The establishment of robust laser-cooling techniques for the formation of ultracold atoms has provided a test-bed for low-temperature science, with scattering events changing character from incoherent thermal interactions to coherent quantum mechanical events. A natural extension is the pursuit of ultracold molecules in prescribed low-energy internal states. Atomic cooling techniques, however, do not generalize to the molecular regime due to the complex energy-level structure afforded by its extra degrees of motion. An indirect approach to ultracold molecule formation - photoassociation using ultrafast laser pulses - is the focus of this thesis. A broadband field associates atom pairs into a localized molecular wavepacket that evolves within the attractive excited-state potential. A suitably timed dump pulse may thus be applied to stabilize population into deeply bound ground vibrational states. This strategy may be generalized to any species whose spectroscopy matches the pulse spectrum, and offers a coherent population transfer scheme that does not require precise knowledge of the system. This thesis presents experiments using high-energy photoassociation pulses applied to ultracold rubidium atoms. The pulses quench the background ground-state molecular population but form bound dimers within the excited state. A pump-probe experiment was designed to chart the excited-state dynamics; however, the oscillations predicted by theoretical calculations were not evident in the molecular signal. The nature of the dynamics is expected to be strongly dependent on the initial state of the atom pairs addressed by the ultrafast pulse: a bound molecular population provides an additional candidate to free atoms. A spectroscopic measurement characterizes these bound molecules and identifies their formation mechanism. A subsequent experiment provides evidence that the predominant contributor to the pump-probe signal is the unbound initial population. The consequences with regard to both the observation of excited-state dynamics and the subsequent application of a dump pulse are discussed.

This dissertation investigates the origins of tunable and efficient photochemistry in three different photoactive proteins, bacteriorhodopsin (BR), rhodopsin (RHO) and green fluorescent protein (GFP). In all cases, significant differences exist between the photoreactivity of model chromophores in solution and in the protein environment, in terms of excited state lifetime and efficiency of the primary photochemical process (opsin proteins) or the type of reaction (excited state proton transfer versus C=C double bond photoisomerisation for GFP). The work presented here investigates for each case to what extent the protein environment is necessary to alter the photochemistry of model chromophores in solution. For GFP and BR steric and electrostatic interactions between the protein pocket and the chromophore are shown to be likely responsible for shaping the excited state surface along which the photoreactions take place. For RHO it is suggested, contrary to current belief, that selection of a reactive ground state conformer might be the main effect generating the observed differences between solution and protein environment. The solution photochemistry of structurally modified retinal protonated Schiff bases, taken as model chromophores for the opsin proteins, is studied with continuous wave irradiation experiments and ultrafast transient spectroscopies. Surprisingly large differences are observed for the isomerisation reaction depending on the starting configuration (trans or cis) of the photoactive double bond. The current model for BR based on the tuning of the excited state barrier encountered along the isomerisation coordinate is expanded to include the changes in selectivity, speed and efficiency observed for a series of all-trans derivatives. For 11-cis, the photoisomerisation in solution is proposed to take place along a barrierless isomerisation coordinate, in contrast with the models currently available in literature. It is suggested that the protein might be discriminating between ground state conformers rather than significantly changing the topography of the reaction coordinate. For GFP, excited state Raman spectra are recorded for the wild-type protein, two mutants and a model chromophore in solution. It is suggested that the high frequency vibrational modes observed in the excited state spectra of the proteins but not of the model chromophore in DMSO are a sign of a tighter chromophore environment that inhibits the photoisomerisation reaction occurring in solution.

