Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3.

In this study we report on the investigation of epitaxially grown Sb2Te3 by employing Fourier-Transform transmission Spectroscopy (FTS) with laser-induced Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) spectral range. Static spectra in the range between 20 and 120 cm-1 highlight a peculiar softening of an in-plane IR-active phonon mode upon temperature decrease, as opposed to all Raman active modes which instead show a hardening upon temperature decrease in the same energy range. The phonon mode softening is found to be accompanied by an increase of free carrier concentration. A strong coupling of the two systems (free carriers and phonons) is observed and further evidenced by exciting the same phonon mode at 62 cm-1 within an ultrafast pump-probe scheme employing a femtosecond laser as pump and a CSR single cycle THz pulse as probe. Separation of the free carrier contribution and the phonon resonance in the investigated THz range reveals that, both damping of the phonon mode and relaxation of hot carriers in the time domain happen on the same time scale of 5 ps. This relaxation is about a factor of 10 slower than expected from the Lorentz time-bandwidth limit. The results are discussed in the framework of phonon scattering at thermal and laser induced transient free carriers.

Probing the non-equilibrium transient state in magnetite by a jitter-free two-color X-ray pump and X-ray probe experiment.

We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 ± 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices.

X-ray spectroscopy on the active ion in laser crystals.

The active ions in typical laser crystals were studied with Resonant Inelastic X-ray Scattering (RIXS) and Partial Fluorescence Yield X-ray Absorption (PFY-XAS) spectroscopies as solid state model systems for dilute active centers. We analyzed Ti3+ and Cr3+ in α-Al2O3:Ti3+ and LiCaAlF6:Cr3+, respectively. The comparison of experimental data with semi-empirical multiplet calculations provides insights into the electronic structure and shows how measured crystal field energies are related across different spectroscopies.

Equilibration dynamics and conductivity of warm dense hydrogen.

We investigate subpicosecond dynamics of warm dense hydrogen at the XUV free-electron laser facility (FLASH) at DESY (Hamburg). Ultrafast impulsive electron heating is initiated by a ≤ 300-fs short x-ray burst of 92-eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variable time delay. We show that the initial molecular structure dissociates within (0.9 ± 0.2) ps, allowing us to infer the energy transfer rate between electrons and ions. We evaluate Saha and Thomas-Fermi ionization models in radiation hydrodynamics simulations, predicting plasma parameters that are subsequently used to calculate the static structure factor. A conductivity model for partially ionized plasma is validated by two-temperature density-functional theory coupled to molecular dynamic simulations and agrees with the experimental data. Our results provide important insights and the needed experimental data on transport properties of dense plasmas.

Electron rearrangement dynamics in dissociating I(2)

The charge rearrangement in dissociating I_{2}^{n+} molecules is measured as a function of the internuclear distance R using extreme ultraviolet pulses delivered by the free-electron laser in Hamburg. Within an extreme ultraviolet pump-probe scheme, the first pulse initiates dissociation by multiply ionizing I_{2}, and the delayed probe pulse further ionizes one of the two fragments at a given time, thus triggering charge rearrangement at a well-defined R. The electron transfer between the fragments is monitored by analyzing the delay-dependent ion kinetic energies and charge states. The experimental results are in very good agreement with predictions of the classical over-the-barrier model demonstrating its validity in a thus far unexplored quasimolecular regime relevant for free-electron laser, plasma, and chemistry applications.

Resolving ultrafast heating of dense cryogenic hydrogen.

We report on the dynamics of ultrafast heating in cryogenic hydrogen initiated by a ≲300 fs, 92 eV free electron laser x-ray burst. The rise of the x-ray scattering amplitude from a second x-ray pulse probes the transition from dense cryogenic molecular hydrogen to a nearly uncorrelated plasmalike structure, indicating an electron-ion equilibration time of ∼0.9 ps. The rise time agrees with radiation hydrodynamics simulations based on a conductivity model for partially ionized plasma that is validated by two-temperature density-functional theory.

Ultrafast spin transport as key to femtosecond demagnetization.

Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer. A demagnetization effect corresponding to the scenario in which the laser directly excites the Ni film is observed, but with a slight temporal delay. We explain this unexpected observation by means of the demagnetizing effect of a superdiffusive current of non-equilibrium, non-spin-polarized electrons generated in the Au layer.

Spatio-temporal coherence of free electron laser pulses in the soft x-ray regime.

The temporal coherence properties of soft x-ray free electron laser pulses at FLASH are measured at 23.9 nm by interfering two time-delayed partial beams directly on a CCD camera. The partial beams are obtained by wave front beam splitting in an autocorrelator operating at photon energies from h nu = 30 to 200 eV. At zero delay a visibility of (0.63+/- 0.04) is measured. The delay of one partial beam reveals a coherence time of 6 fs at 23.9 nm. The visibility further displays a non-monotonic decay, which can be rationalized by the presence of multiple pulse structure.

