Magnetic field dependence of photo-CIDNP MAS NMR on photosynthetic reaction centers of Rhodobacter sphaeroides WT

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in frozen and quinone depleted photosynthetic reaction centers of the purple bacteria Rhodobacter sphaeroides wild type (WT) by (13)C solid-state NMR at three different magnetic fields. All light-induced signals appear to be emissive at all three fields. At 4.7 T (200 MHz proton frequency), the strongest enhancement of NMR signals is observed, which is more than 10 000 above the Boltzmann polarization. At higher fields, the enhancement factor decreases. At 17.6 T, the enhancement factor is about 60. The field dependence of the enhancement appears to be the same for all nuclei. The observed field dependence is in line with simulations that assume two competing mechanisms of polarization transfer from electrons to nuclei, three-spin mixing (TSM) and differential decay (DD). These simulations indicate a ratio of the electron spin density on the special pair cofactors is 3:2 in favor of the L-BChl during the radical cation state. The good agreement of simulations with the experiments raises expectations that artificial solid reaction centers can be tuned to show photo-CIDNP in the near future.     

Pattern pulses: design of arbitrary excitation profiles as a function of pulse amplitude and offset

A novel class of pulses is presented which can be regarded as a generalization of both frequency-selective pulses and B1-selective pulses. The excitation profile of these pulses forms a pre-defined pattern in two dimensions, which are spanned by pulse offset and radio-frequency (RF) amplitude. The presented pulses were designed numerically based on principles of optimal control theory. For simple test patterns, we demonstrate the flexibility of this approach by simulations and experiments. This previously unknown flexibility may trigger novel applications in NMR spectroscopy and imaging. As a first practical application, we demonstrate a direct approach for calibrating RF pulses.     

Energetic particles emitted from energetic nuclear reactions

Protons and pions emitted with extreme momenta from energetic proton and heavy ion induced nuclear reactions are analysed in terms of two simple phenomenological models: the nuclear phase space model and a simplified multiple collision model. The systematic analysis of the observed spectra over many orders of magnitude for a variety of projectile and target combinations in the beam-energy range of 0.08 to 2 Gev/nucl shows the importance of multiple collision contributions and the neccessity of off-shell scattering effects.     

Temperaturabhängigkeit der anomalen Elektronenabsorption von Wismut-Einkristallen

Polycrystalline bismuth foils could be transformed into single crystal plates by a tempering process. Using the so prepared single crystals of high atomic number and of known thickness the temperature dependence of a dynamical two-beam system for the diffraction of 80 keV electrons has been investigated between 300 K and 80 K. The measured intensities of the (¯2 20)-reflection and the primary beam were compared with the transparency of the polycrystalline foils. They increase exponentially with decreasing temperature and thickness of the crystal. This experimental behaviour can be explained by a dynamical electron-diffraction-theory.     

A measurement of the KL-KS mass difference from the charge asymmetry in semi-leptonic kaon decays

The change asymmetry in semi-leptonic kaon decays has been measured as a function of the kaon lifetime. High statistics data of K_e3⁰ and K_μ3⁰ decay modes agree with each other and with the general expectation of the CP violation phenomenology together with the ΔS?ΔQ rule. The K_L-K_S mass difference obtained is Δm = (0.533 ± 0.004) × 10¹⁰ s^?1.     

Austauschwechselwirkungen,Intrabandschwingungen und Ti3+-Kristallfeldaufspaltung in dotiertem Al2O3

The absorption spectra of ruby with various Cr concentrations have been studied in the wavelength region between 50 μ and 1 mm at liquid helium and nitrogen temperatures. At liquid helium temperatures two strong absorptions have been found at 37.7 and 106 cm^?1 and weak absorptions at 18.8, 30.7, 33.2 and 43.0 cm^?1. These lines depend on the Cr concentration and vanish at N₂ temperatures. The two strong absorptions have also been observed in Al₂O₃ crystals doped with Ti or V. Sapphire did not show any of these absorptions. The origin of the two strong absorptions in Al₂O₃ crystals doped with Ti, V or Cr are discussed by consideration of magnetic dipole transitions of exchange coupled Cr³⁺ - pairs, the crystal field splitting of Ti³⁺ impurities and impurity — induced lattice modes. The weak absorption lines, on the other hand, may be assigned to transitions between the exchange levels of the second-nearest and fourth-nearest Cr³⁺ pairs if one makes use of the data obtained from the EPR and fluorescence spectra. For the second-nearest Cr³⁺ pairs group theory of the exchange interaction predicts transitions withΔS=2, whereas for the fourth-nearest neighbours no restriction forΔS exists.     

An Iron Complex with a Bent,O‐Coordinated CO2 Ligand Discovered by Femtosecond Mid‐Infrared Spectroscopy

The activation of carbon dioxide by transition metals is widely recognized as a key step for utilizing this greenhouse gas as a renewable feedstock for the sustainable production of fine chemicals. However, the dynamics of CO₂ binding and unbinding to and from the ligand sphere of a metal have never been observed in the time domain. The ferrioxalate anion is used in aqueous solution as a unique model system for these dynamics and femtosecond UV‐pump mid‐infrared‐probe spectroscopy is applied to explore its photoinduced primary processes in a time‐resolved fashion. Following optical excitation, a neutral CO₂ molecule is expelled from the complex within about 500 fs to generate a highly intriguing pentacoordinate ferrous dioxalate that carries a bent carbon dioxide radical anion ligand, that is, a reductively activated form of CO₂, which is end‐on‐coordinated to the metal center by one of its two oxygen atoms.