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Colloidal II-VI quantum dots (QD) possess tunable optoelectronic properties that make them well-suited in applications as luminescent materials. However, their performance is often limited by nonradiative exciton quenching processes typically involving interaction of the exciton with surface states, crystalline defects, or other charge carriers. It is the purpose of this dissertation to identify and understand these decay mechanisms at a fundamental level. Two of the dominant processes, Auger recombination (AR) and hole trapping (HT), are explored from several angles. Biexciton lifetimes of alloyed Cd1-xZnxSe go through a minimum at small mole fractions of Cd (x = 0.1 – 0.4), and this behavior is attributed to partial hole localization in Cd-rich regions of the particle. For small x, the biexciton times are nonexponential, whereas the pure materials and alloys with x ≥ ~ 0.5 show single exponential decays. A model of the alloy valence band potential is used to approximate hole density gradients (taken to be proportional to AR rates) for different alloy compositions, and the model reproduces both the faster and broader distribution of rates in the small Cd concentration regime. The positive trion excitonic state is selectively prepared in CdSe and CdSeCdS core/shell particles by manipulating the surface chemistry. Comparison of the TA and PL decays shows that the trion lowest exciton bleach magnitude is ~ 1.5x greater than the exciton on account of the differences in fine structure. Stimulated emission is increased because all of the trion fine structure levels have fully allowed transitions to the ground state, while the trion excited state absorbance is weaker since the additional hole decreases the degeneracy of the transitions between the first and second multiexciton states, i.e. valence band state filling. Trion PL is blueshifted ~ 6 nm from the excitonic PL in the core/shells and ~ 0 - 1 nm in the cores, closely matching the sum of the additional e-h stabilization and h-h repulsion terms calculated in the effective mass approximation using first-order perturbation theory. The effects of Ag doping on the photophysical properties of CdSe and CdSe/CdS quantum dots (QDs) are studied in terms of surface composition. TBP-ligated samples that suffer from hole trapping show a rise then fall of PLQY with increasing [Ag+], while higher QY particles passivated with TBP and amine or a thin CdS shell decrease upon doping. The initial increase is assigned to the passivation of pre-existing surface hole traps by interaction of interstitial Ag+ with surface Se2- ions, while the subsequent decrease is due to the introduction of substitutional Ag+, which act as a new source of hole traps. Well-passivated samples don’t have high energy filled surface states, so the dopants remain in the +1 oxidation state, act only as hole traps, and lower QYs. Both CdSe and CdSeCdS particles ligated with amine but no phosphine have Fermi levels within kT of the VB, giving rise to the positive trion through thermal population of surface states. In this case, some fraction of the Ag+ is reduced to Ag by the amine, which then shuts off trion Auger recombination by interacting with empty Se(0) orbitals. Ligation of CdSe QDs with alkylamines raises QYs by decreasing the extent of hole trapping. This cannot be explained by a simple MO theory type interaction between the filled Se2- and the N lone pair of the amine, so a new passivation mechanism is proposed that considers the electrostatics of L-type ligand binding. Electron donation by the amine creates a surface dipole layer that raises the VB and CB levels of the particle having little effect on the surface state energetics, thereby lowering -ΔG for hole trapping. The mechanism is corroborated with the observed increase in CB electron transfer rates to methylviologen and decreased hole transfer rates to 4-methylbenzenethiol upon treatment with the amine. Transient absorption (TA) spectra of CdSe nanoplatelets (NPL) are analyzed in terms of the exciton fine structure and population of low-lying lateral excited states. Treatment with an external hole trap decreases the NPL HH bleach, while the analogous spherical particles show no spectral manifestations of hole trapping in TA measurements. The differences are attributed to the strong z-quantum confinement breaking the LH/HH degeneracy as well to the reduced e-h exchange interaction in NPL. Because the X  XX triplet-triplet transition proceeds from the LH and is forbidden in the HH band, splitting off the LH results in reduced PA and increased bleach of the NPL lowest exciton band. The weak e-h exchange interaction allows significant population of the bright state and hence more stimulated emission in NPL compared with QD. The relative bleach magnitudes of the same 4.5 ML nanoplatelets are measured by transient absorption spectroscopy and are found to be much less than those of their spherical counterparts. Analysis of the relative bleach magnitudes (change in optical density at the HH exciton normalized to the absorbance) shows that absorption of a photon bleaches ~ 10-25 % of the GSA, with the relative bleach getting smaller with increasing platelet lateral dimensions. This suggests that the size of the exciton is smaller than the platelet and is independent of the platelet areas. This is an unsurprising result given that the bulk Bohr radii of CdSe excitons is about 5 nm , that the NPL Bohr radii should be significantly less than those seen in bulk due to the nearby layer of organic, low dielectric constant material, and because the platelet dimensions are greater than the Bohr radii. So the lack of significant quantum confinement along x and y causes the excitonic areas to be platelet-size independent. This consideration is taken into account when explaining the reported size independence of the radiative lifetime, which we measure to be about 5 ns for lateral areas between 85 and 320 nm2. Taking into account the singlet-triplet splitting, the less than unity e-h overlap integral, and the fraction of the platelet area occupied by an exciton allows near-quantitative reproduction of the radiative lifetimes using the integrated extinction coefficients as in the Einstein A&B coefficient treatment of radiative rates.

