Light-matter interactions

Understanding light-matter interaction is important to control the electron and nuclear dynamics of quantum-mechanical systems. The present work investigates this in the form of angular dependent tunnel ionization and different control mechanisms for nuclear, electron and coupled dynamics. With the help of close collaboration with experimental groups several control mechanisms could be examined and explained. The refined methods and models for these studies can be expanded for different experiments or more general concepts. The first part of this thesis focuses on tunnel ionization as one of the fundamental quantum-mechanical light-matter interactions while the second and third part investigates the control of nuclear and electron dynamics in depth. The angular dependent tunnel ionization of small hydrocarbons and the impact of their field dressed orbitals are researched in chapter 3. Advanced quantum chemical methods are used to explain experimental findings that could not be recognized by only looking at the Highest Occupied Molecular Orbital (HOMO). The so studied molecules show the importance to consider field dressed instead of field free orbitals to understand the light-matter interaction, to replicate experimental findings with theoretical models and to predict the behavior of different molecules. The influence of Rydberg character in virtual orbitals, that can become populated in a field dressed picture, can explain the difference in the angular dependent tunnel ionization for two similar derivates of Cyclohexadiene (CHD) and the lobed structure for C2H4 . This chapter also shows the success of adapting a previous used model for diatomic systems to polyatomic systems. The second part (chapter 4) investigates the deprotonation and isomerization reaction of acetylene (C2H2) and allene (C3H4) and the potential control with laser pulses over theses reaction. The first control mechanism utilizes the light field to suppress the reaction barrier, which allows molecules with lower energy to undergo isomerization and therefore increase the rate of the reaction. The second scheme controls the asymmetry of the reaction, so that either the left to right or right to left isomerization is preferred. This control is exercised by directly manipulating the nuclear wave packet with the Carrier–Envelope–Phase (CEP) of the laser pulse. The mechanism relies on forming a superposition of different normal modes that are excited by different means and therefore have a phase difference. One or more normal modes are excited by the light field and get the CEP imprinted in their phase while the other important normal modes are indirectly excited by the ionization process. This enables directional control of the nuclear dynamics in symmetric molecules. The concept of forming the superposition is general enough to be used in different reactions and molecules. In the last part (chapter 5) the control of electron dynamics with laser pulses is studied. The test case is the selective population of dressed states (SPODS) in the potassium dimer (K2). There a first pulse will populate an electronic superposition between the ground and first excited state. Depending on the relative phase of the second pulse to the oscillating dipole created by the electronic wave packet, the upper or lower dressed state will be populated. Excitation from the two different dressed states leads to two distinguishable final states. Although the scheme focuses on the control of the electron dynamics, the whole mechanism is also heavily influenced by the associated nuclear dynamics

Light-matter interactions

Understanding light-matter interaction is important to control the electron and nuclear dynamics of quantum-mechanical systems. The present work investigates this in the form of angular dependent tunnel ionization and different control mechanisms for nuclear, electron and coupled dynamics. With the help of close collaboration with experimental groups several control mechanisms could be examined and explained. The refined methods and models for these studies can be expanded for different experiments or more general concepts. The first part of this thesis focuses on tunnel ionization as one of the fundamental quantum-mechanical light-matter interactions while the second and third part investigates the control of nuclear and electron dynamics in depth. The angular dependent tunnel ionization of small hydrocarbons and the impact of their field dressed orbitals are researched in chapter 3. Advanced quantum chemical methods are used to explain experimental findings that could not be recognized by only looking at the Highest Occupied Molecular Orbital (HOMO). The so studied molecules show the importance to consider field dressed instead of field free orbitals to understand the light-matter interaction, to replicate experimental findings with theoretical models and to predict the behavior of different molecules. The influence of Rydberg character in virtual orbitals, that can become populated in a field dressed picture, can explain the difference in the angular dependent tunnel ionization for two similar derivates of Cyclohexadiene (CHD) and the lobed structure for C2H4 . This chapter also shows the success of adapting a previous used model for diatomic systems to polyatomic systems. The second part (chapter 4) investigates the deprotonation and isomerization reaction of acetylene (C2H2) and allene (C3H4) and the potential control with laser pulses over theses reaction. The first control mechanism utilizes the light field to suppress the reaction barrier, which allows molecules with lower energy to undergo isomerization and therefore increase the rate of the reaction. The second scheme controls the asymmetry of the reaction, so that either the left to right or right to left isomerization is preferred. This control is exercised by directly manipulating the nuclear wave packet with the Carrier–Envelope–Phase (CEP) of the laser pulse. The mechanism relies on forming a superposition of different normal modes that are excited by different means and therefore have a phase difference. One or more normal modes are excited by the light field and get the CEP imprinted in their phase while the other important normal modes are indirectly excited by the ionization process. This enables directional control of the nuclear dynamics in symmetric molecules. The concept of forming the superposition is general enough to be used in different reactions and molecules. In the last part (chapter 5) the control of electron dynamics with laser pulses is studied. The test case is the selective population of dressed states (SPODS) in the potassium dimer (K2). There a first pulse will populate an electronic superposition between the ground and first excited state. Depending on the relative phase of the second pulse to the oscillating dipole created by the electronic wave packet, the upper or lower dressed state will be populated. Excitation from the two different dressed states leads to two distinguishable final states. Although the scheme focuses on the control of the electron dynamics, the whole mechanism is also heavily influenced by the associated nuclear dynamics

