Scattering Amplitudes Interpolating Between Instant Form and Front Form of Relativistic Dynamics

Among the three forms of relativistic Hamiltonian dynamics proposed by Dirac in 1949, the front form has the largest number of kinematic generators. This distinction provides useful consequences in the analysis of physical observables in hadron physics. Using the method of interpolation between the instant form and the front form, we introduce the interpolating scattering amplitude that links the corresponding time-ordered amplitudes between the two forms of dynamics and provide the physical meaning of the kinematic transformations as they allow the invariance of each individual time-ordered amplitude for an arbitrary interpolation angle. We discuss the rationale for using front form dynamics, nowadays known as light-front dynamics (LFD), and present a few explicit examples of hadron phenomenology that LFD uniquely can offer from first-principles quantum chromodynamics. In particular, model-independent constraints are provided for the analyses of deuteron form factors and the N Δ transition form factors at large momentum transfer squared Q ². The swap of helicity amplitudes between the collinear and non-collinear kinematics is also discussed in deeply virtual Compton scattering.     

Vibrational dynamics of the bending mode of water interacting with ions

We studied the vibrational relaxation dynamics of the bending mode (ν(2)) of the H(2)O water molecules in the presence of different salts (LiCl, LiBr, LiI, NaI, CsI, NaClO(4), and NaBF(4)). The linear and nonlinear spectra of the bending mode show distinct responses of water molecules hydrating the anions. We observe that the bending mode of water molecules that are hydrogen-bonded to an anion exhibits much slower relaxation rates (T(1)~1ps) than water molecules that are hydrogen-bonded to other water molecules (T(1)=400 fs). We find that the effect of the anion on the absorption spectrum and relaxation time constant of the water bending mode is not only determined by the strength of the hydrogen-bond interaction but also by the shape of the anion.     

Sensitivity of the DVCS Cross Section to the Compton form Factors in Scalar QED

Using deeply virtual Compton scattering as a tool to study the structure of hadrons in an exclusive process, one expresses the amplitudes in terms of invariant quantities: the Compton form factors. In this paper the sensitivity of the hadronic part of the cross section to the Compton form factors is determined.     

Background current elimination in thin layer ion-selective membrane coulometry

A promising method for the elimination of undesired capacitive currents in view of realizing a potentially calibration free coulometric ion detection system is presented. The coulometric cell is composed of a porous polypropylene tube doped with a liquid calcium-selective membrane and a silver/silver chloride wire as an inner electrode, forming a thin layer sample between wire and tubing. The total charge passed through the system during potential controlled electrolysis of the thin layer sample is indeed found to be proportional to the amount of calcium present, but non-Faradaic processes do contribute to the obtained signal. We introduce here a multi-pulse procedure that allows one to perform a second excitation pulse at the same excitation potential after exhaustive ion transfer voltammetry of calcium has taken place. The intercept of the calibration curve after background subtraction is found as 20.6 ± 0.6 μC, significantly lower than the value of 54.1 ± 0.8 μC for the uncorrected curve. Changes in sample temperature (from 23 °C to 38 °C) did equally not affect the background corrected coulometric readings, supporting that the procedure renders the readout principle more robust.     

Redox-Mediated Alcohol Oxidation Coupled to Hydrogen Gas Formation in a Dye-Sensitized Photosynthesis Cell

This work reports a dye-sensitized photoelectrochemical cell (DSPEC) that couples redox-mediated light-driven oxidative organic transformations to reductive hydrogen (H₂) formation. The DSPEC photoanode consists of a mesoporous anatase TiO₂ film on FTO (fluorine-doped tin oxide), sensitized with the thienopyrroledione-based dye AP11 , while H₂ was formed at a FTO-Pt cathode. Irradiation of the dye-sensitized photoanode transforms 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) to the oxidized TEMPO (TEMPO⁺), which acts as a chemical oxidant for the conversion of benzyl alcohol. The TEMPO^0/+ couple, previously used as redox mediator in DSSC, mediates efficient electron transfer from the organic substrate to the photo-oxidized dye. A DSPEC photoreactor was designed that allows in situ monitoring the reaction progress by infrared spectroscopy and gas chromatography. Sustained light-driven oxidation of benzyl alcohol to benzaldehyde within the DSPEC photoreactor, using of TEMPO as mediator, demonstrated the efficiency of the device, with a photocurrent of 0.4 mA cm⁻², approaching quantitative Faradaic efficiency and exhibiting excellent device stability.     