Femtosecond x-ray transient absorption spectroscopy is used in conjunction with selective bond breaking to characterize the electronic structures of substituted radicals. By utilizing the A band photodissociation of iodides, a series of carbon-centered radicals is produced and their frontier molecular orbitals are probed via core-level transitions by x-ray photons produced through high harmonic generation, whose limit has been extended to 390 eV with the apparatus detailed in the second chapter.Two types of substituted radicals are studied in this work to demonstrate substitution effects: halogenated radicals and alkyl substituted radicals. The third chapter focuses on the halogenated radicals, in particular ·CH2Cl and ·CH2Br, which are produced under ultraviolet radiation of CH2ICl and CH2IBr, respectively. The ·CH2Cl radical is probed via the carbon K edge and the chlorine L2,3 edges to reveal the electron withdrawing effect impacting different atoms in the photodissociation of CH2ICl. Core-level transitions in ·CH2Br are studied in comparison to ·CH3 and ·CH2Cl to understand possible stabilization of the frontier molecular orbital by π bonding and the strength of such stabilization. The fourth chapter turns to the alkyl substituted radicals, where the methyl, ethyl, isopropyl and tert-butyl radical are produced from methyl, ethyl, isopropyl and tert-butyl iodide, respectively. While the characteristic carbon 1s→σ*(C-I) transition takes place in the parent iodides at similar energies, the transition from carbon 1s to the frontier singly occupied molecular orbital differs drastically in the radicals. This can be explained by either a significant change in the carbon 1s orbital of the radicals because of methyl substitution or complications due to vibrational excitation. The results point to new directions for core-level theoretical investigations.

The goal of a carbon free H$_{2}$ economy using a photoelectrochemical cell to split water is a worthwhile endeavor to solve the looming energy crisis. Many scientists have taken up this cause and an abundance of studies exists characterizing, optimizing, and creating materials to integrate into a full artificial photosynthetic system. However, despite widespread attention, a viable industrial-scaled photoelectrochemical device has yet to emerge due to low efficiencies, slow kinetics, and high energetic barriers. To that end, going back to the fundamentals may be a necessary step to understand what is causing the bottleneck, particularly within the heterogeneous water oxidation catalysts. Herein, the surface electronic structure of transition metal-based semiconductors and their 3$d$ valence electrons that actively participate in the oxygen evolution process are investigated using a number of spectroscopic techniques in an effort to unravel the mechanism and uncover important material properties. I focus on three major properties: 1) photo-excited carrier dynamics affecting the excited state electronic structure, 2) ground state electronic structure including covalent and atomic parameters, and 3) surface state-mediated interfacial hole transport.First, the transient electronic structure of Co$_{3}$O$_{4}$, a promising water oxidation catalyst, is probed via transient absorption spectroscopy. With selective excitation of key optical transitions, both inter- and intravalence transitions involving the 3$d$ electrons, the kinetics and spectrum are investigated. A wide range of pump and probe wavelengths, spanning the ultraviolet to the visible to the near infrared, are employed. Despite this range of pump and probe energies, the carrier dynamics were largely unaffected. Additionally, the kinetics and spectra show a unique independence to fluence and sample morphology.The kinetics reveal a photo-excited carrier density that quickly thermalizes when excited across the charge transfer transitions and converts into $d-d$ excitations. The recombination from these localized midgap $d$ states occurs at a longer, nanosecond time scale.In addition to perturbing the system via photo-excitation, the electronic structure of Co$_{3}$O$_{4}$ was tuned using an applied potential in a technique called spectroelectrochemistry. Spectroelectrochemistry reinforced the results of the transient absorption spectroscopy and confirmed the identity of the midgap $d$ states through determination of the energetics of these 3$d$ states as well as assignment of whether electrons or holes induced the absorptions or bleaches observed within the transient spectrum.Taken together, the results suggest a special type of intrinsic hole trap center that is a potentially promising long-lived state for utilization in photo-activation. Further, since the photo-excited hole is efficiently localized at these 3$d$ sites, the most likely water oxidation reaction intermediate is an oxidized cobalt center, \emph{i.e.