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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 H 3 + 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 H 2 roaming prior to H 3 + formation are uncovered. Despite the increased number of hydrogens and possible combinations leading to H 3 + formation, the yield decreases as the carbon chain length increases. The fundamental mechanistic findings presented here explore the formation of H 3 + , the most important ion in interstellar chemistry, through H 2 roaming occurring in ionic species. H 2 roaming is associated with H 3 + formation when certain organic molecules are exposed to strong laser fields. Here, the mechanistic details and time-resolved dynamics of H 3 + formation from a series of alcohols were obtained and found that the product yield decreases as the carbon chain length increases.

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 3 + and that hydrogen scrambling, involving the exchange of hydrogen between different sites, is important in H 2 O + formation. The technique extends to dynamic variables and could, in principle, be applied to larger non-cyclic hydrocarbons. Excitation of hydrogen-rich molecules often causes hydrogen migration, but characterisation of the individual sites is challenging. Here, the authors show that measurements of several isotopologues of ethanol can identify each hydrogen site’s contribution to the final products.

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 H 3 + 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 CHOH 2+ fragment by the roaming H 2 molecule. This reaction has similarities to the H 2  + H 2 + mechanism leading to formation of H 3 + in the universe. These exotic chemical reaction mechanisms, involving roaming H 2 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.

An experimental route to identify and separate geometric isomers by means of coincident Coulomb explosion imaging is presented, allowing isomer-resolved photoionization studies on isomerically mixed samples. We demonstrate the technique on cis/trans 1,2-dibromoethene (C 2 H 2 Br 2 ). The momentum correlation between the bromine ions in a three-body fragmentation process induced by bromine 3 d inner-shell photoionization is used to identify the cis and trans structures of the isomers. The experimentally determined momentum correlations and the isomer-resolved fragment-ion kinetic energies are matched closely by a classical Coulomb explosion model.

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 H 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 H roaming prior to H formation are uncovered. Despite the increased number of hydrogens and possible combinations leading to H formation, the yield decreases as the carbon chain length increases. The fundamental mechanistic findings presented here explore the formation of H , the most important ion in interstellar chemistry, through H roaming occurring in ionic species.

