On the Connection between Particles and Poles of the S-Matrix

In axiomatic S-matrix theory it is usually assumed that stable particles give rise to simple poles of the S-matrix for real negative energies while unstable particles give rise to poles close to the real axis on an unphysical sheet of the energy Riemann surface. The stable particle — pole association has been known for a long time not to be always true. For example in potential scattering what is relevant in this case in fact is not the S-matrix but the Jost function. The zeroes of this function for real negative energies are in fact in one-to-one correspondence with the bound states, while the correspondence may break down for the poles of the S-matrix. On the other hand it has recently been pointed out that there also is in general no connection between unstable particles and poles of the S-matrix.     

A general method for the existence of periodic solutions of differential systems in the plane

We propose a general method to prove the existence of periodic solutions for planar systems of ordinary differential equations, which can be used in many different circumstances. Applications are given to some nonresonant cases, even for systems with superlinear growth in some direction, or with a singularity. Systems “at resonance” are also considered, provided a Landesman–Lazer type of condition is assumed.     

Time-resolved study of the oxidation of ethanol by cerium(IV) using combined quick-XANES, UV-vis, and Raman spectroscopies

Time-resolved X-ray absorption spectroscopy (Quick-XANES) has been combined with UV-vis and Raman spectroscopies to study the in situ reduction of Ce4+ to Ce3+ in ethanolic solution with a time resolution of ca. 4-5 s. For this purpose, a cam-driven oscillating double-crystal monochromator with a channel-cut crystal was combined with two spectrometers for UV-vis and Raman spectroscopies in a specialized cell which allows one to fit the optical pathways for all three spectroscopies individually. The results show that high-quality results can be obtained simultaneously, thus giving a detailed insight into the mechanisms of the investigated chemical reaction. The continuous release of nitrate and ethanol ligands from the initial Ce4+ into the solution finally leads to a trivalent cerium species which is only coordinated with water molecules after about 1800 s of reaction time.     

The SAMBA quick‐EXAFS monochromator: XAS with edge jumping

Results and performances of the QEXAFS double monochromator of the SAMBA beamline (Synchrotron SOLEIL) are presented. The device is capable of speeds of up to 40 Hz, while giving the user the possibility to choose the amplitude of the scan from 0.1° to 4° in a few seconds. The device is composed of two independent units and it is possible to perform scans alternating between two different crystals, literally jumping from low (4 keV) to high (37 keV) energies.     

Surface science approach to the solid-liquid interface: surface-dependent precipitation of Ni(OH)2 on α-Al2O3 surfaces

Surface-dependent precipitation: The adsorption of Ni(II) complexes in aqueous solution on (0001) and (1102) α-Al(2)O(3) single-crystal surfaces has been studied (see the X-ray absorption spectra obtained for parallel and perpendicular polarization directions). The use of planar model systems emphasizes the crucial role of the Al(2)O(3) orientation for Ni dispersion with practical implications in catalyst preparation procedures.     

Combined X-ray absorption and X-ray diffraction under high pressure

ABSTRACT

X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are two complementary structural techniques. Their combination improves the understanding of the effect of pressure on materials as illustrated by examples taken from studies on different types of materials (semiconductors, molecular solid, ferroelectric perovskite and gas mixture). The introduction of nanopolycrystalline diamonds anvils has extended XAS to high-energy edges with the possibility to use energy-scanning XAS beamlines where XRD can be performed in addition to XAS experiments.     

Delayed fluorescence as a measure of nutrient limitation in Dunaliella tertiolecta

The applicability of the delayed fluorescence (DF) for the purpose of distinguishing the cells growing in different nutrient conditions was researched on the marine unicellular algae Dunaliella tertiolecta Butcher (Chlorophyta). The DF intensity (DFI), as a measure of living algal biomass, was compared with other biomass measures--the cell concentration, chlorophyll a and fluorescence. The photosynthetic activity index (PhAI), a non-dimensional physiological index of photosynthesis calculated from a combination of DFI and F(0) was introduced. The nitrogen deprivation was indicated by more than 50% drop of PhAI. DF decay kinetics was measured with two different illuminations (<600 nm and >650 nm). The measured curves were divided and the resulting peak utilized for the differentiation among nutrient conditions. DF decay kinetics of D. tertiolecta differed among the cells growing in various nutrient conditions, indicating changes in the photosynthesis physiology.