} a Co(IV)=O species.The second investigation probes the ground state 3$d$ electronic structure of active and non-active water oxidation catalysts using X-ray absorption spectroscopy to determine an electronic structure-activity relationship. A set of molecular, homogeneous cobalt polyoxometalates serves as model systems for extracting electronic structure parameters, such as metal valence state (\emph{i.e.} 2+ or 3+), metal coordination environment (\emph{i.e.} tetrahedral or octahedral environments), structural distortions and covalency between the metal and oxygen ligands, from cobalt L-edge spectra. No definitive structure-activity relationship could be established because X-ray absorption spectroscopy could not distinguish between the number of metal atoms within the molecular structure or the identity of the heteroatom surrounding the metal(s) and ligands. These properties are what defines the extent of catalytic activity and point to the importance of using more sensitive techniques such as resonant inelastic X-ray scattering. However, ligand field multiplet theory was able to simulate well the experimental Co L-edge spectra for the well-defined model systems and report the ligand field parameter 10Dq to within an accuracy of $\pm$1~eV and the strength of electron-electron interactions to within $\pm$5$\%$ of atomic values. These parameters were subsequently applied to characterize a lesser-defined heterogeneous sample, a Co$_{3}$O$_{4}$ thin film.Lastly, the final chapter addresses the surface structure dynamics under \emph{in situ} conditions, \emph{i.e.} during the influence of water, on GaN. GaN, a widely studied semiconductor for integration as a photoanode in the water oxidation reaction, possesses a unique surface chemistry and a mobility that is experimentally accessible. Using surface sensitive transient grating spectroscopy, a quantitative value for the hole mobilities of both an undoped and a n-doped GaN film were determined in air and at a semiconductor/electrolyte interface. It was found that interfacial carrier mobility is highly dependent on the surface intermediates. For n-doped GaN, a large density of dark surface states in the form of various adsorbed species exist that can localize holes and, further, allow the holes to hop from metal site to metal site along the surface, possibly assisted by proton-coupled electron transfer with the water molecules. This charge transfer pathway during dark equilibration between the semiconductor electrochemical potential and water oxidation potential more than doubles the interfacial hole diffusivity compared to its value in air. This was confirmed under both acidic and neutral conditions, 0.1~M HBr (pH = 2) and 0.1~M Na$_{2}$SO$_{4}$ (pH = 7). In contrast, the hole diffusivity of undoped GaN with no significant surface state density is unchanged with introduction of an electrolyte solution.The results suggest that charge transport and surface reactivity, which are generally treated independently, are connected phenomena.

Organic semiconductors have the potential to form the basis of the next-generation of electronic devices. Device layers formed from these molecular based materials can be solution processed at room temperature using techniques such as roll-to-roll printing, making them cheaper and more sustainable than inorganic alternatives such as silicon. Furthermore, their properties also enable the development of low-weight and flexible devices to access novel applications. Yet, solution processing also leads to the formation of complex microstructure via self-assembly that strongly impacts the function of such devices. A full understanding of the dependence of device functionality on device structure is needed to direct design of organic semiconductor based devices to drive widespread commercial adoption. The work of this thesis is focused on using time-resolved spectroscopy techniques to study the structure-function relationships of various organic semiconductors as a function of their local environment. Chapters 2-4 focus on using transient absorption microscopy to study structure-function relationships in 6,13-Bis(triisopropylsilylethynyl)pentacene, a model organic semiconductor with applications in organic field-effect transistors and organic solar cells, as a function of the local film microstructure. Chapters 5-6 then turn to a time-resolved fluorescence study of next-generation thermally-activated delayed fluorescence organic light-emitting diodes as a function of the host matrix polarity.Organic electronics are a fascinating field of study due to the intertwined relationship between the chaotic self-assembly of the active layers of these devices and their resulting functionality. Molecular design of the building blocks of these films affects the ensemble electronic properties of the entire film not only through the molecular excited states but also by how those molecular electronic states couple together. Efforts to correlate the impact of the local physical structure of samples on the resulting local electronic structure will inform future design decisions in the organic electronics community.