Molecular transformations triggered by the absorption of light are of tremendous importance in our day-to-day life, science, and technology. Examples of such “photo-induced” reactions include, among many others, photosynthesis, solar energy conversion, and mechanisms behind human vision. Besides knowing the final outcome of such reactions, for many scientific and technological applications it is crucially important to understand how they evolve in time, and how the motion of individual atoms leads to a certain outcome. For decades, resolving these processes in time represented a severe experimental challenge since the atomic motion involved is extremely fast. The availability of ultrashort, femtosecond laser pulses in combination with novel molecular imaging techniques provides experimental tools needed to address this challenge.This thesis describes the application of coincident ion momentum imaging setup, sometimes called “a reaction microscope”, for studies of photo-induced dynamics in halomethane molecules (CH2I2, CH2ICl, CH3I). The main objective of this work is to visualize light-induced breaking, rearrangement and formation of molecular bonds, and to determine relevant mechanisms and time scales. Halomethanes are often considered as model systems for studying such prototypical photochemical events because they are small enough to allow for reasonable electronic structure calculations and for coincidence detection of all molecular fragments, while being large enough to be of chemical relevance and to undergo some fundamental chemical transformations. The work described here covers three different regimes of light-molecule interaction: (1) ionization and fragmentation by an intense near-infrared(NIR) field, (2) excitation of a neutral molecule by a single ultraviolet (UV) photon; and (3)ionization and fragmentation by a single extreme ultraviolet (XUV) photon. We specifically focus on several aspects of halomethane photochemistry that are of general importance, have been actively discussed in literature, and yet are difficult to access using more established imaging or spectroscopic techniques.More specifically, we first characterize molecular response to a single intense femtosecond NIR pulse at 800 nm, identifying and disentangling different ionization and fragmentation channels, and their signatures in various coincident observables. Then we apply multiple ionization and rapid dissociation (“Coulomb explosion”) by such a pulse as a tool to map molecular dynamics in pump-probe experiments. In this approach, the information on molecular geometry at the time when the probe pulse arrives is extracted from the coincident measurement of the 3D momentum vectors of the detected fragment ions. We start with the NIR pump / NIR probe experiments on CH2I2 and CH2ICl molecules, aimed at characterizing bound and dissociating wave packets induced by a strong NIR field. Here, we find that both, dissociation dynamics and molecular halogen elimination (I2or ICl) are mainly governed by the large-scale bending vibrations of the molecule, even though (weak) signatures of stretching vibrations can be also observed in the spectra. Focusing on the I2(or ICl)elimination channel, which requires breaking two carbon-halogen bonds and formation of anew bond between the two halogen atoms, we demonstrate how it can be disentangled from the other fragmentation channels, and find that it is dominated by a direct, “synchronous” pathway.Then we apply the same approach and the same NIR probe pulses to study the photoexcitation of diiodomethane (CH2I2) by a femtosecond UV pulse at 266 nm in a UV pump/ NIR probe experiment. Here, in addition to two-body dissociation and I2 elimination channels, we also observe a significant contribution of three-body dissociation. This channel can be easily separated in our triple-coincidence measurements but is notoriously difficult to identify with most of the other techniques. Besides that, we find signatures of transient CH2I-I isomer formation within the first 100 femtoseconds after the initial photoexcitation.While the picosecond-scale isomerization of CH2I2was clearly demonstrated earlier in the liquid-phase experiments in solution, and was shown to occur due to the interaction with the solvent, the existence of a much faster, intra-molecular isomerization pathway for isolated molecules in a gas phase was debated in the literature. In this work, we provide direct evidence of such ultrafast, sub-100 fs CH2I2 isomerization, and demonstrate that the decay of this short-lived isomer opens up an additional pathway for molecular iodine elimination.Finally, we have performed a complementary study on CH2ICl and CH3I molecules employing short extreme ultraviolet pulses (XUV) from FLASH II free-electron laser facility in Hamburg, Germany. Here, one femtosecond XUV pulse at ∼ 53 nm central wavelength is used to initiate the dynamics, mainly by single-photon ionization, while the second identical pulse is used to probe the evolution of the created ionic-state wave packets. Employing the same ion momentum imaging setup, we map different dissociative ionization channels and observe signature of intramolecular electron transfer between different sites of a dissociating molecular ion. In contrast to the results of earlier FEL experiments on X-ray inner-shell photoionization of dissociating halomethanes, which could be readily explained using the classical over-the-barrier charge transfer model, our data for valence XUV ionization suggest a more subtle dependence of the charge transfer probability on the internuclear distance, likely determined by the delocalization of molecular orbitals.Overall, the work presented in this thesis advances our understanding of different path-ways in strong-field and single-photon induced photochemistry of halomethanes and demonstrates an efficient and visual approach for mapping transient reaction intermediates. The tools and methodology presented here can be applied to study a broad range of ultrafast photochemical reactions, and can be useful for many strong-field imaging and control applications.

Tracking the motion of individual atoms during chemical reactions represents a severe experimental challenge, especially if several competing reaction pathways exist or if the reaction is governed by the correlated motion of more than two molecular constituents. Here we demonstrate how ultrashort X-ray pulses combined with coincident ion imaging can be used to trace molecular iodine elimination from laser-irradiated diiodomethane (CH 2 I 2 ), a reaction channel of fundamental importance but small relative yield that involves the breaking of two molecular bonds and the formation of a new one. We map bending vibrations of the bound molecule, disentangle different dissociation pathways, image the correlated motion of the iodine atoms and the methylene group leading to molecular iodine ejection, and trace the vibrational motion of the formed product. Our results provide a quantitative mechanistic picture behind previously suggested reaction mechanisms and prove that a variety of geometries are involved in the molecular bond formation. The Authors demonstrate imaging of separation and formation of chemical bonds during an elimination reaction by means of intense femtosecond X-rays pulses.

Femtosecond structural dynamics of diiodomethane (\\(\\mathrm{CH_2I_2}\\)) triggered by ultraviolet (UV) photoabsorption at 290 nm and 330 nm are studied using time-resolved coincident Coulomb explosion imaging driven by a near-infrared probe pulse. We map the dominant single-photon process, the cleavage of the carbon-iodine bond producing rotationally excited \\(\\mathrm{CH_2I}\\) radical, identify the contributions of the three-body (\\(\\mathrm{CH_2} + \\mathrm{I} + \\mathrm{I}\\)) dissociation and molecular iodine formation channels, which are primarily driven by the absorption of more than one UV photon, and demonstrate the existence of a weak reaction pathway involving the formation of short-lived transient species resembling iso-\\(\\mathrm{CH_2I{-}I}\\) geometries with a slightly shorter I-I separation compared to the ground-state \\(\\mathrm{CH_2I_2}\\). These transient molecular configurations, which can be separated from the other channels by applying a set of conditions on the correlated momenta of three ionic fragments, are formed within approximately 100 fs after the initial photoexcitation and decay within the next 100 fs.