The design, implementation and commissioning of a time-resolved electron energy spectrometer system are discussed. Since its installation at the FELIX free-electron laser user facility in Nieuwegein, The Netherlands, the spectrometer system has been in regular use as a diagnostic and investigative tool. The system provides 0.2% energy resolution with 32 channels, and time resolution of 50 ns. The spectrometer is positioned immediately following the undulator so that the gain medium—the relativistic electron beam—can be probed immediately following its interaction with the optical field in the laser cavity. The system permits real-time calculation and graphical display of key beam parameters as well as the archiving of raw data, and has been used to provide insight into the operation of an FEL in the high slippage, short pulse regime. In particular, direct measurement of the extraction efficiency is possible from macropulse to macropulse. A systematic study of efficiency as a function of wavelength and cavity desynchronisation has been undertaken. At low values of cavity desynchronisation the efficiencies measured exceed the conventional 1/2/V estimate by between 50% and 100% and these results are shown to be consistent with the formation of ultrashort optical pulses—approximately of 6 optical cycles in length. An investigation into the way in which the electron beam energy can be swept on a microsecond time scale has made it possible to produce given sweeps in wavelength—of up to 2 %, limited only by the constraints of the electron beam transport system—which have been used by molecular spectroscopists to excite target molecules through an anharmonic ladder of states. Further evidence for the recent observation of superradiance in an FEL oscillator has been provided by an investigation which shows that the efficiency and intracavity power of the radiation scale respectively as the inverse square root and the inverse square of the cavity losses, verifying the superradiant scaling laws predicted by the supermode theory. An important consequence of this observation is that it indicates that shorter and more intense optical pulses may be produced by increasing the bunch charge and reducing optical cavity losses.

The ability to diagnose the structure of a material at extreme conditions of high-pressure and high-temperature is fundamental to understanding its behaviour, especially since it was found that materials will adopt complex crystal structures at pressures in the Terapascal regime (1TPa). Static compression, using the diamond anvil cell coupled with synchrotron radiation has to date been the primary method for structural studies of materials at high pressure. However, dynamic compression is the only method capable of reaching pressures comparable to the conditions found in the interior of newly discovered exo-planets and gas giants where such exotic high-pressure behaviour is predicted to be commonplace among materials. While generating extreme conditions using shock compression has become a mature science, it has proved a considerable experimental challenge to directly observe and study such phase transformations that have been observed using static studies due to the lack of sufficiently bright X-ray sources. However, the commissioning of new 4th generation light sources known as free electron lasers now provide stable, ultrafast pulses of X-rays of unprecedented brightness allowing in situ structural studies of shock compressed materials and their phase transformation kinetics in unprecedented detail. Bismuth, with its highly complex phase diagram at modest pressures and temperatures, has been one of the most studied systems using both static and dynamic compression. Despite this, there has been no structural characterisation of the phases observed on shock compression and it is therefore the ideal candidate for the first structural studies using X-ray radiation from a free electron laser. Here, bismuth was shock compressed with an optical laser and probed in situ with X-ray radiation from a free electron laser. The evolution of the crystal structure (or lack there of) during compression and shock release are documented by taking snapshots of successive experiments, delayed in time. The melting of Bi on release from Bi-V was studied, with precise time scans showing the pressure releasing from high-pressure Bi-V phase until the melt curve is reached off-Hugoniot. Remarkable agreement with the equilibrium melt curve is found and the promise of this technique has for future off-Hugoniot melt curve studies at extreme conditions is discussed. In addition, shock melting studies of Bi were performed. The high-pressure Bi - V phase is observed to melt along the Hugoniot where melting is unambiguously identified with the emergence of a broad liquid-scattering signature. These measurements definitively pin down where the Hugoniot intersects the melt curve - a source of some disagreement in recent years. Evidence is also presented for a change in the local structure of the liquid on shock release. The impact of these results are discussed. Finally, a sequence of solid-solid phase transformations is observed on shock compression as well as shock release and is detected by distinct changes in the obtained diffraction patterns. The well established sequence of solid-solid phase transformations observed in previous static studies is not observed in our experiments. Rather, Bi is found to exist in some metastable structures instead of forming equilibrium phases. The implications these results have for observing reconstructive phase transformations in other materials on shock timescales are discussed.

