Mechanisms and time-resolved dynamics for trihydrogen cation (H3+) formation from organic molecules in strong laser fields

Tracking Molecular Fragmentation in Electron-Ionization Mass Spectrometry with Ultrafast Time Resolution

ConspectusMass spectrometry is a powerful analytical method capable of identifying compounds given a minute amount of material. The fragmentation pattern that results following molecular activation serves as a fingerprint that can be matched to a database compound for identification. Over the past half century, studies have addressed and, in many cases, named the chemical reactions that lead to some of the principal fragment ions. Theories have been developed to predict the observed fragmentation patterns, many of which assume that energy redistributes prior to dissociation. However, the existence of rearrangements and nonergodic processes complicates the prediction of fragmentation patterns and the identification of compounds that have yet to be entered into a curated database. To date, very few studies have addressed the time-dependent nature of the fragmentation of radical cations and, in particular, processes occurring with picosecond or shorter time scales where one expects to find nonergodic reactions.This Account focuses on a novel approach that enables tracking of molecular fragmentation in electron-ionization mass spectrometry with ultrafast time resolution. The two challenges that have prevented the time-resolved studies following electron ionization are the random impact parameter and moment of ionization of each molecule. In addition, medium-sized molecules can produce fragmentation patterns with tens if not hundreds of product ions. Spectroscopically interrogating all of these ions as a function of time is another major challenge. We describe strong field disruptive probing, a method that ionizes molecules on a femtosecond time scale and allows us to track in time the formation of all fragment ions simultaneously.Molecular fragmentation following ionization can occur on a very wide range of time scales. Metastable ions can survive from nanoseconds to microseconds; reactions that depend on vibrational energy redistribution can take picoseconds to nanoseconds; and direct dissociation processes and some rearrangements can take place in femtoseconds to picoseconds. All of these processes depend on the dynamics that occur during attoseconds and femtoseconds following the ionization process. Following a discussion of these time scales, we provide three examples of fragmentations that have been studied with femtosecond time resolution. Each of these examples include unforeseen reaction dynamics that involve a nonergodic process, highlighting the importance of time resolution in mass spectrometry. Finally, we explore future challenges and unresolved questions in mass spectrometry and, more broadly, in the domain of electron-initiated chemical reactions.

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.

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.

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.

Deep learning study of tyrosine reveals that roaming can lead to photodamage

Amino acids are among the building blocks of life, forming peptides and proteins, and have been carefully 'selected' to prevent harmful reactions caused by light. To prevent photodamage, molecules relax from electronic excited states to the ground state faster than the harmful reactions can occur; however, such photochemistry is not fully understood, in part because theoretical simulations of such systems are extremely expensive-with only smaller chromophores accessible. Here, we study the excited-state dynamics of tyrosine using a method based on deep neural networks that leverages the physics underlying quantum chemical data and combines different levels of theory. We reveal unconventional and dynamically controlled 'roaming' dynamics in excited tyrosine that are beyond chemical intuition and compete with other ultrafast deactivation mechanisms. Our findings suggest that the roaming atoms are radicals that can lead to photodamage, offering a new perspective on the photostability and photodamage of biological systems.

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.

Ultrafast disruptive probing: Simultaneously keeping track of tens of reaction pathways

Ultrafast science depends on different implementations of the well-known pump-probe method. Here, we provide a formal description of ultrafast disruptive probing, a method in which the probe pulse disrupts a transient species that may be a metastable ion or a transient state of matter. Disruptive probing has the advantage of allowing for simultaneous tracking of the yield of tens of different processes. Our presentation includes a numerical model and experimental data on multiple products resulting from the strong-field ionization of two different molecules, partially deuterated methanol and norbornene. The correlated enhancement and depletion signals between all the different fragmentation channels offer comprehensive information on photochemical reaction pathways. In combination with ion imaging and/or coincidence momentum imaging or as complementary to atom-specific probing or ultrafast diffraction methods, disruptive probing is a particularly powerful tool for the study of strong-field laser-matter interactions.

