Structural dynamics by ultrafast electron crystallography

In order to investigate the response of materials to extreme conditions, there are several approaches to depositing extremely high concentrations of energy into very small volumes of material, including ultrafast laser and swift heavy ion (SHI) irradiation. In this study, crystalline-to-crystalline phase transformations in cubic Gd2O3 and monoclinic ZrO2 have been investigated using ultrafast laser irradiation. The phases produced by the extreme conditions of irradiation were characterized by grazing incidence x-ray diffraction (GIXRD) and Raman spectroscopy. Gd2O3 exhibited a cubic-to-monoclinic phase transformation, as evidenced by the appearance of the monoclinic (402¯), (003), (310), and (112¯) peaks in the GIXRD pattern and of four Ag and three Bg Raman modes. ZrO2 underwent a monoclinic-to-tetragonal phase transformation, as evidenced by the emergence of the tetragonal (101) peak in the GIXRD pattern and of Eg and A1g Raman modes. The new phases formed by ultrafast laser irradiation are high temperature polymorphs of the two materials. No evidence of amorphization was seen in the GIXRD data, though Raman spectroscopy indicated point defect accumulation. These results are identical to those produced by irradiation with SHIs, which also deposit energy in materials primarily through electronic excitation. The similarity in damage process and material response between ultrafast laser and SHI irradiation suggests a fundamental relationship between these two techniques.

Organic photovoltaics (OPV) have the potential to provide low-cost solar-to-electricity converting devices. Improving such devices requires a deeper understanding of the ultrafast photoinduced processes occurring after light absorption. We have performed femtosecond (fs) transient absorption spectroscopy, as well as time-resolved electroabsorption (Stark effect) on model OPV systems to follow electron and hole transfer dynamics, as well as charge carrier motion in the active layer to the electrodes. Solid-state cyanine borate (Cy3-B) films undergo intra-ion pair reductive quenching on the picosecond (ps) timescale. We have found that when intermixing Cy3-B with a fullerene acceptor (PCBM), photoexcitation leads to the appearance of oxidized Cy3 in less than 70 fs. We correlated the rate and yield of Cy3 oxidation to the PCBM loading. We then investigated a cyanine Cy3-P in a bilayer geometry with fullerene C60. In this case, ultrafast electron transfer in less than 100 fs is followed by slower dissociation of interfacial charge transfer states in the presence of an electric field. A photoinduced Stark effect observed in neat C60 enabled to identify an initially delocalized excited state, which localizes in 360 fs. Lastly, we analyzed how microstructural changes in polymer pBTTT:PCBM blends impact free charge carrier formation and motion. We identified that the intermixed phase efficiently produces charge carriers within 100 fs, and that pure fullerene and pBTTT domains attract the charge carriers separating them further apart. This is ascribed to an energy cascade toward the neat domains. This thesis reveals ultrafast charge transfer at donor-acceptor interfaces in such OPV systems by means of fs transient absorption spectroscopy. Results emphasize that a second step is needed for charge separation in order to successfully compete with charge pair recombination in these systems. An electric field applied can act as a driving force to dissociate interfacial charge transfer states. Pure donor or acceptor domains attract holes or electrons, respectively, and reduce geminate charge recombination in blends. These second steps in charge separation were derived from photoinduced electroabsorption dynamics in transient absorption measurements, as well as time-resolved electroabsorption based on the Stark effect.

Interfacial water has unique properties in various functions. Here, using 4-dimensional (4D), ultrafast electron crystallography with atomic-scale spatial and temporal resolution, we report study of structure and dynamics of interfacial water assembly on a hydrophobic surface. Structurally, vertically stacked bilayers on highly oriented pyrolytic graphite surface were determined to be ordered, contrary to the expectation that the strong hydrogen bonding of water on hydrophobic surfaces would dominate with suppressed interfacial order. Because of its terrace morphology, graphite plays the role of a template. The dynamics is also surprising. After the excitation of graphite by an ultrafast infrared pulse, the interfacial ice structure undergoes nonequilibrium "phase transformation" identified in the hydrogen-bond network through the observation of structural isosbestic point. We provide the time scales involved, the nature of ice-graphite structural dynamics, and relevance to properties related to confined water.

