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Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ * to a dark nπ * state. The ππ */ nπ * internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ */ nπ * internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ */ nπ * internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations. Many photo-induced processes such as photosynthesis occur in organic molecules, but their femtosecond excited-state dynamics are difficult to track. Here, the authors exploit the element and site selectivity of soft X-ray absorption to sensitively follow the ultrafast ππ */ nπ * electronic relaxation of hetero-organic molecules.

The description of ultrafast nonadiabatic chemical dynamics during molecular photo-transformations remains challenging because electronic and nuclear configurations impact each other and cannot be treated independently. Here we gain experimental insights, beyond the Born–Oppenheimer approximation, into the light-induced spin-state trapping dynamics of the prototypical [Fe(bpy) 3 ] 2+ compound by time-resolved X-ray absorption spectroscopy at sub-30-femtosecond resolution and high signal-to-noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period), clearly identified as molecular breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramolecular vibronic coupling before a slower vibrational energy dissipation to the solution environment. These findings illustrate how modern time-resolved X-ray absorption spectroscopy can provide key information to unravel dynamic details of photo-functional molecules. Ultrafast nonadiabatic chemical dynamics during molecular photo-transformations remain challenging to describe since electronic/nuclear configurations are coupled. Here the authors use time-resolved X-ray absorption spectroscopy to probe the light-induced spin-state trapping dynamics of [Fe(bpy) 3 ] 2+ beyond the Born-Oppenheimer approximation.

We demonstrate that the isotope shift in isotopomers of diatomic molecules, where the nucleus of one of its constituent atoms is replaced by another isotope, can be expressed as the sum of a field shift and a mass shift similar to the atomic case. We show that a linear relation holds between atomic and molecular isotopes shifts, thus extending the King-plot analysis to molecular isotope shifts. Optical isotope shifts in YbF and ZrO and infrared isotope shifts in SnH are analyzed with a molecular King-plot approach utilizingYb+andZr+ionic isotope shifts and charge radii of Sn obtained with nonoptical methods. The changes in the mean-squared nuclear charge radiiδ⟨r2⟩ofYb170–174,176andZr90–92,94,96extracted from the molecular transitions are found to be in excellent agreement with the values from the spectroscopy ofYb+andZr+, respectively. On the contrary, in the case of the vibrational-rotational transition in SnH, no sensitivity to the nuclear volume can be deduced within the experimental resolution, which makes it unsuitable for the extraction of nuclear charge radii but provides insights into the molecular electronic wave function not accessible via other methods. The new opportunities offered by the molecular King-plot analysis for research in nuclear structure and molecular physics are discussed.

The pulse duration, and, more generally, the temporal intensity profile of free-electron laser (FEL) pulses, is of utmost importance for exploring the new perspectives offered by FELs; it is a nontrivial experimental parameter that needs to be characterized. We measured the pulse shape of an extreme ultraviolet externally seeded FEL operating in high-gain harmonic generation mode. Two different methods based on the cross-correlation of the FEL pulses with an external optical laser were used. The two methods, one capable of single-shot performance, may both be implemented as online diagnostics in FEL facilities. The measurements were carried out at the seeded FEL facility FERMI. The FEL temporal pulse characteristics were measured and studied in a range of FEL wavelengths and machine settings, and they were compared to the predictions of a theoretical model. The measurements allowed a direct observation of the pulse lengthening and splitting at saturation, in agreement with the proposed theory.

This book, edited by two of the most respected researchers in plasmonics, gives an overview of the current state in plasmonics and plasmonic-based metamaterials, with an emphasis on active functionalities and an eye to future developments. This book is multifunctional, useful for newcomers and scientists interested in applications of plasmonics and metamaterials as well as for established researchers in this multidisciplinary area.

Attosecond-scale electron localization The primary event in photoexcitation — involved in processes such as photosynthesis and photoisomerization — is an electronic response that occurs on attosecond (1 as = 10 −18 s) timescales, a realm recently made accessible to spectroscopic investigation by the development of attosecond-scale light pulses. Sansone et al . report an experimental study in which electron localization in molecules is measured on attosecond timescales using pump–probe spectroscopy. H 2 and D 2 are dissociatively ionized by the sequence of 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 on the delay between the pump and probe pulses. This work demonstrates that combined experimental and computational efforts enable the use of attosecond pulses for the exploration of electron localization. Attosecond (10 −18 s) laser pulses make it possible to peer into the inner workings of atoms and molecules on the electronic timescale — phenomena in solids have already been investigated in this way. Here, an attosecond pump–probe experiment is reported that investigates the ionization and dissociation of hydrogen molecules, illustrating that attosecond techniques can also help explore the prompt charge redistribution and charge localization that accompany photoexcitation processes in molecular systems. 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 1 , 2 , 3 , 4 has become possible only with the recent development of isolated attosecond (10 −18 -s) laser pulses 5 . Such pulses have been used to investigate atomic photoexcitation and photoionization 6 , 7 and electron dynamics in solids 8 , and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H 2 and D 2 was monitored on femtosecond timescales 9 and controlled using few-cycle near-infrared laser pulses 10 . Here we report a molecular attosecond pump–probe experiment based on that work: H 2 and D 2 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.

