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The electronic and structural components of charge density waves occurring in layered transition metal dichalcogenides are known to be interdependent, yet have only been probed in separate measurements. Now, a broadband terahertz spectroscopy approach that monitors the evolution of these two order parameters simultaneously is demonstrated. The simultaneous ordering of different degrees of freedom in complex materials undergoing spontaneous symmetry-breaking transitions often involves intricate couplings that have remained elusive in phenomena as wide ranging as stripe formation 1 , unconventional superconductivity 1 , 2 , 3 , 4 , 5 , 6 , 7 or colossal magnetoresistance 1 , 8 . Ultrafast optical, X-ray and electron pulses can elucidate the microscopic interplay between these orders by probing the electronic and lattice dynamics separately 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , but a simultaneous direct observation of multiple orders on the femtosecond scale has been challenging. Here we show that ultrabroadband terahertz pulses can simultaneously trace the ultrafast evolution of coexisting lattice and electronic orders. For the example of a charge density wave (CDW) in 1 T -TiSe 2 , we demonstrate that two components of the CDW order parameter—excitonic correlations and a periodic lattice distortion (PLD)—respond very differently to 12-fs optical excitation. Even when the excitonic order of the CDW is quenched, the PLD can persist in a coherently excited state. This observation proves that excitonic correlations are not the sole driving force of the CDW transition in 1 T -TiSe 2 , and exemplifies the sort of profound insight that disentangling strongly coupled components of order parameters in the time domain may provide for the understanding of a broad class of phase transitions.

The electromagnetic field of visible light performs$\\sim 10^{15}$oscillations per second. Although many instruments are sensitive to the amplitude and frequency (or wavelength) of these oscillations, they cannot access the light field itself. We directly observed how the field built up and disappeared in a short, few-cycle pulse of visible laser light by probing the variation of the field strength with a 250-attosecond electron burst. Our apparatus allows complete characterization of few-cycle waves of visible, ultraviolet, and/or infrared light, thereby providing the possibility for controlled and reproducible synthesis of ultrabroadband light waveforms.

See how they run Electrons move in solids at very high speed — traversing atomic layers and interfaces within tens to hundreds of attoseconds (an attosecond is a billionth of a billionth of a second). These astonishingly brief travel times will ultimately limit the speed of the electronics of the future. Physicists have now experimentally probed such electron dynamics in real time. The cover illustrates the first attosecond spectroscopic measurement in a solid, revealing a 110-attosecond difference in the travel time of two different types of electrons following photoexcitation in a tungsten crystal. The ability to time electrons moving in solids over merely a few interatomic distances makes it possible to probe the solid-state electronic processes occurring at the ultimate speed limit and thus helps to advance technologies such as computation, data storage and photovoltaics, which all rely on exquisite control of electron transport in ever smaller structures of solid matter. When exposing a tungsten crystal to intense light, the travel times of emitted electrons differ by 110 attoseconds, depending on whether they were originally tightly bound to one atom in the crystal or delocalized over many atoms. This ability to directly probe fundamental aspects of solid-state electron dynamics could aid the further development of modern technologies such as electronics, information processing and photovoltaics. Comprehensive knowledge of the dynamic behaviour of electrons in condensed-matter systems is pertinent to the development of many modern technologies, such as semiconductor and molecular electronics, optoelectronics, information processing and photovoltaics. Yet it remains challenging to probe electronic processes, many of which take place in the attosecond (1 as = 10 -18  s) regime. In contrast, atomic motion occurs on the femtosecond (1 fs = 10 -15  s) timescale and has been mapped in solids in real time 1 , 2 using femtosecond X-ray sources 3 . Here we extend the attosecond techniques 4 , 5 previously used to study isolated atoms in the gas phase to observe electron motion in condensed-matter systems and on surfaces in real time. We demonstrate our ability to obtain direct time-domain access to charge dynamics with attosecond resolution by probing photoelectron emission from single-crystal tungsten. Our data reveal a delay of approximately 100 attoseconds between the emission of photoelectrons that originate from localized core states of the metal, and those that are freed from delocalized conduction-band states. These results illustrate that attosecond metrology constitutes a powerful tool for exploring not only gas-phase systems, but also fundamental electronic processes occurring on the attosecond timescale in condensed-matter systems and on surfaces.