Ultrafast dynamics of small quantum systems studied using electron-ion coincidence spectroscopy

Studying how small quantum systems, like molecules and clusters, interact with X-rays is crucial to understanding the ultrafast processes that occur in nature on incredibly short timescales, ranging from femtoseconds to picoseconds. X-rays excite small quantum systems to unstable core hole states, leading to a cascade of phenomena, including Auger decay, nuclear rearrangement, and dissociation. The dissociation of molecules is influenced by the initial site of X-ray excitation, as well as the properties of the Auger populated states, such as charge localization and internal energy. In clusters, the dissociation process depends on intermolecular interactions, cluster size, and geometry. The interplay between electronic and nuclear dynamics in core-excited/ionized molecules and clusters is a critical factor that needs to be assessed. This thesis investigates X-ray-induced fragmentation of molecular adamantane and CO2 clusters using synchrotron radiation. The kinematics of molecular and cluster fragmentation is measured using advanced techniques, such as 3D momentum imaging of the ion fragments and multiparticle coincidence spectroscopy. Site-selective fragmentation of the carbon cage of the adamantane molecule is studied using Auger-electron Photoion coincidence spectroscopy, revealing the influence of the core-hole site on the Auger decay and dissociation process. Statistical data analysis treatment is developed and implemented to remove background contamination in the coincidence data using experimental random coincidences. The results highlight that the fragmentation of adamantane cation and dication is a complex dynamical process with competing relaxation pathways involving cage opening, hydrogen migration, and carbon-carbon bond breaking. Additionally, the thesis investigates the photoreactions of core-ionized CO2 clusters, reporting a significantly increased production of O2+ compared to isolated CO2 molecules. Through quantum chemistry calculations and multi-coincidence 3D momentum imaging, the study determined that the enhanced production of O2+ is due to a size-dependent structural transition of the clusters. The study also proposes two relevant photoreactions involving intermolecular interactions. This thesis highlights the complexity of core-hole dynamics in molecular and cluster chemistry and emphasizes the need for meticulous experimental and theoretical investigations of the underlying mechanisms. It also discusses the relevance of the results in the context of X-ray-induced astrochemistry. Indeed, the experiments presented are conducted in vacuum chambers in a controlled environment and can crudely replicate the conditions found in astrophysical environments. From the adamantane study, we conclude that X-ray absorption emphatically results in dissociation into smaller hydrocarbons and low photostability can play a part in the absence of diamondoids in the interstellar medium. From the CO2 clusters study, we found an enhancement in the O2+ yield, which can significantly influence the ion balance in CO2-rich atmospheres like Mars

The Reactions of Dications with Neutral Species: Understanding Planetary Ionospheres