Femtosecond modification of electron localization and transfer of angular momentum in nickel.

The rapidly increasing information density required of modern magnetic data storage devices raises the question of the fundamental limits in bit size and writing speed. At present, the magnetization reversal of a bit can occur as quickly as 200 ps (ref. 1). A fundamental limit has been explored by using intense magnetic-field pulses of 2 ps duration leading to a non-deterministic magnetization reversal. For this process, dissipation of spin angular momentum to other degrees of freedom on an ultrafast timescale is crucial. An even faster regime down to 100 fs or below might be reached by non-thermal control of magnetization with femtosecond laser radiation. Here, we show that an efficient novel channel for angular momentum dissipation to the lattice can be opened by femtosecond laser excitation of a ferromagnet. For the first time, the quenching of spin angular momentum and its transfer to the lattice with a time constant of 120+/-70 fs is determined unambiguously with X-ray magnetic circular dichroism. We report the first femtosecond time-resolved X-ray absorption spectroscopy data over an entire absorption edge, which are consistent with an unexpected increase in valence-electron localization during the first 120+/-50 fs, possibly providing the driving force behind femtosecond spin-lattice relaxation.

Femtosecond undulator radiation from sliced electron bunches.

At the 1.7-GeV electron storage ring BESSY II, a first source of synchrotron radiation with 100 fs pulse duration, variable (linear and circular) polarization, tunable photon energy (300 to 1400 eV), and excellent signal-to-background ratio was constructed and is now in routine operation.

Femtosecond terahertz radiation from femtoslicing at BESSY.

Femtosecond far-infrared radiation pulses in the THz spectral range were observed as a consequence of the energy modulation of 1.7 GeV electrons by femtosecond laser pulses in the BESSY storage ring in order to generate femtosecond x-ray pulses ("femtoslicing"). In addition to being crucial for diagnostics of the laser-electron interaction, the THz radiation itself is useful for experiments where intense ultrashort THz pulses of well-defined temporal and spectral characteristics are required.

Investigation on the photoreactions of nitrate and nitrite ions with selected azaarenes in water

The photoreactions of selected azaarenes with nitrate and nitrite ions were investigated under irradiation at lambda = 313 nm. The excitation of both anions leads to several photochemical reactions forming mainly hydroxyl radicals and nitrogen oxides. The purification capability of natural waters i.e. the oxidation of inorganic and organic substances results from the formation of hydroxyl radicals. Nitrated isomers of azaarenes were found among the main products of the investigated photoreactions. The nitrogen oxides were responsible for the production of nitrated derivatives which possess a high toxic potential. Their formation was explained by the parallel occurance of two mechanism, a molecular and a radical one. The molecular mechanism became more important with increasing ionisation potentials of the azaarenes. The spectrum of oxidized products corresponded to the one got in the photoreactions of azaarenes with hydrogen peroxide. The formation of several oxidation and nitration products of the pyridine ring with its low electron density was explained by the reaction of excited states of azaarenes. The photoreactions with nitrite ions only led to the formation of oxidized and nitrated products. Nitroso products were not formed. The reactivity of nitrogen monoxide is too low for its reaction with the azaarenes.

Fourier Transform Emission Spectroscopy of the A2Pi-X2Sigma+ Transition of BeD.

The A2Pi-X2Sigma+ Deltav = 0 sequence of BeD was observed in the 19 500-20 800 cm-1 spectral region using a Fourier transform spectrometer. The emission spectrum was excited in a Be hollow cathode discharge lamp with a He/D2 gas mixture. The observed lines were assigned to the 0-0 to 6-6 bands. The Deltav = -1 sequence was too weak to be seen in our Fourier transform spectra. We therefore used a previously recorded but unpublished arc emission spectrum to identify the 0-1 to 5-6 Deltav = -1 bands. Consequently, all of the diagonal bands could be linked together and the vibrational intervals determined. The Deltav = 0 and Deltav = -1 data were fitted together in a global fit and effective constants derived. Using the information available from the study of the C2Sigma+-X2Sigma+ system [R. Colin, C. Drèze, and M. Steinhauer, Can. J. Phys. 61, 641 (1983)], the v = 8-12 vibrational levels of the ground state were added in a Dunham fit. A set of Dunham Y constants was determined for the X2Sigma+ state along with traditional equilibrium parameters for the A2Pi excited state. The equilibrium bond lengths were found to be 1.341742(5) Å for the ground state and 1.33309(4) Å for the excited state. A reanalysis of the previously published A2Pi-X2Sigma+ 0-0 to 3-3 Deltav = 0 bands of BeT [D. De Greef and R. Colin, J. Mol. Spectrosc. 53, 455-465 (1974)] was also performed. Copyright 1998 Academic Press.