Doctor of Philosophy / Department of Physics / Artem Rudenko / Christopher M. Sorensen / The motion of electrons in atoms, molecules, and solids in the presence of intense electromagnetic radiation is an important research topic in physics and physical chemistry because of its fundamental nature and numerous practical applications, ranging from precise machining of materials to optical control of chemical reactions and light-driven electronic devices. Mechanisms of light-matter interactions critically depend on the dimensions of the irradiated system and evolve significantly from single atoms or molecules to the macroscopic bulk. Nanoparticles provide the link between these two extremes. In this thesis, I take advantage of this bridge to study light-matter interactions as a function of nanoparticle size, shape, and composition. I present here three discrete, but interconnected, experiments contributing to our knowledge of nanoparticle properties and their response to intense, short-pulsed light fields. First, I investigate how individual nanoparticles interact with each other in solution, studying their temperature-dependent solubility. The interaction potential between 5.5nm gold nanoparticles, ligated by an alkanethiol was found to be -0.165eV, in reasonable agreement with a phenomenological model. The other two experiments explore ultrafast dynamics driven by intense femtosecond lasers in isolated, gas-phase metallic and dielectric nanoparticles. Photoelectron momentum imaging is applied to study the response of gold, silica, and gold-shell/silica-core nanoparticles (ranging from single to several hundred nanometers in size) with near-infrared (NIR), 25 fs laser pulses in the intensity range of 10¹¹ - 10¹⁴ W/cm². These measurements, which constitute the bulk of my graduate work, reveal the complex interplay between the external optical field and the induced near-field of the nanoparticle, resulting in the emission of very energetic electrons that are much faster than those emitted from isolated atoms or molecules exposed to the same light pulses. The highest photoelectron energies (“cutoffs”) were measured as a function of laser intensity, nanoparticle material and size. We found that the energy cutoffs increase monotonically with laser intensity and nanoparticle size, except for the gold/silica hybrid where the plasmon resonance response modifies this behavior at low intensities. The measured photoelectron spectra for metallic nanoparticles display a large energy enhancement over silica. Finally, the last part of this thesis explores the possibility to apply time-resolved x-ray scattering as a probe of the ultrafast dynamics in isolated nanoparticles driven by very intense (~10¹⁵ W/cm²) NIR laser radiation. To do this, I developed and built a nanoparticle source capable of injecting single, gas-phase nanoparticles with a narrow size distribution into the laser focus. We used femtosecond x-ray pulses from an x-ray free electron laser (XFEL) to map the evolution of the laser-irradiated nanoparticle. The ultrafast dynamics were observed in the single-shot x-ray diffraction patterns measured as a function of delay between the NIR and x-ray pulses, which allows for femtosecond temporal and nanometer spatial resolution. We found that the intense IR laser pulse rapidly ionizes the nanoparticle, effectively turning it into a nanoplasma within less than a picosecond, and observed signatures of the nanoparticle surface softening on a few hundred-femtosecond time scale.