Two- and three-body fragmentation of multiply charged tribromomethane by ultrafast laser pulses

This article provides mechanistic insight into the two- and three-body fragmentation dynamics of CHBr3 after strong-field ionization and discusses the possible isomerization of CHBr3 to BrCHBr–Br (iso-CHBr3) prior to the fragmentation.

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.

Anomalous formation of trihydrogen cations from water on nanoparticles

Regarded as the most important ion in interstellar chemistry, the trihydrogen cation, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\rm{H}}}_{{{3}}}^{+}$$\end{document}H3+, plays a vital role in the formation of water and many complex organic molecules believed to be responsible for life in our universe. Apart from traditional plasma discharges, recent laboratory studies have focused on forming the trihydrogen cation from large organic molecules during their interactions with intense radiation and charged particles. In contrast, we present results on forming \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\rm{H}}}_{{{3}}}^{+}$$\end{document}H3+ from bimolecular reactions that involve only an inorganic molecule, namely water, without the presence of any organic molecules to facilitate its formation. This generation of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\rm{H}}}_{{{3}}}^{+}$$\end{document}H3+ is enabled by “engineering” a suitable reaction environment comprising water-covered silica nanoparticles exposed to intense, femtosecond laser pulses. Similar, naturally-occurring, environments might exist in astrophysical settings where hydrated nanometer-sized dust particles are impacted by cosmic rays of charged particles or solar wind ions. Our results are a clear manifestation of how aerosolized nanoparticles in intense femtosecond laser fields can serve as a catalysts that enable exotic molecular entities to be produced via non-traditional routes.

The H₃⁺ ion plays a key role in interstellar chemistry and can be formed from organic compounds upon interaction with charged particles or radiation. Here the authors demonstrate that H₃⁺ can also be formed from water adsorbed on silica nanoparticles exposed to intense laser pulses, conditions that mimic the impact of charged particles on dust in astrophysical settings.

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.

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.

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

The time scales and formation mechanisms of tri-hydrogen cation products in organic molecule ionization processes are poorly understood, despite their cardinal role in the chemistry of the interstellar medium and in other chemical systems. Using an ultrafast extreme-ultraviolet pump and time-resolved near-IR probe, combined with high-level ab initio molecular dynamics calculations, here we report unambiguously that H₃⁺ formation in double-ionization of methanol occurs on a sub 100 fs time scale, settling previous conflicting findings of strong-field Coulomb explosion experiments. Our combined experimental-computational studies suggest that ultrafast competition, between proton-transfer and long-range electron-transfer processes, determines whether the roaming neutral H₂ dynamics on the dication result in [Formula: see text] or [Formula: see text] fragments respectively.

Ultrafast Proton Transfer Dynamics on the Repulsive Potential of the Ethanol Dication: Roaming-Mediated Isomerization versus Coulomb Explosion

If a molecular dication is produced on a repulsive potential energy surface (PES), it normally dissociates. Before that, however, ultrafast nuclear dynamics can change the PES and significantly influence the fragmentation pathway. Here, we investigate the electron-impact-induced double ionization and subsequent fragmentation processes of the ethanol molecule using multiparticle coincident momentum spectroscopy and ab initio dynamical simulations. For the electronic ground state of the ethanol dication, we observe several fragmentation channels that cannot be reached by direct Coulomb explosion (CE) but require preceding isomerization. Our simulations show that ultrafast hydrogen or proton transfer (PT) can stabilize the repulsive PES of the dication before the direct CE and form intermediate H₂ or H₂O. These neutrals stay in the vicinity of the precursor, and roaming mechanisms lead to isomerization and finally PT resulting in emission of H₃⁺ or H₃O⁺. The present findings can help to understand the complex fragmentation dynamics of molecular cations.

