Weathering Behavior of Dimensionally Stabilized Wood Treated by Heating Under Pressure of Nitrogen Gas

With the goal of improving the weathering behavior of wood by reducing its hygroscopicity and accompanying dimensional changes, samples of spruce and beech were heat-treated under nitrogen pressure at 175 to 195 C and subjected to natural and artificial weathering. Beech had a significantly lower hygroscopicity and improved dimensional stability after heat treatment and was more resistant to weathering than the unheated control. Although the hygroscopicity of spruce was also significantly reduced by heat treatment, weathering resistance was diminished. Heat treatment of either species had small, but measurable effects on the performance and durability of semitransparent and film-forming stains applied to the samples

Attosecond two-photon interferometry for doubly excited states of helium

We show that the correlation dynamics in coherently excited doubly excited resonances of helium can be followed in real time by two-photon interferometry. This approach promises to map the evolution of the two-electron wave packet onto experimentally easily accessible non-coincident single electron spectra. We analyze the interferometric signal in terms of a semi-analytical model which is validated by a numerical solution of the time-dependent two-electron Schr\"odinger equation in its full dimensionality.Comment: 5 pages, 4 figure

Tailored high-contrast attosecond electron pulses for coherent excitation and scattering

Temporally shaping the density of electron beams using light forms the basis for a wide range of established and emerging technologies, including free-electron lasers and attosecond electron microscopy. The modulation depth of compressed electron pulses is a key figure of merit limiting applications. In this work, we present an approach for generating background-free attosecond electron pulse trains by sequential inelastic electron-light scattering. Harnessing quantum interference in the fractional Talbot effect, we suppress unwanted background density in electron compression by several orders of magnitude. Our results will greatly enhance applications of coherent electron-light scattering, such as stimulated cathodoluminescence and streaking

Structural dynamics probed by high-coherence electron pulses

Ultrafast measurement technology provides essential contributions to our understanding of the properties and functions of solids and nanostructures. Atomic-scale vistas with ever-growing spatial and temporal resolution are offered by methods based on short pulses of x-rays and electrons. Time-resolved electron diffraction and microscopy are among the most powerful approaches to investigate nonequilibrium structural dynamics. In this article, we discuss recent advances in ultrafast electron imaging enabled by significant improvements in the coherence of pulsed electron beams. Specifically, we review the development and first application of ultrafast low-energy electron diffraction for the study of structural dynamics at surfaces, and discuss novel opportunities for ultrafast transmission electron microscopy facilitated by laser-triggered field-emission sources. These and further developments will render coherent electron beams an essential component in future ultrafast nanoscale imaging

Probing scattering phase shifts by attosecond streaking

Attosecond streaking is one of the most fundamental processes in attosecond science allowing for a mapping of temporal (i.e. phase) information on the energy domain. We show that on the single-particle level attosecond streaking time shifts contain spectral phase information associated with the Eisenbud-Wigner-Smith (EWS) time delay, provided the influence of the streaking infrared field is properly accounted for. While the streaking phase shifts for short-ranged potentials agree with the associated EWS delays, Coulomb potentials require special care. We show that the interaction between the outgoing electron and the combined Coulomb and IR laser fields lead to a streaking phase shift that can be described classically

Ramsey-type phase control of free-electron beams

Quantum coherent evolution, interference between multiple distinct paths and phase-controlled sequential interactions are the basis for powerful multi-dimensional optical and nuclear magnetic resonance3 spectroscopies, including Ramsey’s method of separated fields. Recent developments in the quantum state preparation of free electrons suggest a transfer of such concepts to ultrafast electron imaging and spectroscopy. Here, we demonstrate the sequential coherent manipulation of free-electron superposition states in an ultrashort electron pulse, using nanostructures featuring two spatially separated near-fields with polarization anisotropy. The incident light polarization controls the relative phase of these near-fields, yielding constructive and destructive quantum interference of the subsequent interactions. Future implementations of such electron–light interferometers may provide access to optically phase-resolved electronic dynamics and dephasing mechanisms with attosecond precision

Characteristics of Free Radicals in Wood

Formation and behavior of free radicals in wood exposed to fluorescent light, terrestrial sunlight, and artificial ultraviolet light have been studied by electron spin resonance (ESR) spectroscopy. Freshly cut loblolly pine specimens (with 69% MC) did not exhibit any detectable ESR signals, strongly implying that intrinsic free radicals may not exist in green wood. Free radicals were readily produced by interaction of wood with electromagnetic radiations. At ambient temperature, for green wood specimens, a large amount of short-lived free radicals was generated by sunlight, whereas a relatively low amount of longer lifetime free radicals was generated by fluorescent light. For air-dried specimens, all light sources used were able to generate free radicals either in air (oxygen) or under vacuum. A large quantity of free radical concentration was normally generated under vacuum but free radicals decayed rapidly at ambient temperature. The interaction of free radicals with oxygen and the decomposition and termination of free radicals leading to the discoloration reaction were considered

Nano-wires with surface disorder: Giant localization lengths and dynamical tunneling in the presence of directed chaos

We investigate electron quantum transport through nano-wires with one-sided surface roughness in the presence of a perpendicular magnetic field. Exponentially diverging localization lengths are found in the quantum-to-classical crossover regime, controlled by tunneling between regular and chaotic regions of the underlying mixed classical phase space. We show that each regular mode possesses a well-defined mode-specific localization length. We present analytic estimates of these mode localization lengths which agree well with the numerical data. The coupling between regular and chaotic regions can be determined by varying the length of the wire leading to intricate structures in the transmission probabilities. We explain these structures quantitatively by dynamical tunneling in the presence of directed chaos.Comment: 15 pages, 12 figure