Doubly charged cations (dications) of molecular and atomic species are predicted to be influential in high-energy environments such as the interstellar medium, the ionospheres of planets and satellites, and plasmas. However, definitive detection of dications in these environments are not yet available and the presence of these ions is often overlooked. Early investigations of dication-neutral collisions, often at high collision energies, only resulted in the observation of electron-transfer reactivity. Modern experiments, using lower collision energies, have revealed a range of exotic chemistry such as bond-formation with rare gas elements. This chemistry, coupled with the significant abundance of dications predicted in ionospheres, suggests that these ions could play important roles in atmospheric processes. For example, dications could be involved in the chemistry of complex molecule assembly. The study of dications and their reactions is clearly important to understanding ionospheric processes in planets and satellites including the prebiotic Earth. This thesis explores the bimolecular reactivity of various dications with neutral species in order to better understand the processes occurring in the ionospheres of planets and satellites. The position-sensitive coincidence mass spectrometry technique employed in this work utilises coincident, position-sensitive, detection of ions to reveal comprehensive information concerning the dynamics and energetics of the consequences of dication-neutral reactions. Specifically, the reactions following collisions of Ar2+, S2+ and CH2CN2+ with atoms and small molecules have been investigated. These dication-neutral collision systems exhibit intriguing reactivity clearly demonstrating the diversity of dication chemistry. For example, many of the electron-transfer reactions observed show evidence of proceeding via collision complexes, contrary to the orthodox (direct) mechanism. Of the bond-forming reactions detected, those generating molecular species containing a rare gas, such as ArO+ and ArN+, are the most notable. Despite the observation of the involvement of collision complexes in electron-transfer, many of the bond-forming reactions described in this thesis have been shown to occur via direct mechanisms. The observation of bond-forming reactions and the involvement of collision complexes clearly shows the facility of dications to form associations despite their often-high potential energies

Electron re-collision dynamics in strong mid-IR fields for diffraction imaging of molecular structure and its fragmentation