The static structure of macromolecular assemblies can be mapped out with atomic-scale resolution by using electron diffraction and microscopy of crystals. For transient nonequilibrium structures, which are critical to the understanding of dynamics and mechanisms, both spatial and temporal resolutions are required; the shortest scales of length (0.1-1 nm) and time (10(-13) to 10(-12) s) represent the quantum limit, the nonstatistical regime of rates. Here, we report the development of ultrafast electron crystallography for direct determination of structures with submonolayer sensitivity. In these experiments, we use crystalline silicon as a template for different adsorbates: hydrogen, chlorine, and trifluoroiodomethane. We observe the coherent restructuring of the surface layers with subangstrom displacement of atoms after the ultrafast heat impulse. This nonequilibrium dynamics, which is monitored in steps of 2 ps (total change </=10 ps), contrasts that of the nanometer substrate. The effect of adsorbates and the phase transition at higher fluences were also studied through the evolution of streaks of interferences, Bragg spots (and their rocking curves), and rings in the diffraction patterns. We compare these results with kinematical theory and those of x-ray diffraction developed to study bulk behaviors. The sensitivity achieved here, with the 6 orders of magnitude larger cross section than x-ray diffraction, and with the capabilities of combined spatial ( approximately 0.01 A) and temporal (300-600 fs) resolutions, promise diverse applications for this ultrafast electron crystallography tabletop methodology.

The Time Dependent Schrödinger Equation In A Strong Laser Field

Biological hard tissues, such as those found in bone and teeth, are complex tissues that build a strong mineral structure over an organic matrix framework. The laser-matter interaction for bone hard tissues holds great interest to laser surgery and laser dentistry; the use of short/ultrashort pulses, in particular, shows interesting behaviors not seen in continuous wave lasers. High laser energy densities in ultrashort pulses can be focused on a small irradiated surface (spot diameter is 10-50 &#956;m) leading to rapid temperature rise and thermal ablation of the bone tissue. Ultrashort pulses, specifically those in the picosecond and femtosecond ranges, impose several challenges in modeling bone tissue response. In the present paper we perform time-dependent thermal simulations of short and ultrashort pulse laser-bone interactions in singlepulse and multipulse (set of ultrashort pulses) modes of laser heating. A comparative analysis for both radiation modes is discussed for laser heating of different types of the solid bone on the nanosecond, picosecond and femtosecond time scales. It is shown that ultrashort laser pulses with high energy densities can ablate bone tissue without heating tissues bordering the ablation creator. This reaction is particularly desirable as heat accumulation and thermal damage are the main factors affecting tissue regrowth rates, and thus patient recovery times.

Protein surface hydration is fundamental to its structure, flexibility, dynamics, and function, but it has been challenging to disentangle their ultimate relationships. Here, we report our systematic characterization of hydration dynamics around a β-barrel protein, rat liver fatty acid-binding protein (rLFABP), to reveal the effect of different protein secondary structures on hydration water. We employed a tryptophan scan to the protein surface one at a time and examined a total of 17 different sites. We observed three types of hydration water relaxation with distinct time scales, from hundreds of femtoseconds to a hundred picoseconds. We also examined the anisotropy dynamics of the corresponding tryptophan side chains and observed two distinct relaxations from tens to hundreds of picoseconds. Integrating our previous findings on α-helical proteins, we conclude the following: (1) The hydration dynamics is highly heterogeneous around the protein surface of both α-helical and β-sheet proteins. The outer-layer water of the hydration shell behave like a bulk and relaxes in hundreds of femtoseconds. The inner-layer water collectively relaxes in two time scales, reorientation motions in a few picoseconds and network restructuring in tens to a hundred picoseconds. (2) The hydration dynamics are always faster than local protein relaxations and in fact drive the protein fluctuations on the picosecond time scale. (3) The hydration dynamics in general are more retarded around β-sheet structures than α-helical motifs. A thicker hydration shell and a more rigid interfacial hydration network are observed in the β-sheet protein. Overall, these findings elucidate the intimate relationship between water-protein interactions and dynamics on the ultrafast time scale.

