High-resolution dissociative recombination of cold H3+and first evidence for nuclear spin effects

The energy-resolved rate coefficient for the dissociative recombination (DR) of H(3)(+) with slow electrons has been measured by the storage-ring method using an ion beam produced from a radiofrequency multipole ion trap, employing buffer-gas cooling at 13 K. The electron energy spread of the merged-beams measurement is reduced to 500 microeV by using a cryogenic GaAs photocathode. This and a previous cold- measurement jointly confirm the capability of ion storage rings, with suitable ion sources, to store and investigate H(3)(+) in the two lowest, (J,G) = (1,1) and (1,0) rotational states prevailing also in cold interstellar matter. The use of para-H(2) in the ion source, expected to enhance para-H(3)(+) in the stored ion beam, is found to increase the DR rate coefficient at meV electron energies.     

Size-dependent electron-impact detachment of internally cold Cn- and Aln- clusters

The cross sections for electron detachment of internally cold Cn- and Aln- clusters were measured using an electrostatic ion beam trap fitted with an internal electron target. The measured electron-impact detachment cross sections for the Cn- (n = 1-9) clusters exhibit even-odd oscillations reflecting the binding energy trend, namely, higher cross sections for weaker binding. Surprisingly, however, these cross sections increase on the average with cluster size, n, in spite of the increase in electron binding. In contrast, the Aln- (n = 2-5) clusters follow the known scaling laws for electron detachment. We suggest that the size-dependent polarizability of these clusters is responsible for the observed behavior.     

Electrical-thermal switching in carbon-black-polymer composites as a local effect

Following the lack of microscopic information about the intriguing well-known electrical-thermal switching mechanism in carbon-black-polymer composites, we applied atomic force microscopy in order to reveal the local nature of the process and correlated it with the characteristics of the widely used commercial switches. We conclude that the switching events take place in critical interparticle tunneling junctions that carry most of the current. The macroscopic switched state is then a result of a dynamic-stationary state of fast switching and slow reconnection of the corresponding junctions.     

Effects of gamma irradiation on the shoot length of Cicer seeds

The effects of radiation on the shoot and root lengths of germinated seedling of irradiated seeds of Cicer species, i.e. three kabuli types and four desi types of cultivated chickpea (Cicer arietinum Ladiz.) and 2 annual wild types (C. reticulatum Ladiz. and C. bijugum K.H. Rech.) were investigated. The seeds were irradiated with a ⁶⁰Co gamma source using 0, 200, 300 and 400 Gy doses at 1.66 kGy h⁻¹. At 200 Gy minor effects could be observed, but at 400 Gy an obvious depression of shoot length was observed. The kabuli types were more affected than the desi ones. The critical dose that prevented the shoot and root elongation varied among species and also ranged from genotypes to genotype within species.     

The YN interactions and K− reactions on deuterium at low energies

The K^?d → π^?Λp reaction, as well as the other three-body K^? reactions on deuterium, are considered for s-wave kaons in the Faddeev formalism for charge-independent separable two-body coupled-channel interactions fitted to the available low-energy data. In particular we study the effect of a hypothetical $\text{I =}\text{1}\text{2}\text{, S = 1, Σ}\text{N}$ unstable bound state near the πΣN threshold on the shape of the calculated Λp invariant mass distribution near the ΣN threshold. This calculated distribution always exhibits a cusp behavior at the ΣN threshold, but considerable structure and broadening may be reached by varying the parameters of the YN interaction. Multiple scatterings in the initial K?NN and intermediate πΣN channels play a dominant role at low energy. The calculation compares favorably with the measurements at rest and at low energies. This comparison suggests that a ΣN bound state does not exist near the ΣN threshold.     

Processing and compressive response of Al/SiC functionally graded composites

The European Physical Journal Plus - The nonlocal defocusing nonlinear Schrödinger (ND-NLS) equation is comparatively studied via the Riemann–Hilbert approach. Firstly, via spectral...     

On the Exciton Coupling between Two Chlorophyll Pigments in the Absence of a Protein Environment: Intrinsic Effects Revealed by Theory and Experiment

Exciton coupling between two or more chlorophyll (Chl) pigments is a key mechanism associated with the color tuning of photosynthetic proteins but it is difficult to disentangle this effect from shifts that are due to the protein microenvironment. Herein, we report the formation of the simplest coupled system, the Chl a dimer, tagged with a quaternary ammonium ion by electrospray ionization. Based on action spectroscopic studies in vacuo, the dimer complexes were found to absorb 50–70 meV to the red of the monomers under the same conditions. First‐principles calculations predict shifts that somewhat depend on the relative orientation of the two Chl units, namely 50 and 30 meV for structures where the Chl rings are stacked and unstacked, respectively. Our work demonstrates that Chl association alone can produce a large portion of the color shift observed in photosynthetic macromolecular assemblies.     

Photodissociation pathways and lifetimes of protonated peptides and their dimers

Photodissociation lifetimes and fragment channels of gas-phase, protonated YA(n) (n = 1,2) peptides and their dimers were measured with 266 nm photons. The protonated monomers were found to have a fast dissociation channel with an exponential lifetime of ~200 ns while the protonated dimers show an additional slow dissociation component with a lifetime of ~2 μs. Laser power dependence measurements enabled us to ascribe the fast channel in the monomer and the slow channel in the dimer to a one-photon process, whereas the fast dimer channel is from a two-photon process. The slow (1 photon) dissociation channel in the dimer was found to result in cleavage of the H-bonds after energy transfer through these H-bonds. In general, the dissociation of these protonated peptides is non-prompt and the decay time was found to increase with the size of the peptides. Quantum RRKM calculations of the microcanonical rate constants also confirmed a statistical nature of the photodissociation processes in the dipeptide monomers and dimers. The classical RRKM expression gives a rate constant as an analytical function of the number of active vibrational modes in the system, estimated separately on the basis of the equipartition theorem. It demonstrates encouraging results in predicting fragmentation lifetimes of protonated peptides. Finally, we present the first experimental evidence for a photo-induced conversion of tyrosine-containing peptides into monocyclic aromatic hydrocarbon along with a formamide molecule both found in space.