One of the grand challenges of modern science is to image chemical reactions and biological functions while they are taking place. To realize these molecular movie, experimental techniques need to resolve the relevant molecular motions on their natural dimensions, namely sub-atomic (≤10 ‾‾10 m) spatial dimensions and in few- to hundreds of femtoseconds (10 ‾‾15 ) in duration. A developing molecular imaging technique is based on laser-induced electron diffraction (LIED). Here, an intense electric field is used to liberate an electron from the target molecule before being accelerated and driven back to its parent ion by the same laser field. Upon return, the electron wave packet re-scatters off the target nuclei and an imprint of the molecular structure is encoded in the resultant diffraction pattern. LIED follows similar principles to those used in conventional electron diffraction techniques but occurs in the presence of a strong laser field. Due to the single optical cycle nature of the strong-field induced re-collision process, LIED inherits an intrinsic temporal resolution that fundamentally lies in the sub-femtosecond temporal regime. In this thesis, a novel experimental approach is presented that allows us to use the LIED technique for dynamic imaging of polyatomic molecular systems, a feat that had never before been realized. Our method involves combining an intense mid-IR field with 3D coincidence momentum detection. On the one hand, the mid-IR laser allows the creation of high energy re-collision electrons, and ionization within the quasi-static regime, hence permitting semi-classical treatment of the process. On the other hand, coincidence detection imaging of the entire momentum space allows for a selective assignment of the molecular reaction channel with sufficient signal-to-noise. Both points have been the main drawbacks to use LIED as a readily available and resilient method that can be utilized for structurally imaging of ultrafast processes in gas phase molecular systems. The experiments conducted throughout this thesis are divided in two main parts: First, the interaction of intense mid-IR waveforms with atomic and molecular targets is investigated and tested against established theories. Here, we measured the photo electron spectrum for very low energies with high resolution and for the first time in full 3D. Thereby, we observed various orders of the recently found ionization surprise being electrons bunching for specifically low energies when crossing the ionization threshold. They predominantly exist when tunnel-ionized at longer wavelength (λ≥1 μm). Next, mid-IR induced double ionization was investigated for xenon atoms as an example of a many-electron system. By probing the transition between different ionization regimes as a function of intensity, we found divergent behavior as compared to 800 nm driving fields. These initial experiments serve as a foundation and confirmation that the main parts of strong-field interactions with mid-IR lasers can be accurately modeled with classical simulations. More importantly, this re-insures the interpretation of LIED as re-scattering electrons on classical trajectories which sets the scene for our work on molecular imaging with LIED. Then in the second part, mid-IR LIED imaging is implemented with our experimental methodology. This allowed us to image aligned acetylene (C2H2) molecules and generate enough momentum transfer to successfully probe its entire structure with few picometer spatial resolution. Acetylene offers reaction channels of prototypical, ultrafast molecular dynamics like e.g. dissociation and isomerization. In order to investigate the combined spatio-temporal capabilities of mid-IR LIED, we conducted measurements to image the molecule while it undergoes a breakup of one of its carbon-hydrogen bonds. These snapshots of a proton escape are taken on a time scale below 10 fs after population of the dissociating state.Uno de los grandes desafíos de la ciencia moderna es captar reacciones químicas y funciones biológicas mientras están teniendo lugar. Estas películas moleculares, donde se rastrearían las trayectorias espacio-temporales exactas de los átomos constituyentes, proporcionaría una multitud de perspectivas viables sobre cómo funciona la naturaleza. Para realizar una película molecular, las técnicas experimentales necesitan medir los movimientos moleculares relevantes en sus magnitudes naturales -las distancias subatómicas (≤10 ‾‾10m) y duraciones desde unos pocos a cientos de femtosegundos (10 ‾‾15 s). Una técnica para captar imágenes moleculares en desarrollo se basa en la difracción de electrones inducida por láser (LIED, en inglés: laser-induced electron diffraction). Aquí, un campo eléctrico intenso se utiliza para liberar un electrón de un átomo o molécula antes de ser acelerado y conducido de nuevo a su ión de origen por el mismo campo de láser. Al volver, el paquete de ondas de electrones interactúa con los núcleos objetivos, quienes dejan su huella en el patrón de difracción. LIED sigue principios similares a los utilizados en técnicas convencionales de difracción de electrones, pero ocurre en presencia de un campo láser fuerte. Debido a que la re-colisión inducida por el laser ocurre en una escala menor a la duración de un período, LIED hereda una resolución temporal intrínseca, que se encuentra fundamentalmente en una escala menor a un femtosegundo. En esta tesis se presenta un nuevo enfoque experimental que nos permite utilizar la técnica LIED para captar de sistemas moleculas poliatómicas y dinámicas que nunca antes se había realizado. Nuestro método consiste en combinar un intenso laser de infrarrojo medio con detección 3D de iónes y electrónes en coincidencia. Por un lado, esto permite la creación de electrones de re-colisión de alta energía que operan dentro del régimen cuasi-estático, el cual a la vez atmite un tratamiento semiclasico del proceso de la ionizacón. Por otra parte, la detección en coincidencia de todo el espacio de momento de los partículas permite determinar selectivamente el canal de reacción molecular correspondiente con suficiente relación señal - ruido. Ambos puntos han sido hasta ahora las principales limitaciones al utilizar LIED como un método de imagen fiable y robusto que puede ser utilizado para la imagen de estructuras de moléculas durante procesos ultra-rápidos. Los experimentos llevados a cabo a lo largo de esta tesis se dividen en dos partes principales, ambos aprovechando los beneficios de una fuente infrarroja ultrarápida de 160 kHz, combinada con un sistema de detección de “microscopio de reacción”. En primer lugar, se presenta y analiza la interacción de las formas de onda infrarroja de alta intensidad con objetivos atómicos y moleculares respecto a las teorías establecidas. Aquí, medimos el espectro fotoelectrónico a muy bajas energías con una alta resolución y por