Author: Travis Severt
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Languages : en
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When a molecule absorbs energy from its surrounding environment, the molecule's structure begins to evolve. Understanding this evolution at a fundamental level can help researchers, for example, steer chemical reactions to more favorable outcomes. The research reported in this thesis aims to further knowledge about molecular fragmentation dynamics using coincidence three-dimensional momentum imaging. To achieve this goal, we use a combination of ultrafast, intense laser pulses and vacuum-ultraviolet single-photon absorption to initiate and probe molecular dynamics. Specifically, ultrafast lasers allow researchers to follow and control molecular dynamics on their natural time scales. To complement such studies, we also use vacuum-ultraviolet single-photon absorption, in conjunction with the coincidence momentum imaging of all ejected fragments including electrons, to pinpoint state-selective dynamics occurring in various molecular targets. Throughout the thesis, we are interested in several different classes of molecular dynamics. First is the sequential fragmentation of molecules, where two or more bonds break in a step-wise manner. Specifically, we developed the native-frames analysis method, which is used to systematically reduce the dimensionality of multi-body fragmentation using the conjugate momenta of Jacobi coordinates. Applying this framework, we identify the signature of sequential fragmentation and separate its distribution from other competing processes. Moreover, we highlight the method's strengths by following fragmentation dynamics step-by-step and state-selectively using the single-photon double-ionization of D2O as an example. In addition, we explore how the signature of sequential fragmentation within the native-frames method may change under different initial conditions and demonstrate the first steps toward expanding the method to four-body breakup using formic acid as an example. In the future, we hope to identify exotic sequential fragmentation pathways where two or more metastable intermediates are formed together. We also explore molecular isomerization and roaming dynamics leading to bond rearrangement. Specifically, we demonstrate that bond-rearrangement branching ratios in several triatomic molecules are approximately the same order of magnitude. Furthermore, we highlight that the formation of H3 in various alcohol molecules can occur via roaming of H2 molecules. In addition, we study the coherent control of several molecular ions, demonstrating that the CS2+ molecule fragments via a pump-dump mechanism that occurs in a single laser pulse. We also explore the two-color control of D2+ dissociation. Specifically, we observe phase shifts between pathways originating from different initial vibrational levels corresponding to "time-delays" of 10's of attoseconds, showing that such time-scales are not just accessible via electron dynamics. Since single vacuum-ultraviolet photon absorption experiments have proven to be powerful in studying molecular fragmentation dynamics, we investigate the enhancement of lab-based high-order harmonic generation photon sources driven by two-color laser fields. Specifically, we show that two-color 800-400-nm and 800-266-nm driving fields outperform the single-color 800-nm driver by more than an order of magnitude for the plateau harmonics. Furthermore, we demonstrate that the 800-266-nm bichromatic field can control the excursion time of an electron's trajectory by as much as a factor of 2. This result is important for techniques that use the rescattering electron wavepacket as a probe for molecular dynamics, such as in laser-induced electron diffraction (LIED) and high-harmonic spectroscopy (HHS) techniques. Finally, we highlight an upgrade of our coincidence three-dimensional momentum imaging method to measure breakup channels of molecular ions where the fragments have large mass-to-charge ratio differences. Specifically, we detect the light ions, such as H+ and H2+, by adding a second movable offset detector closer to the interaction region. Meanwhile, the heavy ions and neutral fragments fly underneath the new detector and are measured using the original downstream detector, as demonstrated with preliminary CD2+ measurements. In closing, this thesis covers a variety of topics with the common theme of better understanding molecular fragmentation dynamics, ranging from multi-body fragmentation dynamics to isomerization, roaming, and coherent control. In addition, we discuss enhancing high-harmonic-generation-based photon sources to help assist in such studies in the future. Overall, we believe the results presented throughout this thesis contribute to the advancement of molecular dynamics research
Author: Benjamin Berry
Publisher:
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Languages : en
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The use of ultrafast lasers allows one to study and even control quantum mechanical systems on their natural timescales. Our aim is to study the fragmentation of small molecules in strong laser fields as a means to gain understanding of molecular dynamics and light-matter interactions. Our research group has utilized fast, positively charged molecular ion beams as targets to study and control fragmentation by strong laser fields. This approach allows for detection of all molecular fragments including neutrals, and a coincidence three-dimensional momentum imaging technique is used to characterize the fragmentation. A natural extension of these types of studies is to expand the types of molecular systems that can be studied, from positively charged molecules to neutral and negatively charged molecules. To that end, the primary technical development of this dissertation involved the generation and use of fast, negatively charged molecular beams. Using fast molecular anion beams as targets allows for the study of fragmentation in which all fragments are neutral. As a demonstration, we employ this capability to study F2- dissociation and photodetachment. The dissociation pathways are identified and used to evaluate the initial vibrational population of the F2- beam. The role of dissociation in photodetachment is also explored, and we find that it competes with other dissociative (F+F) and non-dissociative (F2) photodetachment mechanisms. Also highlighted are studies of fragmentation of LiO-, in which the dissociation into Li+O- fragments provides information about the structure of Li O-, including the bond dissociation energy, which was found to be larger than values based on theory. Studies of the autodetachment lifetimes of Li O- were also performed using a pump-probe technique. Additional experimental advancements have made successful pump-probe studies of the ionization of HD+ and Ar2+ possible. Enhancement in the ionization of dissociating HD+ and Ar2+ was observed at surprisingly large internuclear separation where the fragments are expected to behave like separate atoms. The analysis methods used to quantify this enhancement are also described. Finally, the production of excited Rydberg D* fragments from D2 molecules was studied utilizing a state-selective detection method. The carrier-envelope phase dependence of D* formation was found to depend on the range of excited final states of the atomic fragments. We also measured the excited state population of the D* fragments. Together, the studies presented in this work provide new information about fragmentation of positive, negative, and neutral molecules in strong laser fields, and the experimental developments serve as building blocks for future studies that will lead to a better understanding of molecular dynamics.