High-resolution structures of membrane and soluble proteins can be obtained by cryo-electron crystallography, given highly-ordered cryo-preparations of perfectly flat 2D crystals are available. Studies of membrane proteins, which are embedded into a lipid membrane, mimicking the native cell membrane, are of particular biological interest. However there are multiple reasons why electron crystallography is far from being a mainstream protein structure determination technique. In this thesis we address three major difficulties of electron crystallography: (i) resolution loss due to not perfectly flat crystals, (ii) reliable high-throughput automatic image processing and (iii) correction of electron beam-induced motion of the sample.&#13;\n&#13;\nThe conventional electron crystallography image processing procedure assumes perfectly flat 2D crystals, which are almost impossible to obtain. Our new processing approach, described in Chapter 2, weakens this assumption tremendously. Traditional processing assigns the same tilt geometry to all proteins of one 2D crystal. Thus local tilt geometry variations, due to not perfectly flat crystals, are neglected. We developed an algorithm that optimizes the tilt geometry of each protein separately, while exploiting the correlation between neighboring proteins. The new method proves the feasibility of this approach, improves the achieved resolution and opens the doors to new studies, i.e. structural studies of membrane proteins embedded into lipid vesicles.&#13;\n&#13;\nRecently a new generation of digital detectors tremendously changed the cryo-electron microscopy field. Beside a significantly increased signal-to-noise ratio these detectors record dose-fractionated movies of the sample under the electron beam. Previous cameras only recorded one image instead. This new exposure mode enables the computational correction for beam-induced sample movements. In Chapter 3 we describe a new “real-time” automation pipeline for electron crystallography using direct electron detectors. The novel pipeline automatically corrects for homogeneous sample drift on frame level and processes the acquired images automatically. Both, the time-to-solution and the quality of the obtained 3D reconstructions are significantly improved.&#13;\n&#13;\nHeterogeneous beam-induced sample movements are the most severe resolution- limiting factor in modern cryo-electron crystallography. In Chapter 4 we present an algorithm, termed movie-mode unbending, which corrects for inhomogeneous beam- induced sample drift. In contrast to the previous homogeneous drift-correction, the novel algorithm can correct for locally varying beam-induced sample motion. This novel approach significantly increases the resolution of electron cryo-crystallographic studies recorded on the latest detectors.&#13;\n&#13;\nChapter 5 covers multiple successful applications of the methods introduced above. For instance the real-time drift-correction module of the developed automation pipeline was used for multiple near-atomic resolution single particle and helical image processing projects. The high-throughput crystal image processing was applied to different kinds of 2D crystals and enabled the qualitative assessment of different sample preparation methods. Additionally we describe the implementation of a high-throughput single particle automation pipeline, which will enable the generation of near-atomic resolution single particle cryo-electron microscopy density maps on a daily basis.

Ultrafast laser processing of zirconia/alumina nanocomposite ceramics, the current gold standard material for ceramic bearing components in orthopedics, was investigated. Instead of considering the substrate as a homogeneous material, as commonly assumed in laser micromachining, the damage behavior of different phases around the laser ablation threshold upon ultrafast laser irradiation was investigated. Under appropriate experimental conditions, the zirconia phase was selectively ablated while the alumina phase remained intact. The origin of this selective ablation behavior and its relationship with the material band gaps were discussed. Due to the nonlinear absorption mechanisms under ultrafast laser irradiation, the zirconia phase, with its band gap of 5.8 eV, can absorb more laser energy than the alumina phase which has a larger band gap of 8.8 eV. The negligible heat diffusion length ensures that the absorbed laser energy remains confined in the individual phases, leading to the selective ablation of zirconia phase under the given laser fluence. Based on this observation, an ultrafast laser selective phase removal method which can be used to modify the surface composition of nanocomposite materials consisting of phases with different band gaps was proposed.

This chapter focuses on how cyclodextrins (CDs) can be used to produce advanced polymeric materials and surfaces. It is intended as a general presentation of the current status of the field and is by no means exhaustive in its treatment. The reader will find an overview of the possible synthetic routes to cyclodextrins that are used to produce functional polymers and surfaces, their physicochemical properties, as well as their abilities to recognize various organic guest molecules. Additionally, various examples of possible applications are also provided. In relation to surfaces, the chapter summarizes the main strategies to produce CDs that can self-assemble as Langmuir and Langmuir-Blodgett (LB) films and as self-assembled monolayers (SAMs). The properties of these systems are discussed with an emphasis on their molecular recognition/inclusion properties and use as supramolecular immobilization platforms. Keywords: cyclodextrin; functional surfaces; Langmuir films; Langmuir-Blodgett films; polymeric materials; self-assembled monolayer

This thesis is a primarily experimental study of the spectroscopy and dynamics of highly vibrationally excited molecules in the ground and excited electronic states. Linear spectroscopic studies have been performed on CH-stretching overtones of medium and large sized molecules in the gas and solid phases. Also new time-resolved methods based on nonlinear uultiphoton ionization spectroscopy have been applied on the picosecond time-scale to the study of ultrafast intramolecular vibrational-energy redistribution (IVR) in isolated molecules. The first observation of high-energy CH stretching overtones of molecules in low-temperature solids is reported. Spectra of the νCH = 5 transitions of many aromatic molecules at ~2K reveal homogeneously broadened bands which are assigned to inequivalent local modes by studies of same isotopically substituted molecules. Typical linewidths give population lifetimes of ~50 fs for aromatic compared to 100-200 fs for methyl CHs in these large molecules. The polarization ratios of durene CH overtones are in good agreement with those predicted with the localized bond-mode transition moments. Overtone spectra of some gas-phase deuterated methanes and tetramethyl compounds have been studied. Spectra of CHD3 have shown severe vibrational mixing at νCH = 6 and 7 with an apparent tuning of the Fermi-resonance between |νCH> and discrete stretch-bend combination states, e.g., |(ν-1)CH, 2bend>. Nonetheless, the lifetime of the CHD3 overtones must be long (≥5 ps) as evidenced by the narrow resolution limited linewidths. The splitting of the |6,0>± states of CH2D2 is inferred to be Picosecond pump and probe multiphoton ionization is developed for the study of IVR in isolated molecules. Experimental evidence for IVR is presented. Nonexponential decay of vibronic states of S1 trans-stilbene is analyzed in terms of a population rate equation and time-dependent Franck-Condon factor description of IVR. The thermally-averaged IVR time constant is about 2 ps. Also, the excess vibrational energy dependence of the S1 trans-stilbene isomerization rate is determined for total vibrational energy of 2000 to 4500 cm-1 and good agreement is found with calculated thermally averaged rates based on independent results on jet-cooled molecules.

