Doctor of Philosophy / Department of Physics / Igor V. Litvinyuk / The goal of this dissertation project was to study the dynamics of nuclear motion in diatomic (H[subscript]2, N[subscript]2, O[subscript]2, CO) and triatomic (CO[subscript]2) molecules initiated by the ionization and/or excitation of these molecules with near-IR few-cycle laser pulses. This dynamics includes vibrational and rotational motion on the electronic potential surfaces of the molecules and their molecular ions. The experimental techniques used included the pump-probe approach, laser Coulomb explosion imaging and the COLTRIMS technique. The results are presented in four chapters. A study of rotational and vibrational nuclear dynamics in H[subscript]2 and D[subscript]2 molecules and ions initiated by 8 fs near-IR pulses is presented in Chapter 4. Transient alignment of the neutral molecules was observed and simulated; rotational frequency components contributing to the rotational wavepacket dynamics were recovered. Chapter 5 is dedicated to revealing the contribution of excited dissociative states of D[subscript]2[superscript]+ ions to the process of fragmentation by electron recollision. It was shown that it is possible to isolate the process of resonant excitation and estimate the individual contributions of the [superscript]2sigma[subscript]u[superscript]+ and [superscript]2pi[subscript]u states. In Chapter 6 the subject of investigation is the nuclear dynamics of N[subscript]2, O[subscript]2 and CO molecules initiated by ionization of a neutral molecule by a short intense laser pulse. It was shown that the kinetic energy release of the Coulomb explosion fragments, measured as a function of the delay time between pump and probe pulses, reveals the behavior of nuclear wave packet evolution on electronic states of the molecular ions. It was shown that information on the dissociation and excitation pathways can be extracted from the experimental spectra and the relative contributions of particular electronic states can be estimated. Chapter 7 is focused on studying the fragmentation of CO[subscript]2 following the interaction of this molecule with the laser field. The most important result of this study was that it presented direct experimental evidence of charge-resonant enhanced ionization (CREI), a phenomenon well-studied for diatomic molecules and predicted theoretically for triatomic molecules. The critical internuclear distance, the relevant ionic charge state and a pair of charge-resonant states responsible for the CREI were also found.

An algorithm for extracting event information from a Coulomb Explosion Imaging (CEI) position sensitive detector (PSD) is developed and compared with previously employed schemes. The PSD is calibrated using a newly designed grid overlay and validates the quality of the described algorithm. Precision calculations are performed to determine how best the CEI apparatus at The University of Waterloo can be improved. An algorithm for optimizing coincidence measurements of polyatomic molecules in CEI experiments is developed. Predictions of improved efficiency based on this algorithm are performed and compared with experiments using a triatomic molecule. Analysis of an OCS targeted CEI experiment using highly charged Argon ions to initiate ionization is performed. The resulting measurements are presented using a variety of visualization tools to reveal asynchronous and sequential fragmentation channels of OCS3+.

By means of N-body simulations we study the ion and electron dynamics in molecular first-row hydride clusters when exposed to intense and short X-ray pulses. We find that, for a particular range of X-ray intensities, fast protons are ejected from the system on a considerably shorter time scale than that of the screened core. As a consequence, the structure of heavy atoms is kept intact", which may be relevant in the context of X-ray based molecular imaging. Moreover the final charge states of the heavy ions are considerably lower than those of the ions in pristine atomic clusters exposed to the same laser pulses, which is in agreement with recent measurement of methane cluster at the LCLS in Stanford.