In Bohr's model of the hydrogen atom, the electron takes about 150 attoseconds (1 as = 10 -18  s) to orbit around the proton, defining the characteristic timescale for dynamics in the electronic shell of atoms. Recording atomic transients in real time requires excitation and probing on this scale. The recent observation of single sub-femtosecond (1 fs = 10 -15  s) extreme ultraviolet (XUV) light pulses 1 has stimulated the extension of techniques of femtochemistry 2 into the attosecond regime 3 , 4 . Here we demonstrate the generation and measurement of single 250-attosecond XUV pulses. We use these pulses to excite atoms, which in turn emit electrons. An intense, waveform-controlled, few cycle laser pulse 5 obtains ‘tomographic images’ of the time-momentum distribution of the ejected electrons. Tomographic images of primary (photo)electrons yield accurate information of the duration and frequency sweep of the excitation pulse, whereas the same measurements on secondary (Auger) electrons will provide insight into the relaxation dynamics of the electronic shell following excitation. With the current ∼750-nm laser probe and ∼100-eV excitation, our transient recorder is capable of resolving atomic electron dynamics within the Bohr orbit time.

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution. Tunnelling out in real time Atoms exposed to intense light lose one or more electrons and become ionized. In strong light fields this type of ionization occurs through electron tunnelling, which happens when the electrons receive so much energy that they can 'tunnel' through the potential barrier binding them to the nucleus. This processes has now been observed in real time, on a timescale measured in hundreds of attoseconds (an attosecond is a billionth of a billionth of a second). The target neon and xenon atoms were ionized with ultrafast far-ultraviolet pulses, and were then observed using near-infrared pulses tailor-made for the purpose. This technique of 'attosecond tunnelling' spectroscopy could offer control over the dynamics of electrons inside atoms and molecules. A time-resolved observation of electron tunnelling and the short-lived electronic states that subsequently appear is useful as a new approach to obtain insights in multi-electron dynamics inside atoms and molecules. This technique of 'attosecond tunnelling' is applied to study the cascade of electronic transitions that occur in xenon atoms as a result of their ionization.

The characteristic time constants of the relaxation dynamics of core-excited atoms have hitherto been inferred from the linewidths of electronic transitions measured by continuous-wave extreme ultraviolet or X-ray spectroscopy. Here we demonstrate that a laser-based sampling system, consisting of a few-femtosecond visible light pulse and a synchronized sub-femtosecond soft X-ray pulse, allows us to trace these dynamics directly in the time domain with attosecond resolution. We have measured a lifetime of 7.9 -0.9 +1.0  fs of M-shell vacancies of krypton in such a pump–probe experiment.

The generation of ultrashort pulses is a key to exploring the dynamic behaviour of matter on ever-shorter timescales. Recent developments have pushed the duration of laser pulses close to its natural limit the wave cycle, which lasts somewhat longer than one femtosecond (1 fs = 10-15 s) in the visible spectral range. Time-resolved measurements with these pulses are able to trace dynamics of molecular structure, but fail to capture electronic processes occurring on an attosecond (1 as = 10-18 s) timescale. Here we trace electronic dynamics with a time resolution of less than or equal to 150 as by using a subfemtosecond soft-X-ray pulse and a few-cycle visible light pulse. Our measurement indicates an attosecond response of the atomic system, a soft-X-ray pulse duration of 650 plusminus 150 as and an attosecond synchronism of the soft-X-ray pulse with the light field. The demonstrated experimental tools and techniques open the door to attosecond spectroscopy of bound electrons.

