36 research outputs found
X-Ray sum frequency generation; direct imaging of ultrafast electron dynamics
X-ray diffraction from molecules in the ground state produces an image of
their charge density, and time-resolved X-ray diffraction can thus monitor the
motion of the nuclei. However, the density change of excited valence electrons
upon optical excitation can barely be monitored with regular diffraction
techniques due to the overwhelming background contribution of the core
electrons. We present a nonlinear X-ray technique made possible by novel free
electron laser sources, which provides a spatial electron density image of
valence electron excitations. The technique, sum frequency generation carried
out with a visible pump and a broadband X-ray diffraction pulse, yields
snapshots of the transition charge densities, which represent the electron
density variations upon optical excitation. The technique is illustrated by ab
initio simulations of transition charge density imaging for the optically
induced electronic dynamics in a donor/acceptor substituted stilbene
Monitoring Nonadiabatic Electron-Nuclear Dynamics in Molecules by Attosecond Streaking of Photoelectrons
Streaking of photoelectrons has long been used for the temporal
characterization of attosecond extreme ultraviolet pulses. When the
time-resolved photoelectrons originate from a coherent superposition of
electronic states, they carry an additional phase information, which can be
retrieved by the streaking technique. In this contribution we extend the
streaking formalism to include coupled electron and nuclear dynamics in
molecules as well as initial coherences and demonstrate how it offers a novel
tool to monitor non-adiabatic dynamics as it occurs in the vicinity of conical
intersections and avoided crossings. Streaking can enhance the time resolution
and provide direct signatures of electronic coherences, which affect many
primary photochemical and biological events
Molecular Structure and Modeling of Water-Air and Ice-Air Interfaces Monitored by Sum-Frequency Generation.
From a glass of water to glaciers in Antarctica, water-air and ice-air interfaces are abundant on Earth. Molecular-level structure and dynamics at these interfaces are key for understanding many chemical/physical/atmospheric processes including the slipperiness of ice surfaces, the surface tension of water, and evaporation/sublimation of water. Sum-frequency generation (SFG) spectroscopy is a powerful tool to probe the molecular-level structure of these interfaces because SFG can specifically probe the topmost interfacial water molecules separately from the bulk and is sensitive to molecular conformation. Nevertheless, experimental SFG has several limitations. For example, SFG cannot provide information on the depth of the interface and how the orientation of the molecules varies with distance from the surface. By combining the SFG spectroscopy with simulation techniques, one can directly compare the experimental data with the simulated SFG spectra, allowing us to unveil the molecular-level structure of water-air and ice-air interfaces. Here, we present an overview of the different simulation protocols available for SFG spectra calculations. We systematically compare the SFG spectra computed with different approaches, revealing the advantages and disadvantages of the different methods. Furthermore, we account for the findings through combined SFG experiments and simulations and provide future challenges for SFG experiments and simulations at different aqueous interfaces
Monitoring Spontaneous Charge-density Fluctuations by Single-molecule Diffraction of Quantum Light
Homodyne X-ray diffraction signals produced by classical light and classical
detectors are given by the modulus square of the charge density in momentum
space , missing its phase which is
required in order to invert the signal to real space. We show that quantum
detection of the radiation field yields a linear diffraction pattern that
reveals itself, including the phase. We further show that
repeated diffraction measurements with variable delays constitute a novel
multidimensional measure of spontaneous charge-density fluctuations. Classical
diffraction, in contrast, only reveals a subclass of even-order correlation
functions. Simulations of two dimensional signals obtained by two diffraction
events are presented for the amino acid cysteine
Chiral Four-Wave-Mixing signals with circularly-polarized X-ray pulses
Chiral four-wave-mixing signals are calculated using the irreducible tensor
formalism. Different polarization and crossing angle configurations allow to
single out the magnetic dipole and the electric quadrupole interactions. Other
configurations can reveal that the chiral interaction occurs at a given step
within the nonlinear interaction pathways contributing to the signal.
Applications are made to the study of valence excitations of S-ibuprofen by
chiral Stimulated X-ray Raman signals at the Carbon K-edge and by chiral
visible 2D Electronic Spectroscopy.teraction pathways contributing to the
signal.Comment: 33 pages, 10 figure
Opportunities for Gas-Phase Science at Short-Wavelength Free-Electron Lasers with Undulator-Based Polarization Control
Free-electron lasers (FELs) are the world's most brilliant light sources with
rapidly evolving technological capabilities in terms of ultrabright and
ultrashort pulses over a large range of accessible photon energies. Their
revolutionary and innovative developments have opened new fields of science
regarding nonlinear light-matter interaction, the investigation of ultrafast
processes from specific observer sites, and approaches to imaging matter with
atomic resolution. A core aspect of FEL science is the study of isolated and
prototypical systems in the gas phase with the possibility of addressing
well-defined electronic transitions or particular atomic sites in molecules.
Notably for polarization-controlled short-wavelength FELs, the gas phase offers
new avenues for investigations of nonlinear and ultrafast phenomena in spin
orientated systems, for decoding the function of the chiral building blocks of
life as well as steering reactions and particle emission dynamics in otherwise
inaccessible ways. This roadmap comprises descriptions of technological
capabilities of facilities worldwide, innovative diagnostics and
instrumentation, as well as recent scientific highlights, novel methodology and
mathematical modeling. The experimental and theoretical landscape of using
polarization controllable FELs for dichroic light-matter interaction in the gas
phase will be discussed and comprehensively outlined to stimulate and
strengthen global collaborative efforts of all disciplines