primera vez completamente en 3D. En ello, hemos observado varios órdenes de la recientemente encontrada “sorpresa de ionisación”, es decir, electrones agrupados para energías específicamente bajas al cruzar el umbral de ionización. Estos existen predominantemente cuando se ioniza a través de ionización-efecto-túnel con longitudes de onda más larga (λ≥1 μm). Estudiamos estás “estructuras de electrones” en función de la duración del pulso del infrarrojo medio. A continuación, se investigó el proceso de ionización doble inducida por el infrarrojo medio en átomos de xenón, como modelo de un sistema de muchos electrones. Midiendo la transición entre diferentes regímenes de ionización en función de la intensidad, encontramos un comportamiento divergente en comparación con campos de 800 nm. En segundo lugar, la tecníca de LIED con campos infrarrojos medios fue implementado con nuestra metodología experimental. Esto nos permitió realizar imágenes de moléculas de acetileno (C2H2) alineadas y generar suficiente transferencia de momento para medir con éxito su estructura con una resolución espacial de unos pocos picómetros. El acetileno ofrece canales de reacción de dinámicas moleculares prototípicas y ultrarrápidos como disociación e isomerización. Para investigar la combinación de las capacidades espacio-temporales de LIED, obtuvimos imágenes de la molécula durante la ruptura de uno de sus enlaces carbono-hidrógeno. Estas imágenes instantáneas del escape de protones son tomadas en una escala de tiempo por debajo de los 10 fs después de que el estado de disociación se ha poblado.Eine der größten Herausforderungen der modernen Wissenschaft besteht darin, chemische Reaktionen und biologische Funktionen zu visualisieren, während sie stattfinden. Diese molekularen Filme, in denen die exakten Bahnen der einzelnen Atome räumlich und zeitlich verfolgt werden, würden eine Vielzahl von wertvollen Einblicken in die Funktionsweise der Natur liefern. Um einen molekularen Film realisieren zu können, müssen experimentelle Techniken entwickelt werden, die molekulare Bewegungen in ihren natürlichen Dimensionen auflösen können. Dazu wird eine subatomare räumliche Auflösung (≤10 ‾‾10m) benötigt, sowie zeitliche Rasterung binnen weniger Hunderte von Femtosekunden (10 ‾‾15 s). Eine molekulare Bildgebungstechnik, die viel Potential in sich birgt, basiert auf laserinduzierter Elektronenbeugung (engl. laser-induced electron diffraction [LIED]). Dabei wird ein intensives elektrisches Laserfeld verwendet, um ein Elektron aus dem Molekül derWahl freizusetzen, es zu mit Hilfe desselben Laserfeldes zu beschleunigen und danach zu seinem Mutterion zurückzubringen. Bei seiner Rückkehr streut das Elektronenwellenpaket an den Molekülkernen, und ein Abdruck der Molekülstruktur wird im resultierenden Beugungsmuster einkodiert. LIED folgt ähnlichen Prinzipien wie denen konventioneller Elektronenbeugungstechniken, tritt jedoch in Gegenwart eines starken Laserfeldes auf. Aufgrund der Natur des Rekollisionsprozesses, welcher innerhalb eines Zyklusses des starken Laserfeldes stattfindet, weist LIED eine intrinsische zeitliche Auflösung im Subfemtosekundenbereich auf. In dieser Arbeit wird ein neuartiger experimenteller Ansatz vorgestellt, der es uns erlaubt, die LIED-Technik für die dynamische Bildgebung von mehratomigen molekularen Systemen zu verwenden. Dies konnte bisher noch nie erreicht werden. Die angewandte experimentelle Methodik zeichnet sich durch die Kombination eines intensiven mittleren infraroten Laserfeldes mit 3D-Koinzidenzimpulsdetektion aus. Auf der einen Seite erlaubt das mittlere infrarote Laserfeld die Erzeugung von hochenergetischen Rekollisionselektronen sowie einer Ionisation innerhalb des Tunnel- oder quasi-statischen Regimes, wodurch eine semiklassische theoretische Beschreibung des Prozesses möglich wird. Auf der anderen Seite gewärt die Koinzidenzdetektion des gesamten Impulsraums eine selektive Zuordnung der molekularen Reaktionskanäle bei einem ausreichendem Signal-Rausch-Verhältnis. Beide Punkte waren bisher die Haupthindernisse, LIED als standardmäßiges Verfahren zur Abbildung ultraschnellerProzesse in molekularen Systemen zu verwenden. Die in dieser Dissertation durchgeführten Experimente sind in zwei Hauptteile gegliedert, wobei beide die Vorteile der Kombination einer ultraschnellen Laser-quelle im mittleren infraroten Wellenlängenbereich mit einem “Reaktionsmikroskops” nutzen: Im ersten Teil der Arbeit wird die Wechselwirkung von Wellenformen im mittleren infraroten Wellenlängenbereich mit atomaren und molekularen Targets untersucht und mit etablierten Theorien verglichen. Dabei konnten wir erstmals ein Photoelektronenspektrum für sehr niedrige Energien mit hoher Auflösung und in 3D messen. Hierbei fanden wir verschiedene Ordnungen der erst kürzlich entdeckten “Ionisationsüberraschung” (engl. ionization surprise), bei der sich niederenergetische Elektronen beim Überschreiten der Ionisationsschwelle zusammenbündeln. Diese gebündelten Elektronenstrukturen treten hauptsächlich auf, wenn sie mit langwelligen Laserfeldern erzeugt werden (λ≥1 μm). Weiterhin untersuchten wir die Elektronenstrukturen als eine Funktion der Laserpulsdauer. Als Nächstes fokussierten wir uns auf die Doppelionisation von Xenonatomen durch langwellige Laserfelder. Xenon gilt als Beispiel für ein Vielelektronensystem. Unser Fokus lag hierbei auf dem Übergang zwischen verschiedenen Ionisationsregimen als Funktion der Laserintensität. Dabei fanden wir ein divergentes Verhalten im Vergleich zu Laserfeldern mit einer Wellenlänge von 800 nm. Diese ersten Experimente dienen als Grundlage und Bestätigung dafür, dass die Wechselwirkung von Atomen mit starken mittleren infraroten Laserfeldern anhand von klassischen Theorien genau beschrieben werden können. Umso wichtiger ist, dass diese Erkenntnisse die Interpretation von LIED als streuende Elektronen auf klassischen Bahnen sicherstellt, welche die Grundlage für unsere Experimente zur molekularen Bildgebung bildet. Im zweiten Teil widmen wir uns dann der Implementierung von LIED als molekulare Bildgebungsmethodik mit Hilfe von mittleren infraroten Laserfeldern. Unser Ziel war es Acetylen (C2H2) Moleküle abzubilden und dabei mittels genügend Impulsübertragung ihre komplette Struktur mit einer räumlichen Auflösung von wenigen Pikometern zu untersuchen. Acetylen ist interessant, da es via verschiedener ultraschneller und prototypischer Kanäle wie z. B. Dissoziation und Isomerisierung reagieren kann. Um die Leistungsfähigkeit von LIED im Sinne ihrer gleichzeitigen räumlichen und zeitlichen Auflösung zu überpüfen, haben wir Messungen durchgeführt, die das Acetylenmolekül abbilden, während eine seiner Kohlenstoff-Wasserstoff-Bindungen aufbricht. Diese Schnappschüsse einer “Protonenflucht” wurden in einem Zeitraum von weniger als 10 fs nach der Besetzung des spezifischen dissoziierenden Zustands aufgenommen