With the ability to disentangle electronic transitions that occur on different elements and local electronic structures, time-resolved extreme ultraviolet (XUV) spectroscopy has emerged as a powerful tool for studying ultrafast dynamics in condensed phase systems. In this dissertation, a visible-pump/XUV-probe transient absorption apparatus with femtosecond resolution was constructed to investigate the carrier relaxation dynamics in semiconductors after photo-excitation. This includes timescales for carrier thermalization by carrier-carrier and carrier-phonon scattering. The 30 – 72 eV photon energy coverage (17 – 40 nm wavelength) generated by a table-top XUV light source is suitable for probing the 3p-to-3d core level absorptions of various transition metal oxides (TMOs) with specificities to elements and oxidation states.In Chapter 1, a brief introduction to charge carrier dynamics in semiconductor-based materials is given. In addition, fundamentals of core-level spectroscopy and the high harmonic generation (HHG) process are also addressed in this introductory chapter. Specifications of the experimental apparatus that was constructed are summarized in Chapter 2, including the design concepts and characterization of performance. Chapter 3 presents the spectral tunability of the XUV pulses generated from a semi-infinite gas cell (SIGC), as well as the data acquisition procedures. Charge carrier relaxation dynamics in Co3O4 following the charge transfer excitation pathway at 400 nm are documented in Chapter 4. In Chapter 5, various visible pump wavelengths are used to excite Co3O4 and the differences in the carrier dynamics versus excitation wavelength are considered. After selectively photoexciting a Si/TiO2 heterojunction, the resulted electron transfer process is observed and reported in Chapter 6. The concluding remarks of the dissertation are made in Chapter 7, while several ongoing time-resolved experiments are addressed in the Appendix sections.

Isolated attosecond pulses with photon energies in the extreme ultraviolet have provided a new capability to study few-femtosecond and sub-femtosecond dynamics in atomic and molecular systems. A novel regime of attosecond transient absorption measurements, in which the near-infrared pulse follows the attosecond pulse, is reviewed. The observed timescales of the decay of an absorption feature and oscillations due to quantum beating are derived and experimental considerations for these measurements are discussed. Attosecond transient absorption spectroscopy is then applied to observe quantum beating between the 2s22p5(2P3/2)3d and 2s22p5(2P1/2)3d electronically excited states of neon. The quantum beating is observed more prominently in one of the absorption features, and theoretical models of the effect of the near-infrared pulse are proposed to explain this asymmetry and characterize the observed beating. In the model that most closely agrees with the measurement, the beating is detected via Rabi cycling induced by the near-infrared pulse through an intermediate state (likely one of the 3p states), and this mechanism is shown to be strongly dependent on the spectrum of the near-infrared pulse. The isolated attosecond pulses used in these experiments are characterized using the photoelectron streaking method and the duration of these pulses is measured by iteratively reconstructing the streaking spectrograms. The effect on the streaking spectrogram of various pulse characteristics, such as duration, chirp, and the presence of satellite pulses, is described. Streaking spectrograms using low photon energy attosecond pulses are typically asymmetric. The reasons for this asymmetry are discussed and the challenges of reconstructing these low energy pulses are addressed. Finally, attosecond pulses are used to investigate superexcited states, or doubly excited states above the ionization potential, of the nitrogen molecule (N2). These states can decay via two competing channels, autoionization and predissociation. Time-of-flight mass spectrometry is applied to measure lifetimes of these states. Preliminary results are presented and improvements to the experimental design are proposed.