Magnetic excitations on the picosecond timescale

Microcavity polaritons are hybrid quasiparticles emerging from the strong coupling between quantum well excitons and light in the resonator. Their unique half-light half-matter nature brings in specific properties like low effective mass, nonlinearity due to the repulsive interactions, and ability to directly access the polaritonic wavefunction through optical means. Due to their specific features, polaritons can form macroscopic quantum states and even undergo Bose-Einstein condensation, which makes them very interesting system for studying the quantum phenomena occurring on a macroscopic scale. This thesis is devoted to the investigation of dynamics of the second-order coherence of the microcavity polaritons. The evolution of the coherence of the polariton system occurs on a timescale of few picoseconds only, which required assembly of a new ultrafast photon correlation setup based on the streak-camera. The operation of this setup and requirements for high-quality measurements are discussed in details. After this, the experiments under both nonresonant and resonant excitation of polaritons are reported. Under nonresonant excitation, the formation of the polariton Bose-Einstein condensate is observed, and the change of the polariton coherence during this phase transition is measured. This indicates the time which is required to formthe condensate, and also allows to estimate its propagation velocity. The latter is compared with the measurements of the first-order spatial correlation function. Finally, polaritons confined in two coupled potential traps are studied. The traps are fabricated using the wet etching of the microcavity. In the resulting Josephson junction, polaritons are excited resonantly and demonstrate clear Josephson oscillations between the two traps. At the same time, the correlation measurements demonstrate strong perturbations of the light statistics in phase with the oscillations. This behavior is well represented with the quantum simulations, which also indicate presence of a significant quadrature squeezing in the system. The origin of the squeezing, the perspective for generating the nonclassical light, and other features of this phenomenon are discussed.

Carbonate-based lithium-ion electrolytes are of great importance due to their close relationship with the resulting battery efficiency and safety. Modifying the organic electrolyte has been paramount for achieving more efficient and safer lithium-ion batteries. However, the molecular picture of the electrolyte is still under scrutiny. Lately, ultrafast infrared spectroscopic studies have investigated the solvation structure and dynamics of the lithium ion (Li+) in both linear and cyclic carbonates. However, theoretical studies describing the molecular arrangements and transformation occurring in such time scales are scarce. In this study, ab initio molecular dynamics simulations were used to obtain the molecular structure and dynamics of the Li+ solvation shell in cyclic and linear carbonates. The theoretical results showed that molecular arrangement of the carbonates directly coordinating Li+ is not significantly altered by the carbonate chemical nature. However, the cyclic and linear carbonates showed significant different pictures of the overall solvation shell due to the intercalation phenomenon observed for cyclic carbonates, which significantly alters the motions of coordinated solvent. In addition, the intercalation appears to affect the propensity of ion pair formation and/or solvent exchange. Finally, the dynamics of the geometrical changes of the carbonates solvating Li+ is found to occur with characteristic times of tenths of picoseconds, while ion pair and solvent chemical exchange appear to happen in time scales which are at least an order of magnitude larger. Our study provides a comprehensive picture of the structure and dynamics of the molecular components in different carbonate-based lithium-ion electrolytes occurring in picosecond time scales.

The data used to generate figures 3 - 7. Including: zip file of spectral csv files used to generate the heat map in figure 3; Excel file containing normalised spectra examples from three experimental cavities for figure 4; Excel file containing auto correlation traces from three experimental cavities including a longer time scale trace for the figure inset for figure 5; Excel file containing the measured mode locking and multi-pulsing thresholds of six experimental cavities with theoretical plot for figure 6; and an Excel file containing the multi-pulsing thresholds expressed in nonlinear phase shift normalised to pi shown in figure 7.