Primary processes: from atoms to diatomic molecules and clusters

International audienceThis article presents a short review of the main progresses achieved at the GANIL facilities during the last thirty years in the field of ion-atom and ion-diatomic molecule collisions. Thanks to the wide range of projectile energies and species available on the different beam lines of the facility, elementary processes such as electron capture, ionization and excitation have been extensively studied. Beside primary collision mechanisms, the relaxation processes of the collision partners after the collision have been another specific source of interest. Progresses on other fundamental processes such as Young type interferences induced by ion-molecule collisions or shake off ionization resulting from nuclear beta decay are also presented. 1. Introduction For the electronic structures of atoms and molecules, precise theoretical knowledge and high-resolution experimental data are available. But the complete understanding of dynamic processes in atomic collisions remains a challenge, due to large theoretical problems in describing time-dependent many-particle reactions, and to experimental difficulties in performing complete experiments in which all relevant quantities are accessible. Elementary collisions involving ions, atoms and molecules play an important role in many gaseous and plasma environments, where they provide both the heating and cooling mechanisms. The study of such collisions is thus not only of fundamental importance, it is also essential for the understanding of large-scale systems such as astrophysical plasmas, planetary atmospheres, gas discharge lasers, semiconductor processing plasmas, and fusion plasmas. Collisions between ions and atoms (or simple molecules) give also access to the elementary processes responsible for energy transfer in ion-matter and ion-biological molecule collisions. Complete knowledge of these elementary processes is thus of primordial importance for ion induced modification of materials as well as for radiolysis, radiotherapy and biological damages due to radiation exposure