Ultrashort pulses with durations less than one femtosecond advance understanding of chemistry and physics by facilitating time-resolved measurements on the timescale of electronic motion. A very pragmatic type of experiment incorporating isolated attosecond (10&minus18 s) pulses is transient absorption measurements that utilize the broad bandwidth of attosecond pulses. In these measurements the dispersed spectrum of the isolated attosecond pulse is recorded after the attosecond pulse and a time-delayed optical pulse pass through an experimental target of interest. This thesis focuses on the development of an attosecond transient absorption spectrometer with the ultimate goal of using the spectrometer to measure atomic and molecular dynamics. Two experimental studies are presented: one focusing on the behavior of helium atoms bathed in a strong near-infrared optical field, and one focusing on measurement of the lifetimes of autoionizing states in xenon. Measurement of the absorption spectrum of atomic helium bathed in a near-infrared (780 nm) field exhibits new features when the attosecond pulse and the optical pulse are overlapped in the gas target. The features near the 1s2p transition at 21.21 eV are identified as intermediates in two-photon transitions to reach nearby s and d helium excited states that are inaccessible via one photon transitions from the 1s2 ground state of He. Initial measurements identified three intermediate states, and further experiments investigated the behavior of the states as the near infrared field intensity is increased. Attosecond time-resolved measurement of ultrashort lifetimes of autoionizing states is also investigated. The 5s5p66p and 5s5p67p autoionizing states of xenon are visible in the extreme-ultraviolet absorption spectrum of Xe as Fano resonances. The Fano resonances are suppressed by a time-delayed near-infrared pulse and recover with a time-constant that is related to the lifetime of the state. The studies here identified key parameters for time-resolved measurements of the autoionization lifetime, including spectral resolution of the attosecond transient absorption spectrometer.

Ultrafast femtosecond (10-15 s) dynamics of atomic xenon, vinyl bromide and carbon disulfide molecules are studied using a newly developed method of extreme ultraviolet (XUV) transient absorption spectroscopy. This research work is devoted to a deeper understanding of the fundamental electronic and nuclear dynamics using XUV light pulses from a high-order harmonic generation source. The produced XUV light pulses are utilized to selectively probe the chemical reaction coordinate with femtosecond temporal resolution. The experimental apparatus for transient absorption pump-probe spectroscopy is described in detail. The research described in this thesis contains four different gaseous atomic and molecular systems. The first system in the study is motivated by a goal to gain a better understanding of the core-excited state couplings of atomic xenon near zero delay between an intense NIR pump pulse (780 nm) and an XUV probe pulse. Secondly, the ionization and dissociation dynamics of molecular vinyl bromide (C2H3Br) under the influence of strong-field ionization are investigated. Finally, an ongoing research project of CS2 and thiophene molecules is presented for future studies of spin-orbit wavepacket and ring-opening dynamics, respectively.The NIR induced core-excited state coupling of atomic xenon is studied using femtosecond XUV transient absorption spectroscopy with photon energies between 50 eV to 70 eV. Coupling of the core-excited states 4d-1(2D5/2)6p(2P3/2) (65.1 eV) and 4d-1(2D3/2)6p(2P1/2) (67.0 eV) to neighboring states by the NIR field results in a threefold enhancement of XUV transmission. The induced transmission at 65.1 eV (67.0 eV) changes from 3.2 ± 0.4% (5.9 ± 0.5%) without the coupling laser to 9 ± 2% (22 ± 5%) at the maximum of the NIR field. A NIR field induced broad XUV absorption feature ranging from 60 eV to 65 eV is explained by the splitting of the field free absorption lines into multiplets when the Rabi frequencies of the coupling transitions higher than the NIR frequency. This assignment is supported by a numerical integration of the von Neumann equation for a few level quantum system.The dissociative ionization dynamics of vinyl bromide, C2H3Br, initiated by a strong laser field ionization are investigated. XUV light pulses with photon energy between 50 eV and 72 eV are utilized to detect the subsequent dynamics. Several dynamic features are observed including the neutral C2H3Br depletion, the formation of C2H3Br+ ions (X and A states), the production of C2H3Br++ dications, and the emergence of neutral Br (2P3/2) atoms from dissociative ionization. Free Br (2P3/2) atoms appear on a timescale of 330±150 fs. The singly charged ionic A state displays a time-dependent XUV absorption energy shift of ~0.4 eV during the first 300 fs after strong-filed ionization. The signal intensity from Br atoms correlates with the signal intensity from singly charged parent ions in the A state as a function of NIR laser peak intensity. The experimental observations suggest that vibrationally excited C2H3Br+ (A) ions possibly undergo ultrafast intramolecular vibrational energy redistribution concurrent with the C-Br bond dissociation within a time scale of 330±150 fs. The C2H3Br+ (X) and C2H3Br++ ions are relatively stable as a consequence of deeper potential wells and a high dissociation barrier, respectively.Two ongoing experiments of sulfur-containing molecules are presented that are aimed at future studies of a molecular spin-orbit wavepacket in CS2+ ions and ultrafast ring-opening dynamics of thiophene. Strong-field ionization can coherently populate two spin-orbit states in CS2+ ions. The spin-orbit splitting originates from the atomic sulfur (~60 meV). The small splitting offers the possibility to probe a coherent beating on a time scale of 69 fs, well beyond our temporal resolution of 25 fs. For thiophene, an ultrafast ring-opening process initiated by one-photon excitation at 193 nm is studied through multiphoton ionization at 780 nm. The parent ion population exhibits a fast decay on a time scale of 200±30 fs. This offers a reference for the future XUV transient absorption experiments using one-photon excitation at 193 nm and sulfur (2p) L-edge detection at 165 eV.