Strong-field control of H3 + production from methanol dications: Selecting between local and extended formation mechanisms

Using the CD₃OH isotopologue of methanol, the ratio of D₂H⁺ to D₃ ⁺ formation is manipulated by changing the characteristics of the intense femtosecond laser pulse. Detection of D₂H⁺ indicates a formation process involving two hydrogen atoms from the methyl side of the molecule and a proton from the hydroxyl side, while detection of D₃ ⁺ indicates local formation involving only the methyl group. Both mechanisms are thought to involve a neutral D₂ moiety. An adaptive control strategy that employs image-based feedback to guide the learning algorithm results in an enhancement of the D₂H⁺/D₃ ⁺ ratio by a factor of approximately two. The optimized pulses have secondary structures 110-210 fs after the main pulse and result in photofragments that have different kinetic energy release distributions than those produced from near transform limited pulses. Systematic changes to the linear chirp and higher order dispersion terms of the laser pulse are compared to the results obtained with the optimized pulse shapes.

Time-resolved molecular dynamics of single and double hydrogen migration in ethanol

Being the lightest, most mobile atom that exists, hydrogen plays an important role in the chemistry of hydrocarbons, proteins and peptides and most biomolecules. Hydrogen can undergo transfer, exchange and migration processes, having considerable impact on the chemical behavior of these molecules. Although much has been learned about reaction dynamics involving one hydrogen atom, less is known about those processes where two or more hydrogen atoms participate. Here we show that single and double hydrogen migrations occurring in ethanol cations and dications take place within a few hundred fs to ps, using a 3D imaging and laser pump-probe technique. For double hydrogen migration, the hydrogens are not correlated, with the second hydrogen migration promoting the breakup of the C-O bond. The probability of double hydrogen migration is quite significant, suggesting that double hydrogen migration plays a more important role than generally assumed. The conclusions are supported by state-of-the-art molecular dynamics calculations.

Proton migration in hydrocarbons induced by slow highly charged ion impact

Different from most of the previous studies using light or photons, we use highly charged ions as projectiles to activate proton migration in the smallest saturated and unsaturated hydrocarbon molecules, i.e., CH₄ and C₂H₂. The H₃ ⁺ formation channel (H₃ ⁺ + CH⁺) and isomerization channel (C⁺ + CH₂ ⁺), serving as indicators of proton migration, are observed in the fragmentation of CH₄ and C₂H₂ dications. Corresponding kinematical information, i.e., kinetic energy release, is for the first time obtained in the collisions with highly charged ions. In particular, for the C⁺ + CH₂ ⁺ channel, a new pathway is identified, which is tentatively attributed to the isomerization on high-lying states of acetylene dication. The kinetic energy release spectra for other two-body breakup channels are also determined and precursor dication states could thus be identified.

Making Sense of Coulomb Explosion Imaging

A multifaceted agreement between ab initio theoretical predictions and experimental measurements, including branching ratios, channel-specific kinetic energy release, and three-body momentum correlation spectra, leads to the identification of new mechanisms in Coulomb-explosion (CE) induced two- and three-body breakup processes in methanol. These identified mechanisms include direct nonadiabatic Coulomb explosion responsible for CO bond-breaking, a long-range " inverse harpooning" dominating the production of H₂⁺ + HCOH⁺, a transient proton migration leading to surprising energy partitioning in three-body fragmentation and other complex dynamics forming products such as H₂O⁺ and H₃⁺. These mechanisms provide general concepts that should be useful for analyzing future time-resolved Coulomb explosion imaging of methanol as well as other molecular systems. These advances are enabled by a combination of recently developed experimental and computational techniques, using weak ultrafast EUV pulses to initiate the CE and a high-level quantum chemistry approach to follow the resulting field-free nonadiabatic molecular dynamics.

Quantum coherent control of H3 + formation in strong fields

Quantum coherent control (QCC) has been successfully demonstrated experimentally and theoretically for two- and three-photon optical excitation of atoms and molecules. Here, we explore QCC using spectral phase functions with a single spectral phase step for controlling the yield of H₃ ⁺ from methanol under strong laser field excitation. We observe a significant and systematic enhanced production of H₃ ⁺ when a negative 34 π phase step is applied near the low energy region of the laser spectrum and when a positive 34 π phase step is applied near the high energy region of the laser spectrum. In some cases, most notably the HCO⁺ fragment, we found the enhancement exceeded the yield measured for transform limited pulses. The observation of enhanced yield is surprising and far from the QCC prediction of yield suppression. The observed QCC enhancement implies an underlying strong field process responsible for polyatomic fragmentation controllable by easy to reproduce shaped pulses.