[Potential cardiotonic agents. 8. 2-acyloxyalkylamino-, 2-acyloxyalkoxy-, and 2-acylaminoalkylamino-3-cyan-5-(pyrid-4-yl) pyridine]. / Potentielle Kardiotonika. 8. Mitteilung: 2-Acyloxyalkylamino-, 2-Acyloxyalkoxy- und 2-Acylaminoalkylamino-3-cyan-5-(pyrid-4-yl) pyridine.

By acylation of our previously described cardiotonic active 2-hydroxyalkylamino, 2-hydroxyalkoxy, 2-aminoalkyl-amino and 2-piperazino substituted 3-cyano-5-(4-pyridinyl) pyridines with acetic anhydride, propionic anhydride or aroyl and heteroaroyl chlorides, respectively, the corresponding in position 2 O- or N-acylated 3-cyano-5-(4-pyridinyl)pyridines were prepared. Cardiovascular activity of the obtained derivatives is discussed in comparison with that of the parent compounds.

[Potential cardiotonics. 7. Synthesis and cardiovascular effects of 5-(4-pyridinyl)-, 6-methyl-4-(4-pyridinyl) and 6-methyl-5-phenyl-substituted 3-cyano-2-aminoalkylamino-pyridines]. / Potentielle Kardiotonika. 7. Mitteilung5: Darstellung und kardiovaskuläre Eigenschaften von 5-(pyrid-4-yl)-, 6-methyl-5-(pyrid-4-yl)- und 6-methyl-5-phenyl-substituierten 2-Aminoalkylamino-3-cyan-pyridinen.

The title compounds were synthesized by treating of 2-chloro-pyridines 1-3 with the appropriate aminoalkylamines or piperazines. In isolated guinea pig atria some compounds showed greater positive inotropic activity than amrinone. Heart rate was decreased or remained unchanged. In anesthetized dogs some derivatives exerted a dose-dependent increase in myocardial contractility and, additionally, a decrease in blood pressure.

[Potential cardiotonic agents. 6. Synthesis and cardiovascular properties of 5-(4-pyridinyl)-, 6-methyl-5-(4-pyridinyl)- and 6-methyl-5-phenyl-substituted 3-cyano-2-oxaalkyoxy-pyridines]. / Potentielle Kardiotonika. 6. Mitteilung: Darstellung und kardiovaskuläre Eigenschaften von 5-(pyrid-4-yl)-, 6-methyl-5-(pyrid-4-yl)- und 6-methyl-5-phenylsubstituierten 3-Cyan-2-oxaalkoxy-pyridinen.

The headline compounds were prepared by the reaction of 2-chloropyridines 1-3 with the appropriate alcohols in presence of potassium hydroxide and the sodium alkoxides, respectively. Especially some of the 3-cyano-2-hydroxyalkoxy-5-(4-pyridinyl)pyridines showed remarkable positive inotropic potency and, additionally, a vasodilator activity. In spontaneously beating isolated guinea pig atria they had a greater activity than amrinone.

[Potential cardiotonic agents. 1. Synthesis and pharmacologic properties of 3-cyano-2-oxaalkylamino-5-(4-pyridinyl)pyridines]. / Potentielle Kardiotonika. 1. Mitteilung: Darstellung und pharmakologische Eigenschaften von 3-Cyan-2-oxaalkylamino-5-(pyrid-4-yl)pyriden.

Synthesis, physicochemical properties and evaluation of positive inotropic and vasodilator activities are described in a series of substituted 2-amino-3-cyano-5-(4-pyridinyl)pyridines. Some of the 2-oxaalkylamino derivatives showed remarkable positive inotropic activities and, additionally, a decrease in blood pressure. The most potent compounds 11 and 13 have a greater activity than amrinone.

[Potential cardiotonic agents. 2. Synthesis and pharmacologic properties of 5-(pyri-4-yl)- and 5-phenyl substituted 3-cyano-6-methyl-2-oxaalkylaminopyridines]. / Potentielle Kardiotonika. 2. Mitteilung: Synthese und pharmakologische Eigenschaften von 5-(pyrid-4-yl)- und 5-phenylsubstituierten 3-Cyan-6-methyl-2-oxaalkylamino-pyridinen.

The authors describe the preparation, the physicochemical properties and the results of evaluation of positive inotropic and vasodilator activities in a series of 5-(4-pyridinyl)- and 5-phenyl-substituted 3-cyano-6-methyl-2-oxaalkylamino-pyridines. Some of the compounds are comparable in their positive inotropic potency to that of amrinone and cause, additionally, a decrease in blood pressure.