Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: In this thesis, a femtosecond electron diffraction (FED) set-up that is capable of investigating the photo-induced switching of Cu(DCNQI)2 from being an insulator to being a conductor is presented. Movies of atomic structural changes with temporal resolution within the typical photo-switching transition timescales (sub-picoseconds) are obtainable with this set-up by employing a femtosecond laser. The experimental technique and the design of a crucial instrument of the machine, the electron gun, are extensively described and characterised both numerically and experimentally. The interest in observing atomic structural changes of Cu(DCNQI)2 in real time is because of the rich variety of the radical salts available that show alloy-specific Charge Density Wave (CDW) transitions. Valuable insights about the driving mechanisms behind these structural changes that are responsible for a change in conductivity are obtainable, as well as the relation between crystal alloys and their transition characteristics. Electron diffraction patterns of crystals in their metallic phase (room temperature) are shown in this thesis, but diffraction patterns of cryo-cooled Cu(DCNQI)2 in its insulating phase are still to be acquired. The temporal resolution of the atomic movie can be improved by recompression of electron pulses that are debunched due to Coulomb repulsion and electron energy spread within a pulse. Numerical and preliminary experimental results presented in this work expose the potential of a simple compression technique. In this way, more electrons in a single electron pulse can be afforded which allows to perform experiments at shorter integration time or lower repetition rate. / AFRIKAANSE OPSOMMING: In hierdie tesis word ’n femtosekonde elektron diffraksie opstelling aangebied wat daartoe in staat is om die foto-geïnduseerde omskakeling in Cu(DCNQI)2 van nie-geleier tot geleier te ondersoek. Deur gebruik te maak van ’n femtosekonde laser in hierdie opstelling, is ’rolprente’ van strukturele veranderinge op atoomskaal met ’n tyd resolusie beter as die tipiese foto-omskakelings tydskaal (sub-pikosekonde) verkrygbaar. Die eksperimentele tegniek en die ontwerp van ’n noodsaaklike instrument van die masjien, die elektron geweer, word breedvoerig beskryf en numeries en eksperimenteel gekenmerk. Die belangstelling om strukturele veranderinge in Cu(DCNQI)2 op atoom skaal in reële tyd waar te kan neem is as gevolg van die ryke verskeidenheid van radikale soute, wat allooispesifieke ladings digtheid golf (CDW) oorgange toon, wat beskikbaar is. Waardevolle insigte oor die meganismes wat hierdie strukturele veranderinge wat ’n verandering in geleiding veroorsaak dryf is verkrygbaar, sowel as die verwantskap tussen die kristal allooi en die oorgang kenmerke. Diffraksie patrone van kristalle in die metaalagtige fase (kamer temperatuur) word in hierdie tesis getoon, maar diffraksie patrone van cryo-verkoelde Cu(DCNQI)2 in die niegeleier fase moet nog verkry word. Die tyd resolusie van die atomiese rolprent kan verbeter word deur die elektron puls — wat deur Coulomb afstoting en elektron energie spreiding versprei is — weer saam te pers. Numeriese en voorlopige eksperimentele resultate toon die potensiaal van ’n eenvoudige kompressie tegniek. Hierdeur kan meer elektrone in ’n elektron puls gegun word en so die integrasie tyd of die herhalingstempo van die eksperimente verkort kan word.

Ultrafast electron diffraction is a novel pump-probe technique which aims to determine transient structures during photoinduced chemical reactions and other structural transitions. This technique provides structural information at the atomic level of inspection by using an electron pulse as a diffractive probe. The atomic motions of interest happen on the 100 fs = 10^(−13) s time scale. To observe these atomic motions, a probe which matches this time scale is required. In this thesis, I describe the development of an electron diffractometer which is capable of 200 fs temporal resolution while maintaining high signal level per electron pulse. This was made possible by the construction of an ultra-compact photoactivated 60 keV femtosecond electron gun. Traditional electron pulse characterization methods are unsuitable for high number density femtosecond electron pulses such as the pulses produced by this electron gun. I developed two techniques based on the laser ponderomotive force to reliably determine the duration of femtosecond electron pulses into the sub-100 fs range. These techniques produce a direct cross-correlation trace between the electron pulse and a laser pulse. The results of these measurements confirmed the temporal resolution of the newly developed femtosecond electron diffractometer. This cross-correlation technique was also used to calibrate a method for the determination of the temporal overlap of electron and laser pulses. These techniques provide the pulse diagnostics necessary to utilize the temporal resolution provided by femtosecond electron pulses. Owing to their high charge-to-mass ratio, electrons are a sensitive probe for electric fields. I used femtosecond electron pulses in an electron deflectometry experiment to directly observe the transient charge distributions produced during femtosecond laser ablation of a silicon (100) surface. We found an electric field strength of 3.5 × 10^6 V/m produced by the emission of 5.3 × 10^11 electrons/cm^2 just 3 ps after an excitation pulse of 5.6 J/cm^2 . This observation allowed us to rule out Coulomb explosion as the mechanism for ablation under the conditions present in this experiment.