Author: Balram Kaderiya
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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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 (I2 or 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 a new 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 CH2I2 was 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 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 pathways 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.
Author: Shashank Pathak
Publisher:
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Languages : en
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Imaging molecular structures evolving at their natural timescales, during a chemical reaction, with an atomic-scale resolution has been a long-standing goal for physicists and chemists. With the recent developments in experimental techniques, as well as the light sources, such as synchrotron radiation sources, free-electron lasers (FELs), ultrafast lasers, and high-harmonic sources, it is now possible to study the molecular dynamics and structural changes with femtosecond (in some cases attosecond) time-resolution, for near-infrared to x-ray wavelengths. These advancements are particularly useful in studying a wide range of photoinduced chemical reactions and photoinduced fragmentation. In this thesis, some of the advanced techniques are used to study photoinduced isomerization and fragmentation. This thesis also partly focuses on developing the tools and techniques which can be used to study these molecular structural changes. Several molecular systems are studied throughout the thesis. Some of them are studied with the goal of understanding the chemistry post photoexcitation and photo-fragmentation, while others were aiming for method development for future experiments. Specifically, some of the experiments are performed on a prototypical heterocyclic ring molecule, thiophenone. One such experiment studies photochemistry after ultraviolet light absorption, using time-resolved photoelectron spectroscopy at a free-electron laser. The experimental results are combined with ab-initio molecular dynamics and electronic structure calculation for the ground state and excited state molecules, which revealed insights about the electronic and nuclear dynamics. Ring-opening is the most dominant process upon photoexcitation, driven by a ballistic extension of C-S bond, and is completed within ~350 fs. The ground state trajectories also confirm the formation of three ring-opened products, providing detailed insights into this reaction. Ring-opening reactions of similar types are considered as candidates for designing fast molecular switches. In another study, the fragmentation pathways of thiophenone are studied using ion-electron coincidence experiments. With these experiments, it is observed that some of the fragmentation pathways may be decoupled purely based upon the photoelectron energy, which is also a measure of the internal energy of an ion. Another method, which is often used to study dissociation, fragmentation, and isomerization pathways, is coincident ion momentum imaging. The sensitivity of this method in distinguishing similar-looking structures is demonstrated by distinguishing conformational isomers of 1,2-dibromoethane, which only differ by a rotation around a single bond and coexist in a particular ratio at any given temperature. Sequential and concerted breakup pathways were disentangled using a newly developed Native frames method to obtain information about the initial molecular geometry. These experiments may trigger future time-resolved studies to monitor subtle molecular structural changes using coincidence ion momentum imaging. The work presented in this thesis uses a wide variety of techniques to understand light-induced isomerization and fragmentation dynamics, from simple molecules to moderately complex systems. This work contributes to the understanding developed for the prototypical systems, which may help formulate general principles underlying some light-induced reactions and processes.