This thesis describes studies on nanohybrid systems consisting of single-walled carbon nanotubes (SWNTs) with monolayer coatings of semiconducting polymers. Steady-state and time-resolved optical and high-resolution microscopy experiments were used to investigate the blends. These materials show promise for use in organic photovoltaics (OPVs) owing to the high carrier mobilities and large aspect ratios of SWNTs, the controllable solubilisation of tubes with various polymers and the broad light-harvesting abilities of organic polymers. Chapters 1 and 2 introduce the theory and background behind the work and present a literature review of previous work utilising carbon nanotubes in OPV devices, revealing poor performances to date. The experimental methods used during the thesis are detailed in Chapter 3 and the solution processing techniques used to prepare the polymer–nanotube blend samples are described in Chapter 4. Chapter 5 describes a study on a nanotube blend with a thiophene polymer, a system previously unsuccessfully implemented into OPV devices. Ultrafast spectroscopic measurements showed that electrons can transfer on a 400 fs time scale from the polymer to nanotubes and the conditions to allow long-lived free charges to be produced were found. The study is extended in Chapter 6 to show that nanostructures consisting of a nanotube coated in one polymer can then be coated by a second polymer and that these nano-engineered structures could be implemented into OPV devices. The use of a competition binding process to isolate purely semiconducting nanotubes dispersed with any desired polymer is then described in Chapter 7. Finally, Chapter 8 introduces systems consisting of chains of porphyrin units, nature’s light-harvesting systems, bound to nanotubes and the blends were found to exhibit the required electronic alignment for use in OPVs. The work described in this thesis provides an explanation for the poor device behaviour of nanotube–polymer blends to date and, in particular, demonstrates several nanohybrid systems that show particular promise for improved OPV applications.

This thesis presents experimental studies of three different chemical reaction steps relevant for heterogeneous catalysis: dissociation, desorption, and oxidation. CO on single-crystal metal surfaces was chosen as the model systems. X-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) provide information about the electronic structure, and were performed on CO/Fe to measure both a non-dissociative, and a pre-dissociative state. The measurement on the pre-dissociative state showed a π →  π* excitation, which implies a partly broken internal π bond in the molecule. Ultrafast laser-induced reactions were used to examine the dynamic properties of desorption and oxidation. Here CO/Ru and CO/O/Ru were used as model systems. Desorption of CO from a Ru surface involve both hot electrons and phonons. In the case of CO oxidation from CO/O/Ru a pronounced wavelength dependence of the branching ratio between desorption and oxidation was observed. Excitation with 400 nm showed a factor of 3-4 higher selectivity towards oxidation than 800 nm. This was attributed to coupling to transiently excited, non-thermalized electrons. Finally, by performing optical pump/x-ray probe XAS and XES changes in the electronic structure during the reaction could be followed, both for desorption and oxidation. In the CO/Ru experiment, two different transient excitation paths were observed, one leading to a precursor state, and one where CO moves into a more highly coordinated site. Using selective excitation in XES, these were shown to coexist on the surface. In the oxidation experiment, probing the reacting species located near the transition state region in an associative catalytic surface reaction was demonstrated for the very first time.

Nous avons étudié la cohérence excitonique dans le poly[N- 9’-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2’,1’,3’-benzothiadiazole] (PCDTBT). À l’aide d’un modulateur spatial de lumière, nous avons forgé des impulsions lasers ultracourtes permettant de sonder les cohérences du système. Nous nous sommes concentrés sur les propriétés cohérentes des états excitoniques, soit le singulet et l’état à transfert de charge. Nous avons observé que 35 fs après l’excitation, le singulet et l’état à transfert de charge sont toujours cohérents. Cette cohérence se mesure à l’aide de la visibilité qui est de respectivement environ 10% et 30%. De plus, nous avons démontré que les mécanismes permettant de générer du photocourant dans de tels dispositifs photovoltaïques ne sont déjà plus cohérents après 35 fs. Ces mesures révèlent une visibilité inférieure à 3%, ce qui est en deçà de la précision de nos instruments. Nous concluons donc que les états à transfert de charge ne sont pas les états précurseurs à la génération de photocourant, car ceux-ci se comportent très différemment dans les mesures de cohérences. / We have studied the excitonic coherences in poly[N- 9’-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2’,1’,3’-benzothiadiazole] (PCDTBT). Using a spacial light modulator, we shaped ultrafast laser pulses allowing us to probe the system’s coherences. We focused our strength on studying coherences of excitonic states : the singlet and the charge transfer state. We have observed that that 35 fs after excitation, these states are still coherent. This coherence is measured via the visibility that is of abot 10% and 30% respectively. Furthermore, we have shown that the coherence in states generating photocurrent has already vanished after 35 fs. Our data reveals a visibility smaller than 3%, which is lower our instruments resolution. We therefore conclude that the charge transfer states are not precursor states to photocurrent generation, because of their different visibility behavior.

