Fragmentation dynamics of the methanol dication

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.

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.

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.

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⁺.

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.

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.

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.

Insights Into Chemical Reactions at the Beginning of the Universe: From HeH+ to H3. Front Chem 2021;9:679750. [PMID: 34222195 PMCID: PMC8249737 DOI: 10.3389/fchem.2021.679750] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

At the dawn of the Universe, the ions of the light elements produced in the Big Bang nucleosynthesis recombined with each other. In our present study, we have tried to mimic the conditions in the early Universe to show how the recombination process would have led to the formation of the first ever formed diatomic species of the Universe: HeH+, as well as the subsequent processes that would have led to the formation of the simplest triatomic species: H3 +. We have also studied some special cases: higher positive charge with fewer number of hydrogen atoms in a dense atmosphere, and the formation of unusual and interesting linear, dicationic He chains beginning from light elements He and H in a positively charged atmosphere. For all the simulations, the ab initio nanoreactor (AINR) dynamics method has been employed.

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.

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.

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.

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

Strong-field laser-matter interactions often lead to exotic chemical reactions. Trihydrogen cation formation from organic molecules is one such case that requires multiple bonds to break and form. We present evidence for the existence of two different reaction pathways for H₃⁺ formation from organic molecules irradiated by a strong-field laser. Assignment of the two pathways was accomplished through analysis of femtosecond time-resolved strong-field ionization and photoion-photoion coincidence measurements carried out on methanol isotopomers, ethylene glycol, and acetone. Ab initio molecular dynamics simulations suggest the formation occurs via two steps: the initial formation of a neutral hydrogen molecule, followed by the abstraction of a proton from the remaining CHOH²⁺ fragment by the roaming H₂ molecule. This reaction has similarities to the H₂ + H₂⁺ mechanism leading to formation of H₃⁺ in the universe. These exotic chemical reaction mechanisms, involving roaming H₂ molecules, are found to occur in the ~100 fs timescale. Roaming molecule reactions may help to explain unlikely chemical processes, involving dissociation and formation of multiple chemical bonds, occurring under strong laser fields.

Communication: long-lived neutral H2 in hydrogen migration within methanol dication

The ejection of triatomic hydrogen molecular ions HD2(+) and D3(+) from CD3OH(2+) is investigated by first-principle molecular dynamics simulation. Two C-D chemical bonds are found to be broken to form a neutral D2 moiety that vibrates, rotates, and moves for a relatively long period of time (20-330 fs) towards a transition state leading to the ejection of HD2(+) or D3(+). The formation of such a long-lived neutral D2 moiety within a hydrocarbon molecule interprets well the recent experimental findings of the long lifetime of doubly charged energized hydrocarbon molecules prior to the ejection of H3(+).