Femtosecond time-resolved extreme-ultraviolet core-level absorption spectroscopy has developed into a powerful tool for investigating chemical dynamics due to its sensitivity for detecting changes in electronic structure. By probing the core-levels of atoms and molecules, dynamics may be monitored with elemental specificity, as well as localized sensitivity to the oxidation state around the atomic absorber. Previous experiments with this technique demonstrated the capability to quantitatively resolve strong-field ionization dynamics including the complete |j,m> quantum state distribution of Xe+ and the dissociative ionization of CH2Br2, both monitored in real-time with femtosecond resolution. In this thesis, the complete reconditioning of the experimental apparatus is described and its application to the study of vibrational wave packets created by strong-field ionization of Br2 is discussed. Initially, the experimental apparatus was meant as a proof of principle demonstration for table-top extreme ultraviolet transient absorption spectroscopy based on a high-order harmonic generation source. To achieve the experiments discussed here, substantial changes were required to its construction concerning the high harmonic source and overall stability of the apparatus, which are discussed in terms of increasing the operation of the experiment to several days as opposed to a few hours, yielding an unprecedented spectral resolution after Fourier Transform of the signal versus pump-probe time delay for a table-top core-level spectrometer, better than 1.5 cm-1. The improvements to the apparatus are exploited in terms of examining the vibrational wave packet dynamics in Br2 induced by optical strong-field ionization.First, extreme-ultraviolet transient absorption is established as a bond-length specific probe capable of resolving vibrational motion in Br2 and due to a change in the transition amplitude and energy versus bond length of the 3d5/2,3/2 core-level transitions to unoccupied molecular orbitals. Furthermore, the degree of coherence and beat composition of the 1&Sigma+ g ground state vibrational wave packet is established. Second, the preparation of vibrational wave packets by strong-field ionization of Br2 is analyzed as a function of field intensity. At a field intensity near the threshold of ionization, selective depletion of the vibrational amplitude is observed near the inner turning point, corresponding to the minimum in the ionization energy for the unperturbed molecular potential energy curves. The observed superposition is determined to be primarily composed of the &nu0&nu1 vibrational beat. As the field intensity is increased, perturbation of the potential curves results in preparation of the wave packet at longer internuclear separations, and the distortion of the potentials is directly observed by a shift in the core-level transition energy within the intense laser pulse. Moreover, the composition of the wave packet is found to involve higher vibrational levels, indicating the involvement of a Raman pumping mechanism in the preparation of the superposition. Third, several ongoing experiments utilize the unique instrumental capabilities including the elemental specificity and polarization dependence of the absorption. The elemental specificity and nuclear motion dependence of the core-level probe is utilized to investigate the correlated detection of vibrational wave packet dynamics in IBr on the iodine 4d and bromine 3d core-level absorptions simultaneously. Also, polarization dependent absorption of Xe2+ looks to extend the quantitative measurements conducted for Xe+ to determine the complete |j,m> state populations after sequential ionization. Lastly, polarization dependent measurements of the strong-field ionization of Br2 suggest alignment between the molecular axis and the electric field of the ionizing pump at field intensities near the threshold for ionization, which is lost at higher field strengths. These studies expand the repertoire of capabilities for high-order harmonic generation beyond the proof of principle experiments and establish the technique as a practical tool for monitoring complex, ultrafast molecular dynamics by core-level probing.The capabilities of femtosecond transient absorption have since been expanded here to include the investigation of strong-field generated vibrational wave packets in molecular bromine. To this end, a theoretical model for determining the sensitivity of XUV absorption to valence vibrational motion generated by strong-field ionization was developed utilizing Restricted Excitation Window Time-Dependent Density Functional Theory coupled with internuclear separation, vibrational state sensitive Ammosov-Delone-Krainov tunnel ionization rates. This theoretical model, including details of traditional Ammosov-Delone-Krainov tunnel ionization rates, is