In this work we study thin epilayers of new antiferromagnetic semimetal CuMnAs by time- resolved magneto-optical experiments. In 10 nm layers of CuMnAs, we observed a harmonic dependence of the dynamical magneto-optical signal on the orientation of probe pulse linear polarization. This shows that in this 10 nm layer there is an in-plane uniaxial magnetic anisotropy which can be detected due to a quadratic magneto-optical effect - magnetic linear dichroism. From the measured data we also estimated the Néel temperature and the spectral variation of the magneto-optical coefficient describing the magnitude of the magnetic linear dichroism in this sample.

Doctor of Philosophy / Department of Physics / Itzik Ben-Itzhak / One ultimate goal of ultrafast, strong- field laser science is to coherently control chemical reactions. Present laser technology allows for the production of intense (>10[superscript]13 W/cm[superscript]2), ultrashort ( 5 fs), carrier-envelope phase-stabilized pulses. By knowing the electric field waveform, sub-cycle resolution on the order of 100's of attoseconds (1 as=10[superscript]-18 s) can be reached -- the timescale for electron motion. Meanwhile, the laser field strengths are comparable to that which binds electrons to atoms or molecules. In this intense-field ultrashort-pulse regime one can both measure and manipulate dynamics of strong-field, quantum-mechanical processes in atoms and molecules. Despite much progress in the technology, typical durations for which lasers can be reliably locked to a specific carrier-envelope phase ranges from a few minutes to a few hours. Experiments investigating carrier-envelope phase effects that have necessarily long data acquisition times, such as those requiring coincidence between fragments originating from the same atom or molecule, are thus challenging and uncommon. Therefore, we combined the new technology for measuring the carrier-envelope phase of each and every laser shot with other single-shot coincidence three-dimensional momentum imaging techniques to alleviate the need for carrier-envelope phase stabilized laser pulses. Using phase-tagged coincidence techniques, several targets and laser-induced processes were studied. One particular highlight uses this method to study the recollision process of non-sequential double ionization of argon. By measuring the momentum of the two electrons emitted in the process, we could study their energy sharing. Furthermore, by selecting certain carrier-envelope phase values, and therefore laser pulses with a particular waveform, events with single recollision could be isolated and further analyzed. Another highlight is our studies of carrier-envelope phase effects in the dissociation of the benchmark H[subscript]2[superscript[+] ion beam. Aided by near-exact quantum mechanical calculations, we could identify interfering pathways which lead to the observed spatial asymmetry. These and other similar experiments are described in this thesis as significant steps toward their ultimate control.

Doctor of Philosophy / Department of Physics / Chii-Dong Lin / Imaging of molecules has always occupied an essential role in physical, chemical and biological sciences. X-ray and electron diffraction methods routinely achieve sub-angstrom spatial resolutions but are limited to probing dynamical timescales longer than a picosecond. With the advent of femtosecond intense lasers, a new imaging paradigm emerges in last decade based on laser-induced electron diffraction (LIED). It has been placed on a firm foundation by the quantitative rescattering theory, which established that large-angle e-ion elastic differential cross sections (DCS) can be retrieved from the LIED spectrum. We further demonstrate that atomic potentials can be accurately retrieved from those extracted DCSs at energies from a few to several tens of electron volts. Extending to molecules, we show mid-infrared (mid-IR) lasers are crucial to generate high-energy electron wavepackets (> 100 eV) to resolve the atomic positions in a molecule. These laser-driven 100 eV electrons can incur core-penetrating collisions where the momentum transfer is comparable to those attained in conventional keV electron diffraction. Thus a simple independent atom model (IAM), which has been widely used in conventional electron diffractions, may apply for LIED. We theoretically examine and validate the applicability of IAM for electron energies above 100 eV using e-molecule large-angle collision data obtained in conventional experiments, demonstrating its resolving powers for bond lengths about 0.05 angstrom. The Validity of IAM is also checked by an experimental LIED investigation of rare gas atoms in the mid-IR regime. We show that the electron’s high energy promotes core-penetrating collisions at large scattering angles, where the e-atom interaction is dominated by the strong short range atomic-like potential. Finally, we analyze the measured LIED spectrum of N[subscript]2 and O[subscript]2 at three mid-IR wavelengths (1.7, 2.0, and 2.3 μm). As expected, the retrieved bond lengths of N[subscript]2 at three wavelengths are about same as the equilibrium N[subscript]2 bond length. For O[subscript]2, the data is also consistent with a bond length contraction of 0.1 angstrom within 4-6 fs after tunnel ionization. This investigation establishes a foundation for this novel imaging method for spatiotemporal imaging of gas-phase molecules at the atomic scale.