The combined effect of cations and anions on the dynamics of water

We studied the orientational relaxation of the OD-stretch vibration of HDO molecules in concentrated solutions of alkali-halide salts (NaCl, NaI, CsCl and KI) in isotopically diluted water (4% D(2)O in H(2)O), using polarization-resolved femtosecond infrared pump-probe spectroscopy (fs-IR). We were able to distinguish the orientational dynamics of the water molecules solvating the halide ions from the dynamics of the bulk water and the water solvating the cations. We found that the reorientation of the halide-bound molecules shows two strongly different components (2.0 ± 0.3 ps and 9 ± 1 ps), related to a wiggling motion of the OD group hydrogen-bonded to the anion, and rotational diffusion of the molecule over the charged anion surface, respectively. The relative amplitudes of the two components are dependent on the nature of both the anion and cation, and on the concentration. These results show that cations can have a profound effect on the solvation shell dynamics of their counter-ions.     

Vibrational spectroscopy of gas-phase neutral and cationic phenanthrene in their electronic groundstates

Various experimental methods are applied to retrieve the vibrational structure of phenanthrene in its neutral and cationic groundstates. The linear infrared (IR) absorption spectra in the 400-1650 cm(-1) range of jet-cooled phenanthrene and its cation, both clustered with either an argon or a neon atom, are obtained via photo-induced cluster dissociation spectroscopy. The spectra observed are in good agreement with calculated spectra of the bare species. However, the observed spectrum of cationic phenanthrene shows more lines and lines with different intensities in the 900-1400 cm(-1) range than expected from calculations. Additional spectra of the perdeuterated phenanthrene Ar cation, and the warm (T approximately > room temperature) bare phenanthrene cation are recorded. Also the mass-analyzed threshold ionization spectra of bare phenanthrene and phenanthrene-Ar are recorded and compared with each other. Comparison of the spectral data recorded to calculated spectra of bare neutral, cationic and cationic perdeuterated phenanthrene, as well as to IR spectra recorded in matrix-isolation experiments, explicitly demonstrates that cluster dissociation spectroscopy is a valid and powerful method to obtain IR spectroscopic information of bare neutral and cationic jet-cooled poly-aromatic hydrocarbons.     

Differences between eastern and western-type nuclear reactor pressure vessel steels as probed by Mössbauer spectroscopy

Mössbauer spectra (MS) at room temperature have been collected for non-irradiated Eastern- and Western-type nuclear reactor pressure vessel (RPV) steels. All samples showed a typical Mössbauer spectrum for steels with a low alloy-element concentration. Analysis with distributed hyperfine parameters revealed that the spectra consist of two magnetically split subspectra and that only for the Western-type RPV steels a small doublet is present. The analysis of the resulting H_hf-distribution profiles showed that for the Eastern-type steels the relative area for the ''perturbed'' component is more pronounced, and that it has a more complex structure than the corresponding profile for the Western-type steels. The additional doublet present in the MS of the Western-type steels could be assigned to Mn and/or Cr-substituted cementite, while no carbide doublet was observed for the Eastern-type RPV steel, Cr₂₃C₆, Cr₇C₃ and VC being the principal carbides. The distinctions between the two types of steel are due to compositional differences. The results further show that Mössbauer spectroscopy is sensitive to small changes in composition and hence is capable of distinguishing between different types of steel.