Theoretical and experimental studies on hydrogen migration in dissociative ionization of the methanol monocation to molecular ions H3+ and H2O+

The dissociative ionization processes of the methanol monocation CH₃OH⁺ to H₃⁺ + CHO and H₂O⁺ + CH₂ are studied by ab initio method, and hydrogen migration processes are confirmed in these two dissociation processes. Due to the positive charge assignment in dissociation processes, the fragmentation pathways of CH₃OH⁺ to H₃ + CHO⁺ and CH₃OH⁺ to H₂O + CH₂⁺ are also calculated. The calculation results show that a neutral H₂ moiety in the methanol monocation CH₃OH⁺ is the origin of the formation of H₃⁺, and the ejection of fragment ions H₃⁺ and H₂O⁺ is more difficult than CHO⁺ and CH₂⁺ respectively. Experimentally, by using a dc-slice imaging technique under an 800 nm femtosecond laser field, the velocity distributions of fragment ions H₃⁺, CHO⁺, CH₂⁺, and H₂O⁺ are calculated from their corresponding sliced images. The presence of low-velocity components of these four fragment ions confirms that the formation of these ions is not from the Coulomb explosion of the methanol dication. Hence, the four hydrogen migration pathways from the methanol monocation CH₃OH⁺ to H₃⁺ + CHO, CHO⁺ + H₃, H₂O⁺ + CH₂, and CH₂⁺ + H₂O are securely confirmed. It can be observed in the time-of-flight mass spectrum of ionization and dissociation of methanol that the ion yields of fragment ions H₃⁺ and H₂O⁺ are lower than CHO⁺ and CH₂⁺ respectively, which is consistent with the theoretical results according to which dissociation from the methanol monocation to H₃⁺ and H₂O⁺ is more difficult than CHO⁺ and CH₂⁺ respectively.

Hydrogen migration processes of methanol monocation CH₃OH⁺ to H₃⁺, COH⁺, H₂O⁺ and CH₂⁺ were studied theoretically and experimentally.

Substituent effects on H3 + formation via H2 roaming mechanisms from organic molecules under strong-field photodissociation

Roaming chemical reactions are often associated with neutral molecules. The recent findings of roaming processes in ionic species, in particular, ones that lead to the formation of H₃ ⁺ under strong-field laser excitation, are of considerable interest. Given that such gas-phase reactions are initiated by double ionization and subsequently facilitated through deprotonation, we investigate the strong-field photodissociation of ethanethiol, also known as ethyl mercaptan, and compare it to results from ethanol. Contrary to expectations, the H₃ ⁺ yield was found to be an order of magnitude lower for ethanethiol at certain laser field intensities, despite its lower ionization energy and higher acidity compared to ethanol. In-depth analysis of the femtosecond time-resolved experimental findings, supported by ab initio quantum mechanical calculations, provides key information regarding the roaming mechanisms related to H₃ ⁺ formation. Results of this study on the dynamics of dissociative half-collisions involving H₃ ⁺, a vital cation which acts as a Brønsted-Lowry acid protonating interstellar organic compounds, may also provide valuable information regarding the formation mechanisms and observed natural abundances of complex organic molecules in interstellar media and planetary atmospheres.

Double Photodetachment of F-·H2O: Experimental and Theoretical Studies of [F·H2O]. J Phys Chem Lett 2018;9:6808-6813. [PMID: 30433784 DOI: 10.1021/acs.jpclett.8b02562]

Double photodetachment of the cluster F^-·H₂O in a strong laser field is explored in a combined experimental-theoretical study. Products are observed experimentally by coincidence photofragment imaging following double ionization by intense laser pulses. Theoretically, equation of motion coupled cluster calculations (EOM-CC), suitable for modeling strong correlation effects in the electronic wave function, shed light on the Franck-Condon region, and ab initio molecular dynamics simulations also performed using EOM-CC methods reveal the fragmentation dynamics in time on the lowest-lying singlet and triplet states of [F·H₂O]⁺. The simulations show the formation of H₂O⁺ + F, which is the predominant experimentally observed product channel. Suggestions are proposed for the formation mechanisms of the minor products, for example, the very interesting H₂F⁺, which involves significant geometrical rearrangement. Analysis of the results suggests interesting future directions for the exploration of photodetachment of anionic clusters in an intense laser field.