This thesis presents experimental studies of water and ice at near-atmospheric pressures using intense x-rays only accessible at synchrotrons and free-electron lasers. In particular, it focuses on the deeply supercooled, metastable state and its implications on ice nucleation. The local structure of the liquid phase was studied by x-ray scattering over a wide temperature range extending from 339 K down to 227 K. In order to be able to study the deeply supercooled liquid, micron-sized water droplets were evaporatively cooled in vacuum and probed by ultrashort x-ray pulses. This is to date the lowest temperature at which measurements of the structure have been performed on bulk liquid water cooled from room temperature. Upon deep supercooling, the structure evolved toward that of a low-density liquid with local tetrahedral coordination. At ~230 K, where the low-density liquid structure started to dominate, the number of droplets containing ice nuclei increased rapidly. The estimated nucleation rate suggests that there is a “fragile-to-strong” transition in the dynamics of the liquid below 230 K, and its implications on water structure are discussed. Similarly, the electronic structure of deeply supercooled water was studied by x-ray emission spectroscopy down to 222 K, but the spectral changes expected from the structural transformation remained absent and explanations are discussed. At high fluence, the non-linear dependence of the x-ray emission yield indicated that there were high valence hole densities created during the x-ray pulse length due to Auger cascades, resulting in reabsorption of the x-ray emission. Finally, the hydrogen-bonded network in water was studied by x-ray absorption spectroscopy and compared to various ices. It was found that the pre-edge absorption cross-section, which is associated with distorted hydrogen bonds, could be minimized for crystalline ice grown on a hydrophobic BaF2(111) surface with low concentration of nucleation centers. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript. Paper 6: Manuscript.</p>

In the European XFEL, electrons bunches are accelerated up to 20 GeV and thenenter undulators where coherent X-rays are produced which can be used for imaging atva molecular level. Electrons may stray from the path and hit the permanent magnets inthe undulators. It is well known that ionizing radiation can affect the magnetic characteristics of permanent magnets. The undulators are therefore equipped with a type of semiconductor dosimeters, RADFETs, so that the potential damage from ionizing radiation to the magnets can be measured and corrected for. It is also known that heat will be generated from air-coils in the accelerator which can change the ambient temperature around the dosimeters up to 25 K. All semiconductor technology is highly susceptible totemperature. This report investigates the fading characteristics of the RADFET under different temperatures and times after irradiation. It also investigates the dose responseunder dierent temperatures and estimates the magnitude of errors in measured dose which can be expected if temperatures are not accounted for. It is seen that a delta T of a few K can have a large impact on RADFETs' ability to both record and retain dose. A strong time dependence is also seen. The fading is the largest during irradiation andthen slows down exponentially, stabilizing after 1 to 2 months. An increase from 20 deg C to 26 deg C will increase the fading by 2 Gy/h during irradiation, and 0.015 Gy/h weeks afterirradiation. It is estimated that dose measurements at XFEL can have errors of up to 14% if long-term fading is not accounted for. A model for estimating long-term fading as a function of temperature is proposed.

Hard x-ray free-electron lasers are novel sources of coherent x-rays with unprecedented brightness and very short pulses. The radiation from these sources enables a wide range of new experiments that were not possible with previous x-ray sources. Many of these experiments require the possibility to focus the intense x-ray beam onto small samples. This Thesis investigates the possibility to use diffractive zone plate optics to focus the radiation from hard x-ray free-electron lasers. The challenge for any optical element at free-electron laser sources is that the intensity in a single short pulses is high enough to potentially damage the optics. This is especially troublesome for zone plates, which are typically made of high Z elements that absorb a large part of the incident radiation. The first part of the Thesis is dedicated to simulations, where the temperature behavior of zone plates exposed to hard x-ray free-electron laser radiation is investigated. It is found that the temperature increase in a single pulse is several hundred Kelvin but still below the melting point of classical zone plate materials, such as gold, tungsten, and iridium. Even though the temperature increases are not high enough to melt a zone plate it is possible that stresses and strains caused by thermal expansion can damage the zone plate. This is first investigated in an experiment where tungsten gratings on diamond substrates are heated to high temperatures by a pulsed visible laser. It is found that the gratings are not damaged by the expected temperature fluctuations at free-electron lasers. Finally, a set of tungsten zone plates are tested at the Linac Coherent Light Source where they are exposed to a large number of pulses at varying fluence levels in a prefocused beam. Damage is only observed at fluence levels above those typically found in an unfocused x-ray free-electron laser beam. At higher fluences an alternative is to use a diamond zone plate, which has significantly less absorption and should be able to survive much higher fluence. Damage in diamond structures is investigated during the same experiment, but due to a remaining tungsten etch mask on top of the diamond the results are difficult to interpret. Additionally, we also demonstrate how the classical Ronchi test can be used to measure aberrations in focusing optics at an x-ray free-electron laser in a single pulse. The main result of this Thesis is that tungsten zone plates on diamond substrates can be used at hard x-ray free-electron laser sources. / <p>QC 20130514</p>