The ultrafast structural dynamics of water following inner-shell ionization is a crucial issue in high-energy radiation chemistry. We have exposed isolated water molecules to a short X-ray pulse from a free-electron laser and detected momenta of all produced ions in coincidence. By combining experimental results and theoretical modeling, we can image dissociation dynamics of individual molecules in unprecedented detail. We reveal significant molecular structural dynamics in H2O2+, such as asymmetric deformation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a few femtoseconds. We thus reconstruct several snapshots of structural dynamics at different time intervals, which highlight dynamical patterns that are relevant as initiating steps of subsequent radiation-damage processes.

The theoretical framework of energy-density functionals has been used to show that the depth of the confining nuclear potential has an important role in cluster formation, with a pronounced effect for relativistic functionals. Clustering behaviour of nucleons Nucleonic matter — the protons and neutrons that comprise atomic nuclei — acts predominantly as a quantum liquid, but lighter nuclei behave more like molecules composed of clusters of protons and neutrons. Clustering is related to the overall nuclear interaction, but its detailed mechanism is not fully understood. These authors use theoretical modelling to calculate the conditions that cause clustering in neon-20, a small nucleus thought to favour clustering. They find that the depth of the confining nuclear potential plays an important part in cluster formation, with a particularly pronounced effect for relativistic functionals. More generally, clustering is considered as a transitional phenomenon between crystalline and quantum liquid phases. Nucleonic matter displays a quantum-liquid structure, but in some cases finite nuclei behave like molecules composed of clusters of protons and neutrons. Clustering is a recurrent feature in light nuclei, from beryllium to nickel 1 , 2 , 3 . Cluster structures are typically observed as excited states close to the corresponding decay threshold; the origin of this phenomenon lies in the effective nuclear interaction, but the detailed mechanism of clustering in nuclei has not yet been fully understood. Here we use the theoretical framework of energy-density functionals 4 , 5 , encompassing both cluster and quantum liquid-drop aspects of nuclei, to show that conditions for cluster formation can in part be traced back to the depth of the confining nuclear potential. For the illustrative example of neon-20, we show that the depth of the potential determines the energy spacings between single-nucleon orbitals in deformed nuclei, the localization of the corresponding wavefunctions and, therefore, the degree of nucleonic density clustering. Relativistic functionals, in particular, are characterized by deep single-nucleon potentials. When compared to non-relativistic functionals that yield similar ground-state properties (binding energy, deformation, radii), they predict the occurrence of much more pronounced cluster structures. More generally, clustering is considered as a transitional phenomenon between crystalline and quantum-liquid phases of fermionic systems.

The response of cryogenic high-resolution detectors to a short-pulse laser consists of a Poisson-distributed set of equidistant peaks that correspond to integer numbers of absorbed photons. Since the laser has a negligible intrinsic line width, the peaks can be used for detailed characterization of both the detector and the data acquisition system. We have characterized our superconducting tunnel junction (STJ) photon detectors in the UV and soft X-ray range with a pulsed 355-nm laser at rates up to 5000 counts/s. The observed peaks are described by a Gaussian to very high accuracy, with a width between ~ 1 and ~ 3 eV FWHM depending on the detector area and the absorbed energy. For high statistics, centroids can be determined with a precision of order 1 meV over an energy range of several 100 eV. This allows identifying and correcting for non-linearities in the digitizer that can otherwise limit the calibration accuracy.

The generation of attosecond pulse trains at free-electron lasers opens new opportunities in ultrafast science, as it gives access, for the first time, to reproducible, programmable, extreme ultraviolet (XUV) waveforms with high intensity. In this work, we present a detailed analysis of the theoretical model underlying the temporal characterization of the attosecond pulse trains recently generated at the free-electron laser FERMI. In particular, the validity of the approximations used for the correlated analysis of the photoelectron spectra generated in the two-color photoionization experiments are thoroughly discussed. The ranges of validity of the assumptions, in connection with the main experimental parameters, are derived.