In this thesis we have utilized few-cycle pulses in the range 10-15s, to initiate CE to allow us to image the structure, dynamics, and kinetics of ionization and dissociation of triatomic molecules. We have made a series of measurements of this process for CO2 and N2O, by varying the laser pulse duration from 7 to 500 fs with intensity ranging from 2.5×1014 to 4×1015 (W/cm2), in order to identify the charge states and time scales involved. This is a new approach in CEI introducing a multi-dimensional aspect to the science of non-perturbative laser-molecule interaction. We refer to this approach as FEmtosecond Multi-PUlse Length Spectroscopy (FEMPULS). The use of a time and position sensitive detector allow us to observe all fragment ions in coincidence. By representing the final fragmentation with Dalitz and Newton plots, we have identified the underlying break up dynamics. Momentum conservation has been used to extract the correlated fragment ions which come from a single parent ion. This is achieved by considering that the total momentum of all correlated fragments must add up to zero. One of the main outcomes of our study is observation of charge resonance enhanced ionization (CREI) for triatomic molecules. In the case of CO2, we found that for the 4+ and higher charge states, 100 fs is the time scale required to reach the critical geometry RCO= 2.1Å and ӨOCO =163º (equilibrium CO2 geometry is RCO= 1:16Å and ӨOCO =172º. The CO23+ molecule, however, appears always to begin dissociation from closer than 1.7 Å indicating that dynamics on charge states lower than 3+ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO2 and identify the electronic states responsible for CREI. Total kinetic energy (KER) has been measured for channels (1, 1, 1) to (2, 2, 2) and it was found that the (1, 1, 1) channel is not Coulombic, while (2, 2, 2) channel is very close to Coulombic (KER close to 90% of the coulombic potential). As another outcome of our study, for the case of N2O, we observed for the first time that there are two stepwise dissociation pathways for N2O3+: (1) N2O3+ → N++ NO2+ → N+ + N++ O+ and (2) N2O3+ → N22++O+ → N+ + N++ O+ as well as one for N2O4+ → N2++ NO2+ → N2+ + N++ O+. The N22+ stepwise channel is suppressed for longer pulse length, a phenomenon which we attribute to the influence which the structure of the 3+ potential has on the dissociating wave packet propagation. Finally, by observing the KER for each channel as a function of pulse duration, we show the increasing importance of CREI for channels higher than 3+.

An algorithm for extracting event information from a Coulomb Explosion Imaging (CEI) position sensitive detector (PSD) is developed and compared with previously employed schemes. The PSD is calibrated using a newly designed grid overlay and validates the quality of the described algorithm. Precision calculations are performed to determine how best the CEI apparatus at The University of Waterloo can be improved. An algorithm for optimizing coincidence measurements of polyatomic molecules in CEI experiments is developed. Predictions of improved efficiency based on this algorithm are performed and compared with experiments using a triatomic molecule. Analysis of an OCS targeted CEI experiment using highly charged Argon ions to initiate ionization is performed. The resulting measurements are presented using a variety of visualization tools to reveal asynchronous and sequential fragmentation channels of OCS3+.

We have investigated processes of ionization, energy absorption and subsequent explosion of atomic and molecular clusters under intense laser illumination using numerical as well as analytical methods. In particular, we focused on the response of composite clusters, those consisting of different atomic elements, to intense light pulses. Another major theme is the effect of the molecular structure of clusters on their Coulomb explosion. The action of intense laser pulses on clusters leads to fundamental, irreversible changes: they turn almost instantaneously into nanoplasmas and subsequently disintegrate into separate ions and electrons. Due to this radical transformation, remarkable new features arise. Transient cluster nanoplasmas are capable of absorbing enormous amounts of laser energy. In some cases more than 90 % of incident laser energy is absorbed by a gas of clusters with a density much smaller than that of a solid. After the efficient absorption, the energy is transformed into production of energetic ions, electrons, photons, and even neutrons. Composite clusters show especially interesting behavior when they interact with intense laser pulses. Nanoplasmas formed