A novel, no-contact approach to X-ray mirror bending control is presented here, proposed for use on the beamlines of the European X-ray Free Electron Laser (XFEL) project. A set of mirrors with tunable bending radii are desired, that will maintain their optical properties even as the beam incidence causes local heating. Various mechanical bending mechanisms have been proposed and used on other beamlines, which can take up a lot of physical space, demanding more vacuum power, while using expensive high precision servomotors. Rather than bend the mirror by mechanical means, it is proposed to heat the mirror to produce the desired bending. This could work two ways. One scenario calls for a finely tunable heat lamp to irradiate the back surface of the mirror while the X-ray laser heats the front side. With appropriate tuning, simulations show that this approach can keep the mirror flat, and perhaps produce a circular profile. The second scenario is similar to the first, but a thin film of tungsten is added to the back of the silicon mirror. This scenario calls for the temperature of the mirror to change homogenously to affect the desired bending, and in this case the profile should be cylindrical. In both scenarios the uneven nature of the incident radiation causes distortions that may be undesirable. Both scenarios are simulated and it is shown that the stress produced by a metal film may minimize this distortion. The response time of the mirror and configuration of both the heating and cooling mechanism are also considered.

X-ray crystallography is a form of microscopy that allows the three-dimensional arrangement of atoms belonging to molecules within crystals to be determined. In this method, a crystal is illuminated with a beam of X-rays and the diffracted amplitudes resulting from the illumination are measured and computationally processed to enable the electron density of the unit molecule, or the unit cell, constituting the crystal to be calculated. The recent development of the X-ray free-electron laser (XFEL) provides new routes for determining molecular structures via its ability to generate intense but brief X-ray pulses. These new instruments enable diffraction measurements to be obtained from crystals that have a small number of unit cells, referred to as nanocrystals, and molecular structure determination via this technique is known as serial femtosecond nanocrystallography (SFX). This thesis is concerned with the characterisation of diffraction data obtained from SFX experiments and the techniques for reconstructing the electron density of the molecule from such data. The noise characteristics of diffraction measurements from nanocrystals is developed. Methods for directly inverting nanocrystal diffraction to obtain the electron density of the molecule are analysed and an approach to ameliorate the effect of noise is proposed and evaluated by simulation. A model for diffraction by nanocrystals that include the effects of different unit cell arrangements and incomplete unit cells on the crystal surface is also developed and explored by simulation. The diffraction by finite crystals is shown to be equal to the incoherent average over a set of unit cells that contain different molecular arrangements related to the symmetry of the crystal at hand. The problem of image reconstruction under this circumstance is investigated. The more general problem of reconstructing multiple, unrelated, objects from their averaged diffraction is also explored and uniqueness properties along with reconstruction algorithms developed. The problem of reconstructing multiple, related, unit cells is studied and preliminary results are obtained. These results show that iterative phase retrieval algorithms can in principle be adapted to reconstruct the electron density of a crystalline specimen from the data obtained in SFX and the retrieval of phases from the diffracted intensity averaged over multiple objects is feasible.

