Disentangling the Electronic and Lattice Contributions to the Dielectric Response of Photoexcited Bismuth

Elucidating the interplay between nuclear and electronic degrees of freedom that govern the complex dielectric behavior of materials under intense photoexcitation is essential for tailoring optical properties on demand. However, conventional transient reflectivity experiments have been unable to differentiate between real and imaginary components of the dielectric response, omitting crucial electron-lattice interactions. Utilizing thin film interference we unambiguously determined the photoinduced change in complex dielectric function in the Peierls semimetal bismuth and examined its dependence on the excitation density and nuclear motion of the A$_{1g}$ phonon. Our modeled transient reflectivity data reveals a progressive broadening and redshift of Lorentz oscillators with increasing excitation density and underscores the importance of both, electronic and nuclear coordinates in the renormalization of interband transitions.Comment: Manuscript (6 pages) plus supplemental material (6 pages

Fenómenos de solvatación y fotoexcitación de KI en NH3 supercrítico

Esta tesis presenta un estudio experimental y teórico detallado del proceso de fotoexcitación vinculado a la solvatación de las distintas especies que existen en soluciones de KI disuelto en NH3 supercrítico: iones libres I-(sv) y pares iónicos de dos tipos, [K+(sv)I-](sv) y [K+I-](sv). Se realizaron experimentos de espectroscopia de absorción UV de estos sistemas en celdas de alta presión y se modelaron los mismos para utilizar en su descripción procedimientos de simulación de dinámica molecular combinados con cálculos de estructura electrónica. El análisis del conjunto de estos resultados nos permitió relacionar la estructura de solvatación de las especies estudiadas con el tipo de transición electrónica que experimentaban al ser fotoexcitadas. Esto fue posible gracias al empleo de un solvente en condiciones supercríticas, lo que permitió modificar gradualmente el valor de una variable escasamente controlable en solventes líquidos: la densidad de moléculas de solvente ρ1 . Esta estrategia experimental nos permitió una descripción consistente de las observaciones realizadas en el solvente supercrítico, aun a alta densidad del fluido, con aquellas que se observaron en fase vapor; también fue posible proponer su extensión a sistemas confinados, como son los clusters de NH3 conteniendo las mismas especies iónicas.This thesis presents a detailed experimental and theoretical study of the photoexcitation process which, we show, to be linked to the solvation of the different species that exist in supercritical NH3 solutions of KI: free ions I¡(sv) and two types of ion pairs, [K+(sv)I-](sv) and [K+I-](sv). Experiments of UV absorption spectroscopy were done using high pressure optical cells, the systems were modelled in order to use for their description the molecular dynamics simulation procedure together with electronic structure calculations. The global analysis of these results allowed us to relate the solvation structure of the studied species to the type of electronic transition occurring when they were photoexcited. This could be achieved because a supercritical solvent was employed, thus the fluid density ρ1 could be varied gradually; which would not be possible if liquid solvents were used. This experimental approach allowed us to make a consistent description of the observations made in the supercritical solvent, even for high ½1, with those made for the vapour phase; it was also possible to propose its extension to con¯ned systems of NH3 clusters containing the same ionic species.Fil: Sciaini, Alberto Germán. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Recent Advances in Ultrafast Structural Techniques

A review that summarizes the most recent technological developments in the field of ultrafast structural dynamics with focus on the use of ultrashort X-ray and electron pulses follows. Atomistic views of chemical processes and phase transformations have long been the exclusive domain of computer simulators. The advent of femtosecond (fs) hard X-ray and fs-electron diffraction techniques made it possible to bring such a level of scrutiny to the experimental area. The following review article provides a summary of the main ultrafast techniques that enabled the generation of atomically resolved movies utilizing ultrashort X-ray and electron pulses. Recent advances are discussed with emphasis on synchrotron-based methods, tabletop fs-X-ray plasma sources, ultrabright fs-electron diffractometers, and timing techniques developed to further improve the temporal resolution and fully exploit the use of intense and ultrashort X-ray free electron laser (XFEL) pulses

Shaped cathodes for the production of ultra-short multi-electron pulses

An electrostatic electron source design capable of producing sub-20 femtoseconds (rms) multi-electron pulses is presented. The photoelectron gun concept builds upon geometrical electric field enhancement at the cathode surface. Particle tracer simulations indicate the generation of extremely short bunches even beyond 40 cm of propagation. Comparisons with compact electron sources commonly used for femtosecond electron diffraction are made

Static and dynamic scavenging of ammoniated electrons by nitromethane

We studied the time-resolved scavenging efficiency of nitromethane for transient electron species in liquid ammonia, at a temperature of 298 K. UV excitation of iodide ions produced fully solvated electrons, as well as transient (I, e-) and (counterion, e-) pairs, the overall concentration of which was monitored by NIR absorption with subpicosecond time resolution. After the UV pulse, the solution absorbance decays almost completely in a few hundreds of picoseconds due to geminate electron-iodine atom recombination and a competitive annihilation channel involving the scavenger. Recombination of transient (I, e-) pairs follows the well-known kinetic model, while the electron-nitromethane reaction proceeds by two distinct mechanisms: static scavenging (interpreted in terms of the encounter complex model), with a characteristic time shorter than the temporal resolution of the apparatus, or via a diffusion-limited bimolecular reaction, with a rate constant of 1.1 × 1011 M-1 s-1.Fil: Rivas, Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. University of Waterloo; CanadáFil: Sciaini, Germán. University of Waterloo; CanadáFil: Marceca, Ernesto José. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

High-Resolution Bulgeless Liquid-Cell Electron Microscopy

Liquid cell electron microscopy (LCEM) has long suffered from irreproducibility and its inability to confer high-quality images over a wide field of view. LCEM demands the encapsulation of the in-liquid sample between two ultrathin membranes (windows). In the vacuum environment of the electron microscope, the windows bulge, drastically reducing the achievable resolution and the usable viewing region. Herein, we introduce a shape-engineered nanofluidic cell architecture and an air-free drop-casting sample loading technique, which combined, provide robust bulgeless imaging conditions. We demonstrate the capabilities of our approach through the study of in-liquid model samples and quantitative measurements of the liquid layer thickness. The presented LCEM method confers high throughput, lattice resolution across the complete viewing window, and sufficient contrast for the observation of unstained liposomes, paving the way to high-resolution movies of biospecimens in their near native environment

Mapping molecular motions leading to charge delocalization with ultrabright electrons

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or hard X-rays15, 16, 17, 18, 19 as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics20. Here we show that a recently developed, ultrabright femtosecond electron source7, 8, 9 makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition21, 22, 23, 24. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources7, 8, 9, 10, 11, 12, 13, 14 for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology

Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale(1). The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials(2), with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems(3-7). Numerous time-resolved experiments have been performed on CDWs(8-13), probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred(14). Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction(15) to study the quasi two-dimensional CDW system 1T-TaS2. Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of similar to 0.1 angstrom, is suppressed by about 20% on a timescale (similar to 250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (similar to 350 femtoseconds) and are followed by fast recovery of the CDW (similar to 4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems