The calculated kinetic energy release for the H_{3}^{+}+COH^{+} ion pair is in excellent agreement with the experimental result. In contrast, for the dicationic clusters, due to the possibility of distributing the two charges onto different molecules, several fast dissociation channels occur and suppress the roaming of H_{2} and formation of H_{3}^{+}. The present Letter suggests that the quenching of H_{3}^{+} formation by the chemical environment is a general phenomenon in dicationic clusters of organic molecules.

Time-resolved dissociative ionization and double photoionization of CO2

CO₂ single-photon double photoionization, Coulomb explosion, and dissociative ionization are studied with ultrafast extreme-ultraviolet pump and time-delayed near-infrared probe pulses. Kinetic energy release and momentum correlations for the two-body CO⁺ + O⁺ and three-body O⁺ + C⁺ + O fragmentation products are determined by 3D coincidence fragment imaging. The transient enhancement of the ratio of two-body vs three-body Coulomb explosion events and the time dependence of low and high kinetic energy release dissociation events are discussed in terms of dissociative ionization and Coulomb explosion dynamics.

Formation of H3+ from ethane dication induced by electron impact

Hydrogen migration plays an important role in the chemistry of hydrocarbons which considerably influences their chemical functions. The migration of one or more hydrogen atoms occurring in hydrocarbon cations has an opportunity to produce the simplest polyatomic molecule, i.e. H₃⁺. Here we present a combined experimental and theoretical study of H₃⁺ formation dynamics from ethane dication. The experiment is performed by 300 eV electron impact ionization of ethane and a pronounced yield of H₃⁺ + C₂H₃⁺ coincidence channel is observed. The quantum chemistry calculations show that the H₃⁺ formation channel can be opened on the ground-state potential energy surface of ethane dication via transition state and roaming mechanisms. The ab initio molecular dynamics simulation shows that the H₃⁺ can be generated in a wide time range from 70 to 500 fs. Qualitatively, the trajectories of the fast dissociation follow the intrinsic reaction coordinate predicted by the conventional transition state theory. The roaming mechanism, compared to the transition state, occurs within a much longer timescale accompanied by nuclear motion of larger amplitude.

Absence of Triplets in Single-Photon Double Ionization of Methanol

Despite the abundance of data concerning single-photon double ionization of methanol, the spin state of the emitted electron pair has never been determined. Here we present the first evidence that identifies the emitted electron pair spin as overwhelmingly singlet when the dication forms in low-energy configurations. The experimental data show that while the yield of the CH2O+ + H3+ Coulomb explosion channel is abundant, the metastable methanol dication is largely absent. According to high-level ab initio simulations, these facts indicate that photoionization promptly forms singlet dication states, where they quickly decompose through various channels, with significant H3+ yields on the low-lying states. In contrast, if we assume that the initial dication is formed in one of the low-lying triplet states, the ab initio simulations exhibit a metastable dication, contradicting the experimental findings. Comparing the average simulated branching ratios with the experimental data suggests a >3 order of magnitude enhancement of the singlet:triplet ratio compared with their respective 1:3 multiplicities.