Author: Farzaneh Ziaee
Publisher:
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Category :
Languages : en
Pages : 0
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Understanding the photoexcitation of molecules and visualizing the ensuing dynamics on their natural time scale is essential for our ability to describe and exploit many fundamental processes in different areas of science and technology. Prominent examples of such processes include, among many others, the adverse impacts of different classes of molecules on the ozone layer in atmospheric chemistry, light conversion into electricity through photovoltaics, photocatalysis, and some essential biological processes like vision and photosynthesis. Studies of molecular dynamics triggered by photon-molecule interaction underpin our understanding of many of these phenomena by adding the intermediate state to the "before-and-after" view of such photochemical or photobiological reactions. While identifying the initial molecular structure at equilibrium and determining the final products are crucial steps for the reaction characterization, understanding the dynamics connecting these initial and final states is essential for comprehending how the reaction really happens and potentially controlling its outcome. In other words, besides the "static" view of photo-induced reactions, identifying all intermediate states involved and mapping their spatio-temporal evolution are of great interest and importance. Since photoexcitation often induces coupled electron and nuclear motion on Angström spatial and femtosecond time scales, resolving such dynamics in space and time represents a significant scientific and technological challenge. Experimental tools to address this challenge have recently become available with the development of femtosecond lasers and imaging techniques capable of visualizing the evolving molecular structure. The present thesis aims to investigate the photodissociation dynamics of halomethane molecules triggered by ultraviolet (UV) light using coincidence ion momentum imaging as a primary structural characterization tool. Halomethanes are often considered as prototypical systems for molecular photodissociation in the UV domain. Due to the complicated excited-state structure driving the photochemistry of these molecules, they exhibit rich dynamics while being small enough to still allow for a detailed theoretical treatment. The primary goal of this work is to disentangle the photo-induced reaction channels, including direct and indirect dissociation pathways, and to visualize the motion of the individual molecular fragments in each of these channels. The photofragmentation reactions considered here include two- and three-body dissociation, transient isomerization and molecular halogen formation. The experiments are carried out at two different excitation wavelengths, 263 nm and 198 nm, which enables varying the dominant reaction pathways. To carry out these measurements, the 3rd and 4th harmonics of a 790 nm Ti: Sa femtosecond laser are used to initiate the dynamics of interest, which are then probed by multiple ionization and Coulomb explosion induced by an intense 790 nm pulse arriving after a variable time delay. The ions created in such pump-probe experiments are detected employing COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS). To facilitate interpreting the experimental results, they are compared to an extensive set of Coulomb explosion simulations. More specifically, this thesis describes three major studies. The first one is a set of time-resolved measurements on iodomethane (CH3I) photodissociation in the A-band, one of the best-studied reactions in ultrafast photochemistry. Here, the focus is on a detailed characterization of direct dissociation dynamics by Coulomb explosion imaging (CEI) and disentangling the competing reaction pathways involving single- and multi-photon excitations. The coincident measurement mode and an improved time resolution of 40-45 fs allowed us to observe a new feature in the two-body CEI pattern of this well-studied reaction, which was predicted theoretically but not yet observed experimentally, and to identify signatures of two- and three-photon processes populating Rydberg and ionic states. The second part of this work focuses on time-resolved studies of bromoiodomethane (CH2BrI) and chloroiodomethane (CH2ICl) photofragmentation in the A-band at 263 nm and, in particular, on imaging the co-fragment rotation. Here, the main objectives are to evaluate the effects of halogen-atom substitution on molecular dynamics and map the time evolution of individual photodissociation pathways. For these molecules, photoabsorption in the A-band predominantly breaks the C-I bond, with weaker but non-negligible contribution from the C-Br (or C-Cl) bond cleavage. Coincident two-body CEI analysis is used to map both of these channels, as well as a minor contribution from molecular halogen (IBr or ICl) formation. Three-body CEI patterns offer a deeper insight into the dynamics of these reactions and, in addition, reveal clear signatures of the three-body dissociation, which - at this wavelength - is most likely driven by the two-photon absorption. The three-body analysis also suggests that some fragmentation pathways pass through a transient linearized configuration, which is reached within ~100 fs from the initial photoabsorption and decays on a comparably fast time scale. One of the interesting aspects of dihalomethanes photodissociation in the A-band is that, unlike CH3I, where the excess energy is primarily channeled into translational motion, a significant portion of the available energy is partitioned into rotational excitation. Carbon-halogen bond cleavage results in the rotation of the molecular co-fragment, which can be unambiguously traced in the coincident three-body CEI maps for the corresponding dissociation channel. In this work, such rotational motion is directly imaged for the dissociation of either halogen atom, resulting in a "molecular movie" of the dissociating and rotating molecule. The third group of experiments described in this thesis includes time-resolved studies of bromoiodomethane and diiodomethane (CH2I2) photofragmentation in the B-band at 198 nm. In this part, the main goal is to trace the wavelength dependence of the photochemical reaction pathways. For CH2BrI, we observe a reversal of the branching ratio of C-I and C-Br bond cleavage compared to the 263 nm data, in agreement with earlier spectroscopic and theoretical studies. However, at 198 nm, three-body dissociation and molecular halogen formation become dominant photofragmentation channels for both molecules. Finally, the CH3I photodissociation is also studied in the B-band at 198 nm, where the excitation of the lowest-lying Rydberg states is expected to trigger pre-dissociation dynamics. Although no in-depth data analysis and modeling for this reaction have been carried out, the two-body CEI results clearly demonstrate the pre-dissociation nature of CH3I fragmentation at this wavelength, reflected in a broad, diffuse dissociation band, which is very different from distinct dissociation features observed for direct dissociation processes. Moreover, the data exhibit a pronounced oscillatory structure with a periodicity of 130-140 fs, which is visible only within the pre-dissociation lifetime of the excited state (~1.5 ps). While the exact origin of this structure remains unclear and will be a subject of further analysis and theoretical work, it most likely reflects the bound-state vibrational motion, which lasts until it pre-dissociates. The work presented in this thesis represents a significant step towards a better understanding of the UV-driven photochemistry of halomethanes and contains several examples of direct visualization of the atomic motion during these photochemical reactions. Our experimental approach enabled us to identify and disentangle different dissociation pathways and track their time evolution. The experimental methodology described here can be directly applied to investigate the light-driven nuclear motion in other molecular systems with different light sources.