in composite clusters may absorb even more laser energy, than those formed in homogeneous clusters, as we demonstrate in this work. One of the most important results of this thesis is the identification of a novel type of plasma resonance. This resonance is enabled by an unusual ellipsoidal shape of the nanoplasma created during the ionization process in a helium droplet doped with just a few xenon atoms. In contrast to the conventional plasma resonance, which requires significant ion motion, here, the resonant energy absorption occurs at a remarkably fast rate, within a few laser cycles. Therefore, this resonance is not only the most efficient (like the conventional resonance), but also, perhaps, the fastest way to transfer laser energy to clusters. Recently, dedicated experimental studies of this effect were performed at the Max Planck Institute in Heidelberg. Their preliminary results confirm our prediction of a strong, avalanche-like ionization of the helium droplet with a small xenon cluster inside. A conventional plasma resonance, which relies on the cluster explosion, also exhibits interesting new properties when it occurs in a composite xenon-helium cluster with a core-shell geometry. We have revealed an intriguing double plasma resonance in this system. This was the first theoretical study of the influence of the helium embedding on the laser- driven nanoplasma dynamics. Our results demonstrate the important role of the interaction between xenon and helium parts of the cluster. Understanding this interaction is necessary in order to correctly interpret the experimental results. We have elucidated several important properties of Coulomb explosion in atomic and molecular clusters. Specifically, it was found that the kinetic energy distribution of ions after the Coulomb explosion of an atomic cluster is quite similar to the initial potential energy distribution of ions and is only weakly influenced by ion overtake effects, as was believed before. For the case of molecular hydrogen clusters, we have shown that the alignment of molecules inside the cluster affects its Coulomb explosion. Investigation of the dynamical processes in composite and molecular clusters induced by intense laser pulses is a step towards understanding them in more complex nano-objects, such as biomolecules or viruses. This is of great interest in the context of x-ray diffractive imaging of biomolecules with atomic resolution, which is one of the main goals of new x-ray free electron laser facilities.

We have investigated processes of ionization, energy absorption and subsequent explosion of atomic and molecular clusters under intense laser illumination using numerical as well as analytical methods. In particular, we focused on the response of composite clusters, those consisting of different atomic elements, to intense light pulses. Another major theme is the effect of the molecular structure of clusters on their Coulomb explosion. The action of intense laser pulses on clusters leads to fundamental, irreversible changes: they turn almost instantaneously into nanoplasmas and subsequently disintegrate into separate ions and electrons. Due to this radical transformation, remarkable new features arise. Transient cluster nanoplasmas are capable of absorbing enormous amounts of laser energy. In some cases more than 90 % of incident laser energy is absorbed by a gas of clusters with a density much smaller than that of a solid. After the efficient absorption, the energy is transformed into production of energetic ions, electrons, photons, and even neutrons. Composite clusters show especially interesting behavior when they interact with intense laser pulses. Nanoplasmas formed in composite clusters may absorb even more laser energy, than those formed in homogeneous clusters, as we demonstrate in this work. One of the most important results of this thesis is the identification of a novel type of plasma resonance. This resonance is enabled by an unusual ellipsoidal shape of the nanoplasma created during the ionization process in a helium droplet doped with just a few xenon atoms. In contrast to the conventional plasma resonance, which requires significant ion motion, here, the resonant energy absorption occurs at a remarkably fast rate, within a few laser cycles. Therefore, this resonance is not only the most efficient (like the conventional resonance), but also, perhaps, the fastest way to transfer laser energy to clusters. Recently, dedicated experimental studies of this effect were performed at the Max Planck Institute in Heidelberg. Their preliminary results confirm our prediction of a strong, avalanche-like ionization of