Impurity trapped excitons (ITEs) occurring in divalent ytterbium doped calcium and strontium fluoride crystals have been investigated by exploiting the radically different radiative decay rates of the lowest exciton state and higher excited states, utilizing a novel two-colour transient photoluminescence enhancement experiment. The ITE energy levels have been directly measured with the observation of sharp transitions occurring from the changes of states of the localized hole and broad bands associated with changes of state of the delocalized electrons. The dynamic behaviour under excitation by time delayed ultra-violet (UV) and infrared (IR) pulses has been observed allowing for the identification of excitation and decay pathways between the ITE states. The position and transition intensities of the sharp lines within the IR excitation spectrum have been successfully matched using a semi-empirical effective Hamiltonian crystal field model. In CaCaF₂:Yb²⁺ the lines occurring at 249 and 1145 cm⁻¹ were matched with the crystal field parameter B4 = 800 cm⁻¹ and the exchange parameter G3(fs) = 5900 cm⁻¹. In SrCaF₂:Yb²⁺ these lines were observed at 178 and 1284 cm⁻¹ and matched with B4 = 600 cm⁻¹ and G3(fs) = 7278 cm⁻¹. Local heating and direct absorption by intra-excitonic transitions are found to be the causes of the broad band observed in the spectrum and have been deconvolved by studying the dynamic behaviour of the monitored emission at different IR excitation frequencies. Through this modeling, higher lying ITE states have been identified occurring at 785 cm⁻¹ in SrCaF₂:Yb²⁺ and in between 740 - 820 cm⁻¹ in CaCaF₂:Yb²⁺. The dynamic model developed successfully simulates the temporal behaviour of the emission under IR excitation under a variety of parameters including IR fluence, excitation frequency, sample temperature and UV - IR pulse delay. Examination of the SrCaF₂:Yb²⁺ dynamic behaviour over a time scale of 100 ms shows UV driven trap population at a rate of approximately 3% per pulse, which are liberated and recycled to the Yb²⁺ ground state by the IR pulse. The two-colour technique is applied to MgCaF₂:Yb²⁺, a candidate for possible ITE emission. Temperature dependent emission spectra obtained under UV excitation indicates the possibility of an ITE state, independently populated from the 5d level of the Yb²⁺. Typical 5d emission is also observed from this system. Under IR excitation, liberation of shallow traps and possible local heating is observed. No ITE emission is conclusively found with IR probing.

abstract: With the advent of the X-ray free-electron laser (XFEL), an opportunity has arisen to break the nexus between radiation dose and spatial resolution in diffractive imaging, by outrunning radiation damage altogether when using single X-ray pulses so brief that they terminate before atomic motion commences. This dissertation concerns the application of XFELs to biomolecular imaging in an effort to overcome the severe challenges associated with radiation damage and macroscopic protein crystal growth. The method of femtosecond protein nanocrystallography (fsPNX) is investigated, and a new method for extracting crystallographic structure factors is demonstrated on simulated data and on the first experimental fsPNX data obtained at an XFEL. Errors are assessed based on standard metrics familiar to the crystallography community. It is shown that resulting structure factors match the quality of those measured conventionally, at least to 9 angstrom resolution. A new method for ab-initio phasing of coherently-illuminated nanocrystals is then demonstrated on simulated data. The method of correlated fluctuation small-angle X-ray scattering (CFSAXS) is also investigated as an alternative route to biomolecular structure determination, without the use of crystals. It is demonstrated that, for a constrained two-dimensional geometry, a projection image of a single particle can be formed, ab-initio and without modeling parameters, from measured diffracted intensity correlations arising from disordered ensembles of identical particles illuminated simultaneously. The method is demonstrated experimentally, based on soft X-ray diffraction from disordered but identical nanoparticles, providing the first experimental proof-of-principle result. Finally, the fundamental limitations of CFSAXS is investigated through both theory and simulations. It is found that the signal-to-noise ratio (SNR) for CFSAXS data is essentially independent of the number of particles exposed in each diffraction pattern. The dependence of SNR on particle size and resolution is considered, and realistic estimates are made (with the inclusion of solvent scatter) of the SNR for protein solution scattering experiments utilizing an XFEL source. / Dissertation/Thesis / Ph.D. Physics 2011