Ultrafast electron diffraction is a novel pump-probe technique which aims to determine transient structures during photoinduced chemical reactions and other structural transitions. This technique provides structural information at the atomic level of inspection by using an electron pulse as a diffractive probe. The atomic motions of interest happen on the 100 fs = 10^(−13) s time scale. To observe these atomic motions, a probe which matches this time scale is required. In this thesis, I describe the development of an electron diffractometer which is capable of 200 fs temporal resolution while maintaining high signal level per electron pulse. This was made possible by the construction of an ultra-compact photoactivated 60 keV femtosecond electron gun. Traditional electron pulse characterization methods are unsuitable for high number density femtosecond electron pulses such as the pulses produced by this electron gun. I developed two techniques based on the laser ponderomotive force to reliably determine the duration of femtosecond electron pulses into the sub-100 fs range. These techniques produce a direct cross-correlation trace between the electron pulse and a laser pulse. The results of these measurements confirmed the temporal resolution of the newly developed femtosecond electron diffractometer. This cross-correlation technique was also used to calibrate a method for the determination of the temporal overlap of electron and laser pulses. These techniques provide the pulse diagnostics necessary to utilize the temporal resolution provided by femtosecond electron pulses. Owing to their high charge-to-mass ratio, electrons are a sensitive probe for electric fields. I used femtosecond electron pulses in an electron deflectometry experiment to directly observe the transient charge distributions produced during femtosecond laser ablation of a silicon (100) surface. We found an electric field strength of 3.5 × 10^6 V/m produced by the emission of 5.3 × 10^11 electrons/cm^2 just 3 ps after an excitation pulse of 5.6 J/cm^2 . This observation allowed us to rule out Coulomb explosion as the mechanism for ablation under the conditions present in this experiment.

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Cette thèse présente une étude des impulsions électroniques ultra-brèves en utilisant le nouveau microscope électronique en transmission ultrarapide (UTEM) à Strasbourg. La première partie porte sur le mode d’opération stroboscopique, basé sur l’utilisation d’un train d’impulsions d’électrons de l’ordre de la picoseconde pour l’étude des phénomènes réversibles ultrarapides. L’étude paramétrique effectuée a permis de révéler les dynamiques fondamentales des impulsions électroniques. Des mécanismes inconnus jusqu’alors et décisifs dans les caractéristiques des impulsions ont été dévoilés. Il s’agit des effets de trajectoire, qui limitent la résolution temporelle, et du filtrage chromatique, qui impacte la distribution en énergie et l’intensité du signal. Ces connaissances permettent aujourd’hui un paramétrage affiné de l’UTEM de manière à satisfaire les divers besoins expérimentaux. La deuxième partie concerne l’installation du mode d’opération complémentaire : le mode « singel-shot ». Ce mode fait appel à une impulsion unique d’intensité élevé et d’une durée de l’ordre de la nanoseconde pour l’étude des phénomènes irréversibles. L’UTEM de Strasbourg étant le premier instrument single-shot équipé d’un spectromètre de perte d’énergie des électrons (EELS), l’influence de l’aberration chromatique a pu été étudiée en détail. Elle s’est dévoilée être une limitation majeure pour la résolution en imagerie, nécessitant d’ajuster le bon compromis avec l’aberration sphérique d’une part et l’intensité du signal d’autre part. Enfin, la faisabilité de mener des études en EELS ultrarapide avec une seule impulsion nanoseconde a pu être démontrée, ceci constituant une première mondiale. Ce résultat très prometteur ouvre un tout nouveau domaine d’expériences résolu en temps. / This thesis presents a study of ultrashort electron pulses by using the new ultrafast transmission electron microscope (UTEM) in Strasbourg. The first part focuses on the stroboscopic operation mode which works with trains of picosecond multi-electron pulses in order to study ultrafast, reversible processes. A detailed parametric study was carried out, revealing fundamental principles of electron pulse dynamics. New mechanisms were unveiled which define the pulse characteristics. These are trajectory effects, limiting the temporal resolution, and chromatic filtering, which acts on the energy distribution and signal intensity. Guidelines can be given for optimum operation conditions adapted to different experimental requirements. The second part starts with the setup of the single-shot operation mode, based on intense nanosecond electron pulses for the investigation of irreversible processes. Having the first ns-UTEM equipped with an electron energy loss spectrometer, the influence of chromatic aberration was studied and found to be a major limitation in imaging. It has to be traded off with spherical aberration and signal intensity. For the first time, the feasibility of core-loss EELS with one unique ns-electron pulse is demonstrated. This opens a new field of time-resolved experiments.

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