The cardiovascular system is currently considered a target for particulate matter, especially for ultrafine particles. In addition to autonomic or cytokine mediated effects, the direct interaction of inhaled materials with the target tissue must be examined to understand the underlying mechanisms. In the first approach, pulmonary and systemic distribution of inhaled ultrafine elemental silver (EAg) particles was investigated on the basis of morphology and inductively coupled plasma mass spectrometry (ICP-MS) analysis. Rats were exposed for 6 hr at a concentration of 133 μg EAg m3(3× 106cm3, 15 nm modal diameter) and were sacrificed on days 0, 1, 4, and 7. ICP-MS analysis showed that 1.7 μg Ag was found in the lungs immediately after the end of exposure. Amounts of Ag in the lungs decreased rapidly with time, and by day 7 only 4% of the initial burden remained. In the blood, significant amounts of Ag were detected on day 0 and thereafter decreased rapidly. In the liver, kidney, spleen, brain, and heart, low concentrations of Ag were observed. Nasal cavities, especially the posterior portion, and lung-associated lymph nodes showed relatively high concentrations of Ag. For comparison, rats received by intratracheal instillation either 150 μL aqueous solution of 7 μg silver nitrate ( AgNO3) (4.4 μg Ag) or 150 μL aqueous suspension of 50 μg agglomerated ultrafine EAg particles. A portion of the agglomerates remained undissolved in the alveolar macrophages and in the septum for at least 7 days. In contrast, rapid clearance of instilled water-soluble AgNO3from the lung was observed. These findings show that although instilled agglomerates of ultrafine EAg particles were retained in the lung, Ag was rapidly cleared from the lung after inhalation of ultrafine EAg particles, as well as after instillation of AgNO3and entered systemic pathways.

Photoelectrons excited by extreme ultraviolet or x-ray photons in the presence of a strong laser field generally suffer a spread of their energies due to the absorption and emission of laser photons. We demonstrate that if the emitted electron wave packet is temporally confined to a small fraction of the oscillation period of the interacting light wave, its energy spectrum can be up- or downshifted by many times the laser photon energy without substantial broadening. The light wave can accelerate or decelerate the electron's drift velocity, i.e., steer the electron wave packet like a classical particle. This capability strictly relies on a sub-femtosecond duration of the ionizing x-ray pulse and on its timing to the phase of the light wave with a similar accuracy, offering a simple and potentially single-shot diagnostic tool for attosecond pump-probe spectroscopy.

Regulla, D., Schmid, E., Friedland, W., Panzer, W., Heinzmann, U. and Harder, D. Enhanced Values of the RBE and H Ratio for Cytogenetic Effects Induced by Secondary Electrons from an X-Irradiated Gold Surface. Radiat. Res. 158, 505–515 (2002). The low-energy secondary electrons emerging from the entrance surface of an X-irradiated gold foil increase the dose to cells in contact with or at micrometer distances from this surface (Radiat. Res. 150, 92–100, 1998). We examined the effect of the spectrum of these low-energy electrons on the RBE for cytogenetic effects and showed that this RBE was increased. A monolayer of surface-attached human T lymphocytes was exposed to 60 kV X rays in the absence or presence of a gold foil positioned immediately behind the cell layer or separated from it by a Mylar foil 0.9 or 2 μm thick. The enhancement of dose in the cell nuclei caused by the photoelectrons and Auger electrons emerging from the entrance surface of the gold foil was measured by TSEE dosimetry. Dose enhancement factors of 55.7, 46.6 and 37.5 were obtained with 0, 0.9 and 2 μm of Mylar inserted between the gold surface and the cell layer. This large enhancement results from the photoelectric effect in the gold foil, as shown by the accompanying Monte Carlo calculations of the secondary electron spectra at the gold surface. Auger electrons from the gold foil generally were not able to penetrate into the cell nuclei except for that fraction of the cells that had a very thin (< 0.7 μm) layer of cytoplasm and membranes between gold surface and cell nucleus. The dose–yield curves for dicentric chromosomes plus centric rings and for acentric fragments obtained after exposures without or with the gold foil were linear-quadratic. The coefficient α, the slope of the linear yield component, was increased in the presence of the gold foil and showed RBE values ranging from 1.7 to 2.2 compared to exposures in absence of the gold foil. The ratio of the yield of interstitial deletions and dicentrics (H ratio) was significantly increased from about 0.17 in the absence of the gold foil to about 0.22 in the presence of the gold foil. The increases in the RBE and the H ratio are interpreted in microdosimetric terms: The preferred occurrence of electron track ends in the vicinity of the gold surface causes an increase in the dose-mean restricted linear energy transfer in cell nuclei exposed to the photoelectrons and Auger electrons.