Cette thèse présente l'étude de la coexistence métal isolant dans deux systèmes très différents pour la communauté scientifique de la matière condensée : l'isolant topologique 3D Bi2Te3 et le composé prototype de la transition de Mott V2O3. Ces deux systèmes ont été étudiés par des techniques basées sur la spectroscopie de photoélectrons. La première technique utilisée est le TR-ARPES (time and angle resolved photoemission spectroscopy), avec une résolution temporelle de 80 fs, appliquée à l'isolant topologique 3D Bi2Te3 pour distinguer la dynamique ultra-rapide des états métalliques de la surface de celle des états isolants du volume. Cette mesure a permet de comprendre les différents mécanismes de diffusion entre la surface et le volume, ainsi que l'amélioration de la relaxation du cône de Dirac par la préexistence à la sous-surface d'une bande de flexion. La seconde technique utilisée dans cette thèse est le SPEM (scanning photoelectron microscopy), avec une résolution spatiale de 150 nm, permettant d'étudier la coexistence des domaines métalliques et isolants à la transition de Mott dans V2O3 ; cette coexistence a pour origine le caractère 1 er ordre de la transition. La mesure montre une coexistence métal-isolant dans le Cr-dopé : les domaines métalliques sont dus à des centres de nucléations < 150 nm et la forme des domaines est clairement liée à la forme des marches de clivage. / This thesis presents the study of metal-insulator coexistence in two very different systems for the scientific community of condensed matter: the 3D topological insulator Bi2Te3 and the prototype compound of the Mott transition V2O3. Both systems were studied by techniques based on photoelectron spectroscopy. The first technique is the TR- ARPES (time and angle resolved photoemission spectroscopy), with a temporal resolution of 80 fs, applied to the 3D topological insulator Bi2Te3 to distinguish the ultrafast dynamics of metallic surface states from that of the insulating bulk states. This allows us to understand the different mechanisms of scattering between the surface and the bulk, as well as the amelioration on the Dirac cone relaxation due to the preexistence of subsurface band bending. The second technique used in this thesis is the SPEM (scanning photoelectron microscopy), with a spatial resolution of 150 nm, which was used to study the coexistence of metallic and insulating domains at the Mott transition on V2O3. This coexistence takes its origin from the first order character of the transition. The measurement shows the metal-insulator coexistence on the Cr-doped: metal domains are due to nucleation centers < 150 nm and the shape of the domains is clearly linked to the shape of the cleavage steps.

Die Untersuchungen der nichtskalierbaren Eigenschaften von Clustern in dem Größenregime, in dem jedes Atom zählt, zeigten, daß hier neuartige Phänomene und Funktionalität entstehen können. Dadurch motiviert wurden in dieser Arbeit: i) strukturelle und elektronische Eigenschaften sowie die Reaktivität von Metall Clustern, ii) stationäre optische Eigenschaften und iii) zeitabhängige Eigenschaften und optimale Kontrolle von ultraschnellen Prozessen in Edelmetallcluster und in nonstoichiometrischen Natrium-Fluorid Cluster, untersucht. / The study of the nonscalable properties of clusters in the size regime in which each atom counts have shown that fully new phenomena and striking new unexpected properties of small clusters can emerge. In this work three aspects have been addressed: i) the structural and electronic properties and reactivity of metal clusters, ii) stationary optical propertis and iii) real time investigation and control of ultrafast processes in noble metal and in nonstoichiometric sodium fluoride clusters.