Author: Bretislav Friedrich
Publisher: Springer Nature
ISBN: 3030639630
Category : Science
Languages : en
Pages : 639
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This Open Access book gives a comprehensive account of both the history and current achievements of molecular beam research. In 1919, Otto Stern launched the revolutionary molecular beam technique. This technique made it possible to send atoms and molecules with well-defined momentum through vacuum and to measure with high accuracy the deflections they underwent when acted upon by transversal forces. These measurements revealed unforeseen quantum properties of nuclei, atoms, and molecules that became the basis for our current understanding of quantum matter. This volume shows that many key areas of modern physics and chemistry owe their beginnings to the seminal molecular beam work of Otto Stern and his school. Written by internationally recognized experts, the contributions in this volume will help experienced researchers and incoming graduate students alike to keep abreast of current developments in molecular beam research as well as to appreciate the history and evolution of this powerful method and the knowledge it reveals.
Author: Katrin Reininger
Publisher:
ISBN:
Category : Molecular dynamics
Languages : en
Pages :
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Author: Joachim Ullrich
Publisher: Springer Science & Business Media
ISBN: 3662084929
Category : Science
Languages : en
Pages : 529
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This is the first comprehensive treatment of the interactions of atoms and molecules with charged particles, photons and laser fields. Addressing the subject from a unified viewpoint, the volume reflects our present understanding of many-particle dynamics in rearrangement and fragmentation reactions.
Author: Ralf Zimmermann
Publisher: John Wiley & Sons
ISBN: 3527335102
Category : Science
Languages : en
Pages : 448
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Book Description
Provides comprehensive coverage of laser-induced ionization processes for mass spectrometry analysis Drawing on the expertise of the leading academic and industrial research groups involved in the development of photoionization methods for mass spectrometry, this reference for analytical scientists covers both the theory and current applications of photo-induced ionization processes. It places widely used techniques such as MALDI side by side with more specialist approaches such as REMPI and RIMS, and discusses leading edge developments in ultrashort laser pulse desorption, to give readers a complete picture of the state of the technology. Photoionization and Photo-Induced Processes in Mass Spectrometry: Fundamentals and Applications starts with a complete overview of the fundamentals of the technique, covering the basics of the gas phase ionization as well as those of laser desorption and ablation, pulse photoionization, and single particle ionization. Numerous application examples from different analytical fields are described that showcase the power and the wide scope of photo ionization in mass spectrometry. -The first general reference book on photoionization techniques for mass spectrometry -Examines technologies and applications of gas phase resonance-enhanced multiphoton ionization mass spectrometry (REMPI-MS) and gas phase resonance ionization mass spectrometry (RIMS) -Provides complete coverage of popular techniques like MALDI -Discusses the current and potential applications of each technology, focusing on process and environmental analysis Photoionization and Photo-Induced Processes in Mass Spectrometry: Fundamentals and Applications is an excellent book for spectroscopists, analytical chemists, photochemists, physical chemists, and laser specialists.
Author: Ben Zhong Tang
Publisher: Walter de Gruyter GmbH & Co KG
ISBN: 311067307X
Category : Technology & Engineering
Languages : en
Pages : 347
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Book Description
This two volume set introduces the up-to-date high-tech applications of Aggregation-Induced Emission (AIE) luminogens in biosensor, bioimaging, and biomedicine. The 2nd volume presents the applications of AIE materials in biomedicine, including the utilizations in biomedical polymers, organic nanoprobes, photosensitizer, AIEgens-based delivery systems, etc. It is an essential reference for materials scientists, physicists and biological chemists.