presented in Chapter 2. Chapter 3 describes the experimental changes incorporated into the apparatus for successful multi-day experiments for the determination of vibrational dynamics with ~1 cm-1 spectral resolution. Demonstration of the sensitivity of XUV transient absorption to non-stationary ground state vibrational dynamics is then presented in Chapter 4, investigating the vibrational superposition generated in Br2 after irradiation with a 1.6 x 1014 W/cm2 driving pump pulse. Chapter 5 expounds upon the dependence on field intensity to the observed vibrational dynamics in Br2 as well as Br2+. Chapter 6 discusses the currently ongoing experiments being conducted for ground state vibrational wave packet preparation in iodine monobromide, the determination of the complete |j,m> quantum state population of Xe2+, and the alignment of the molecular axis of Br2 with the electric field of the ionizing 800 nm field following strong-field ionization. Chapter 6 also presents the conclusions of the experimental work presented here as well as the future outlook of XUV transient absorption spectroscopy in the broader perspective of investigating chemical reaction and dynamics.

Singlet fission (SF) is a process in organic molecules that is able to create two triplet excitons from one initially excited singlet exciton. Beyond its interest as a unique photophysical process, SF is also worth studying due to its capability to enhance the overall efficiencies of photovoltaic cells beyond the Shockley-Queisser limit. In order to take advantage of this process, a more thorough understanding of how it occurs and how it can be enhanced is needed. Specifically, this work begins to answer some of the larger questions about SF within tetracene. SF was first observed in anthracene in 1965, and has since been found to occur in larger polyacenes and other conjugated organic molecules. This study focuses on crystalline tetracene for several reasons, most notably for its favorable energy levels for the study of the interaction between the singlet and triplet excitons, greater chemical stability, and large body of previous work. In this work, the electronic states of tetracene are characterized with time resolved photoluminescence and transient absorption spectroscopies in order to determine the rate and mechanism of SF within crystalline tetracene, which have not been conclusively shown in prior studies. Using spectroscopy measurements, this work has determined that SF is occurring within crystalline tetracene with a formation time of 80±3 ps, which leads to a triplet yield of 200%. Additionally, the study of the temperature dependence of crystalline tetracene photoluminescence has shown that the fission of a singlet into two triplets may not be thermally activated, a finding which is unexpected based on previous work. Furthermore, observation of quantum beats within the delayed fluorescence of single crystals of tetracene strongly suggests that the singlet fissions directly into two triplets rather than proceeding through a charge-transfer intermediate. Overall, this work has shown that SF occurs with a high efficiency within tetracene, and that tetracene is a viable candidate for use as a SF sensitizer within a photovoltaic cell.

© 2015 IOP Publishing Ltd. Attosecond transient absorption spectra near the energies of autoionizing states are analyzed in terms of the photon coupling mechanisms to other states. In a recent experiment, the autoionization lifetimes of highly excited states of xenon were determined and compared to a simple expression based on a model of how quantum coherence determines the decay of a metastable state in the transient absorption spectrum. Here it is shown that this procedure for extracting lifetimes is more general and can be used in cases involving either resonant or nonresonant coupling of the attosecond-probed autoionizing state to either continua or discrete states by a time-delayed near infrared (NIR) pulse. The fits of theoretically simulated absorption signals for the 6p resonance in xenon (lifetime = 21.1 fs) to this expression yield the correct decay constant for all the coupling mechanisms considered, properly recovering the time signature of twice the autoionization lifetime due to the coherent nature of the transient absorption experiment. To distinguish between these two coupling cases, the characteristic dependencies of the transient absorption signals on both the photon energy and time delay are investigated. Additional oscillations versus delay-time in the measured spectrum are shown and quantum beat analysis is used to pinpoint the major photon-coupling mechanism induced by the NIR pulse in the current xenon experiment: the NIR pulse resonantly couples the attosecond-probed state, 6p, to an intermediate 8s (at 22.563 eV), and this 8s state is also coupled to a neighboring state (at 20.808 eV).