For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10−15-s) timescale. ...However, studying the inner workings of atoms and molecules on the electronic timescale has become possible only with the recent development of isolated attosecond (10−18-s) laser pulses. Such pulses have been used to investigate atomic photoexcitation and photoionization and electron dynamics in solids, and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H2 and D2 was monitored on femtosecond timescales and controlled using few-cycle near-infrared laser pulses. Here we report a molecular attosecond pump-probe experiment based on that work: H2 and D2 are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends-with attosecond time resolution-on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump-probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born-Oppenheimer approximation.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Hole migration is a fascinating process driven by electron correlation, in which purely electronic dynamics occur on a very short time scale in complex ionized molecules, prior to the onset of ...nuclear motion. However, it is expected that due to coupling to the nuclear dynamics, these oscillations will be rapidly damped and smeared out, which makes experimental observation of the hole migration process rather difficult. In this Letter, we demonstrate that the instantaneous ionization of benzene molecules initiates an ultrafast hole migration characterized by a periodic breathing of the hole density between the carbon ring and surrounding hydrogen atoms on a subfemtosecond time scale. We show that these oscillations survive the dephasing introduced by the nuclear motion for a long enough time to allow their observation. We argue that this offers an ideal benchmark for studying the influence of hole migration on molecular reactivity.
The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Ultrashort extreme ultraviolet pulses offer a ...means to reveal molecular dynamics at ultrashort timescales. Here, we report the use of time-resolved electron-momentum imaging combined with extreme ultraviolet attosecond pulses to study highly excited organic molecules. We measure relaxation timescales that increase with the state energy. High-level quantum calculations show these dynamics are intrinsic to the time-dependent many-body molecular wavefunction, in which multi-electronic and non-Born-Oppenheimer effects are fully entangled. Hints of coherent vibronic dynamics, which persist despite the molecular complexity and high-energy excitation, are also observed. These results offer opportunities to understand the molecular dynamics of highly excited species involved in radiation damage and astrochemistry, and the role of quantum mechanical effects in these contexts.
Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments ...provided by attosecond science. Here, we apply experimental and theoretical methods to assess the ultrafast nonadiabatic vibronic processes in a prototypical complex system-the excited benzene cation. We use few-femtosecond duration extreme ultraviolet and visible/near-infrared laser pulses to prepare and probe excited cationic states and observe two relaxation timescales of 11 ± 3 fs and 110 ± 20 fs. These are interpreted in terms of population transfer via two sequential conical intersections. The experimental results are quantitatively compared with state-of-the-art multi-configuration time-dependent Hartree calculations showing convincing agreement in the timescales. By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules.
To describe the microscopic properties of matter, quantum mechanics uses wave functions, whose structure and time dependence is governed by the Schrödinger equation. In atoms the charge distributions ...described by the wave function are rarely observed. The hydrogen atom is unique, since it only has one electron and, in a dc electric field, the Stark Hamiltonian is exactly separable in terms of parabolic coordinates (η, ξ, φ). As a result, the microscopic wave function along the ξ coordinate that exists in the vicinity of the atom, and the projection of the continuum wave function measured at a macroscopic distance, share the same nodal structure. In this Letter, we report photoionization microscopy experiments where this nodal structure is directly observed. The experiments provide a validation of theoretical predictions that have been made over the last three decades.
Modern ultrashort X-ray/XUV (extreme ultraviolet) sources provide unique opportunities to investigate the primary reactions of matter upon energetic excitation. Understanding these processes in ...molecules on ultrafast timescales is required to improve bespoke high-energy radiation detectors, nanomedicine schemes or to study the molecular composition of interstellar media. However, current experiments struggle to provide a general framework because of the uniqueness and complexity of each system. Here we show the universal role of correlation bands—features created by electron correlation. This is done by studying ultrafast energy relaxation of size-scalable two-dimensional molecules following ionization by an ultrashort XUV pulse. We observed long lifetimes that nonlinearly increase with the number of valence electrons. A general law based on solid-like electron–phonon scattering is proposed, which explains both our results and previously reported measurements. This offers new opportunities in attosecond science and high-energy photophysics.The size-dependent lifetimes observed in the ultrafast molecular relaxation dynamics of an entire class of polycyclic aromatic hydrocarbons can be explained by correlation bands and electron–phonon scattering, reminiscent of solid-state systems.
Salmonellosis is a prevalent food-borne illness that causes diarrhea in over 130 million humans yearly and can lead to death. There is an urgent need to find alternatives to antibiotics as many ...salmonellae are now multidrug resistant. As such, specific beneficial bacteria and dietary fibers can be an alternative as they may prevent
Typhimurium (STM) infection and spreading by strengthening intestinal barrier function.
We tested whether immune active long-chain inulin-type fructans and/or
W37,
W63, and
W56 can strengthen barrier integrity of intestinal Caco-2 cells in the presence and absence of a STM. Effects of the ingredients on intestinal barrier function were first evaluated by quantifying trans-epithelial electric resistance (TEER) and regulation of gene expression by microarray. Only
had effects on TEER and modulated a group of 26 genes related to tight-junctions. Inulin-type fructans,
W63 and
W56 regulated other genes, unrelated to tight-junctions.
also had unique effects on a group of six genes regulating epithelial phenotype toward follicle-associated epithelium.
W37 was therefore selected for a challenge with STM and prevented STM-induced barrier disruption and decreased secretion of IL-8.
W37 increases TEER and can protect against STM induced disruption of gut epithelial cells integrity
. Our results suggest that selection of specific bacterial strains for enforcing barrier function may be a promising strategy to reduce or prevent STM infections.
Highly excited molecular species are at play in the chemistry of interstellar media and are involved in the creation of radiation damage in a biological tissue. Recently developed ultrashort extreme ...ultraviolet light sources offer the high excitation energies and ultrafast time-resolution required for probing the dynamics of highly excited molecular states on femtosecond (fs) (1 fs=10(-15) s) and even attosecond (as) (1 as=10(-18) s) timescales. Here we show that polycyclic aromatic hydrocarbons (PAHs) undergo ultrafast relaxation on a few tens of femtoseconds timescales, involving an interplay between the electronic and vibrational degrees of freedom. Our work reveals a general property of excited radical PAHs that can help to elucidate the assignment of diffuse interstellar absorption bands in astrochemistry, and provides a benchmark for the manner in which coupled electronic and nuclear dynamics determines reaction pathways in large molecules following extreme ultraviolet excitation.
Midinfrared strong-field laser ionization offers the promise of measuring holograms of atoms and molecules, which contain both spatial and temporal information of the ion and the photoelectron with ...subfemtosecond temporal and angstrom spatial resolution. We report on the scaling of photoelectron holographic interference patterns with the laser pulse duration, wavelength, and intensity. High-resolution holograms for the ionization of metastable xenon atoms by 7-16 μm light from the FELICE free electron laser are presented and compared to semiclassical calculations that provide analytical insight.
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