the helium droplet with a small xenon cluster inside. A conventional plasma resonance, which relies on the cluster explosion, also exhibits interesting new properties when it occurs in a composite xenon-helium cluster with a core-shell geometry. We have revealed an intriguing double plasma resonance in this system. This was the first theoretical study of the influence of the helium embedding on the laser- driven nanoplasma dynamics. Our results demonstrate the important role of the interaction between xenon and helium parts of the cluster. Understanding this interaction is necessary in order to correctly interpret the experimental results. We have elucidated several important properties of Coulomb explosion in atomic and molecular clusters. Specifically, it was found that the kinetic energy distribution of ions after the Coulomb explosion of an atomic cluster is quite similar to the initial potential energy distribution of ions and is only weakly influenced by ion overtake effects, as was believed before. For the case of molecular hydrogen clusters, we have shown that the alignment of molecules inside the cluster affects its Coulomb explosion. Investigation of the dynamical processes in composite and molecular clusters induced by intense laser pulses is a step towards understanding them in more complex nano-objects, such as biomolecules or viruses. This is of great interest in the context of x-ray diffractive imaging of biomolecules with atomic resolution, which is one of the main goals of new x-ray free electron laser facilities.:1. Introduction 1 2. Interaction of clusters with intense laser pulses 5 2.1. Cluster formation and structure . . . . . . . . . . . . . . . . . . 5 2.1.1. Cluster formation . . . . . . . . . . . . . . . . . . . . . . 5 2.1.2. Cluster structure . . . . . . . . . . . . . . . . . . . . . . 6 2.1.3. Composite clusters . . . . . . . . . . . . . . . . . . . . . 7 2.2. Matter in intense light fields . . . . . . . . . . . . . . . . . . . . 9 2.2.1. Laser sources . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2.2. Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3. Clusters under intense laser pulses . . . . . . . . . . . . . . . . . 11 2.3.1. Three stages of intense laser-cluster interaction . . . . . 12 2.3.2. Pathways of cluster ionization and energy absorption . . 13 2.3.3. Composite clusters in intense laser fields . . . . . . . . . 14 2.4. Scenarios of cluster explosion . . . . . . . . . . . . . . . . . . . 15 2.4.1. Coulomb explosion vs. quasi-neutral expansion . . . . . 15 2.4.2. Anisotropic explosion . . . . . . . . . . . . . . . . . . . . 17 2.5. Comparison between experiment and theory . . . . . . . . . . . 18 3. Theoretical methods for intense laser-cluster interaction 21 3.1. The Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2. Survey of simulation methods . . . . . . . . . . . . . . . . . . . 22 3.2.1. Quantum methods . . . . . . . . . . . . . . . . . . . . . 22 3.2.2. Classical methods . . . . . . . . . . . . . . . . . . . . . . 23 3.3. Our method: classical microscopic molecular dynamics . . . . . 24 3.3.1. Initial configuration . . . . . . . . . . . . . . . . . . . . . 24 3.3.2. Integrating the equations of motion . . . . . . . . . . . . 26 3.3.3. Observables . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.4. The role of quantum effects . . . . . . . . . . . . . . . . . . . . 31 4. Cluster nanoplasma: a statistical approach 33 4.1. Vlasov-Poisson formalism . . . . . . . . . . . . . . . . . . . . . . 33 4.2. Nanoplasma electrons at quasi-equilibrium . . . . . . . . . . . . 34 4.2.1. Self-consistent potential and electron density . . . . . . . 34 4.2.2. Energy distribution of nanoplasma electrons . . . . . . . 36 4.3. Harmonic oscillator model . . . . . . . . . . . . . . . . . . . . . 41 4.3.1. Derivation from kinetic equations . . . . . . . . . . . . . 42 4.3.2. Comparison with the molecular dynamics results . . . . 44 4.4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 5. Ionization and energy absorption in helium droplets doped with xenon clusters 47 5.1. Local ignition and anisotropic nanoplasma growth . . . . . . . . 48 5.1.1. Cluster size dependence . . . . . . . . . . . . . . . . . . 50 5.1.2. Nanoplasma resonance during its anisotropic growth . . 51 5.1.3. Range of laser frequencies and intensities . . . . . . . . . 55 5.1.4. Plasma resonance for circular polarization . . . . . . . . 56 5.1.5. Summary and future work . . . . . . . . . . . . . . . . . 57 5.2. Electron migration and its influence on the cluster expansion . . 59 5.2.1. Charging dynamics . . . . . . . . . . . . . . . . . . . . . 59 5.2.2. Explosion dynamics . . . . . . . . . . . . . . . . . . . . . 