H2 roaming chemistry and the formation of H3+ from organic molecules in strong laser fields

Roaming mechanisms, involving the brief generation of a neutral atom or molecule that stays in the vicinity before reacting with the remaining atoms of the precursor, are providing valuable insights into previously unexplained chemical reactions. Here, the mechanistic details and femtosecond time-resolved dynamics of H₃⁺ formation from a series of alcohols with varying primary carbon chain lengths are obtained through a combination of strong-field laser excitation studies and ab initio molecular dynamics calculations. For small alcohols, four distinct pathways involving hydrogen migration and H₂ roaming prior to H₃⁺ formation are uncovered. Despite the increased number of hydrogens and possible combinations leading to H₃⁺ formation, the yield decreases as the carbon chain length increases. The fundamental mechanistic findings presented here explore the formation of H₃⁺, the most important ion in interstellar chemistry, through H₂ roaming occurring in ionic species.

H₂ roaming is associated with H₃⁺ formation when certain organic molecules are exposed to strong laser fields. Here, the mechanistic details and time-resolved dynamics of H₃⁺ formation from a series of alcohols were obtained and found that the product yield decreases as the carbon chain length increases.

Dissociation of [HCCH]2+ to H2+ and C2+: a benchmark reaction involving H migration, H-H combination, and C-H bond cleavage

We report the formation of H2+ and C2+ from dissociation of acetylene induced by α-particle irradiation. The unusual dissociation channel [C2H2]2+ → H2+ + C2+ is unambiguously identified by measuring the time-of-flight of both fragmented ions in coincidence. Our quantum chemical calculation confirms the existence of this dissociation pathway. It shows that [HCCH]2+ is firstly populated to the 3Π excited electronic state, followed by acetylene-vinylidene isomerization, and finally the vinylidene-like intermediate dissociates to H2+ and C2+. This dissociation channel is the simplest prototypical reaction involving H migration, H-H combination, and C-H bond cleavage. The current study plays an important role for understanding the H2+/H3+ formation reactions from organic di-cations in an interstellar medium.

Identifying the Multielectron Effect on Chemical Bond Rearrangement of CH3Cl Molecules in Strong Laser Fields

Strong field double ionization that triggers the chemical bond rearrangement of CH₃Cl is investigated by impulsive control of the alignment of molecules. The alignment and laser intensity dependent H₂⁺ and H₃⁺ yields in linearly polarized femtosecond laser have been measured, and the obtained data show that the maximum signal of H₂⁺ appears at the laser polarization parallel to the C-Cl axis of molecules and H₃⁺ species are more likely to eject at the laser polarization parallel to the C-Cl axis at low laser intensity while the H₃⁺ signal peaks at laser polarization perpendicular to the C-Cl axis at high laser intensity. The measurements indicate that electrons from HOMO - 1 and HOMO - 2 orbitals have been ionized for the generation of bond rearrangement at different laser intensity. Our results demonstrate the importance of multielectron effects and also provide an effective control method in the process of chemical bond rearrangement of the molecules in strong laser fields.

Theories and simulations of roaming

The phenomenon of roaming in chemical reactions has now become both commonly observed in experiment and extensively supported by theory and simulations. Roaming occurs in highly-excited molecules when the trajectories of atomic motion often bypass the minimum energy pathway and produce reaction in unexpected ways from unlikely geometries. The prototypical example is the unimolecular dissociation of formaldehyde (H₂CO), in which the "normal" reaction proceeds through a tight transition state to yield H₂ + CO but for which a high fraction of dissociations take place via a "roaming" mechanism in which one H atom moves far from the HCO, almost to dissociation, and then returns to abstract the second H atom. We review below the theories and simulations that have recently been developed to address and understand this new reaction phenomenon.