61 5.3. Interplay between nanoplasma expansion and its electronic response 63 5.3.1. Single pulse: time-dependence . . . . . . . . . . . . . . . 64 5.3.2. Two pulses: a pump-probe study . . . . . . . . . . . . . 67 5.4. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . 71 6. Coulomb explosions of atomic and molecular clusters 75 6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 6.2. Analytical treatment of the Coulomb explosion . . . . . . . . . . 76 6.2.1. Steplike density profile . . . . . . . . . . . . . . . . . . . 76 6.2.2. Kinetic approach . . . . . . . . . . . . . . . . . . . . . . 79 6.2.3. Gradually decreasing initial density . . . . . . . . . . . . 83 6.3. Coulomb explosions of atomic and molecular hydrogen clusters: a molecular dynamics study . . . . . . . . . . . . . . . . . . . . 84 6.3.1. Kinetic energy distributions of ions (KEDI) . . . . . . . 85 6.3.2. Information loss during the explosion . . . . . . . . . . . 87 6.3.3. Ion overtake processes . . . . . . . . . . . . . . . . . . . 90 6.3.4. Non-radial motion of ions . . . . . . . . . . . . . . . . . 91 6.3.5. Three-body effects in Coulomb explosion . . . . . . . . . 93 6.4. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . 96 7. Conclusions and outlook 97 7.1. Physical conclusions . . . . . . . . . . . . . . . . . . . . . . . . 97 7.2. Methodological conclusions . . . . . . . . . . . . . . . . . . . . . 99 7.3. Research perspectives . . . . . . . . . . . . . . . . . . . . . . . . 100 A. Suppression of the cluster barrier 101 B. Structure determination for Xen@Hem clusters 103 C. Calculation of the time-dependent phase shift 107 D. Potential of a uniformly charged spheroid 109 E. On the possibility of molecular alignment inside hydrogen clusters 111 Bibliography

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Doctor of Philosophy / Department of Physics / Daniel Rolles / Imaging the structures of molecules, understanding the molecular dynamics in onization and dissociation processes and, most importantly, observing chemical reactions, i.e. the making and breaking of chemical bonds in real time, have become some of the most exciting topics in the atomic and molecular physics. The rapid advances of experimental tools such as synchrotron radiation light sources, free-electron lasers and continuing advances of tabletop femtosecond ultrashort lasers that provide laser pulses at a variety of wavelengths have opened new avenues for understanding the structure of matter and the dynamics of the chemical interactions. In addition, significant improvements in computational techniques and molecular dynamic simulations have provided complementary theoretical predictions on structures and chemical dynamics. The Coulomb explosion imaging method, which has been developed and applied in many studies in the last three decades, is a powerful way to study molecular structures. The method has mostly been applied to small diatomic molecules and to simple polyatomic molecules. In this thesis, Coulomb explosion imaging is applied to study the structure of isomers, molecules that have the same chemical formula but different chemical structures. Specifically, by taking inner-shell photoionization as well as strong-field ionization approaches to ionize and fragment the molecules and by using coincidence electron-ion-ion momentum imaging techniques to obtain the three-dimensional momentum of fragment ions, structures of isomers are distinguished by using the correlations among product ion momentum vectors. At first, the study aims to understand if the Coulomb explosion imaging of geometrical isomers can identify and separate cis and trans structures. Secondly, in order to extend the application of the Coulomb explosion imaging method to larger organic molecules to test the feasibility of the method for identifying structural isomers, photoionization studiesof 2,6- and 3,5-difluoroiodobenzene have been conducted. In addition, using the full three-dimensional kinematic information of multi-fold coincidence channels, breakup dynamics of both cis/trans geometric isomers and structural isomers, and in particular, sequential fragmentation dynamics of the difluoroiodobenzene isomers are studied. Furthermore, for each study, Coulomb explosion model simulations are conducted to complement the experimental results. The results of the Coulomb explosion imaging reseach in this thesis paves the way for future time-resolved Coulomb explosion imaging experiments aiming to understand the transient molecular dynamics such as photoinduced ring opening reactions and cis/trans isomerization processes in gas-phase molecules.