Volume 52, Issue 12

The dependence of fluorescence quantum yields and S 1 -lifetimes of donor-acceptor substituted trans-stilbenes on temperature was measured in the temperature range from 298 to 100 K, using solutions of stilbenes in 3-methylpentane (3-MP). Measurements of fluorescence and phosphorescence spectra show that the triplet energy is almost independent of the acceptor. The S 1 -energy and the S 1 -T 1 energy gap decrease with increasing acceptor strength. For all compounds intersystem crossing (ISC) S 1 → T 1 , is negligible

Jefimenko gave recently [1] new expressions for the electric and magnetic potentials of uniformly moving, time-independent charge distributions. We discuss these potentials for uniformly accelerated distributions. As Jefimenko did. we implement two procedures either converting directly retarded into present position integrals or using relativistic transformations for a stationary charge in an instantaneous comoving inertial frame (Frenet-Serret tetrad). We discuss why, at the difference of what happens for uniform motions [1], both procedures provide different expressions for potentials.

The results of an electron paramagnetic resonance (EPR) study of vanadyl ion VO 2+ doped in single crystals of Ba(ClO 4 ) 2 - 3 H 2 O are reported. The spectra indicated the presence of two substitutional and interstitial sites depending on the orientation. The spin-Hamiltonian parameters were determined from single crystal and powder EPR data. These parameters were found to be axial symmetric for the powder sample, whereas they showed deviations from axial symmetry for the single crystal

The atomic structure of poly-molybdate ions formed in acid Ni-Mo aqueous solutions has been determined by applying anomalous X-ray scattering (AXS) and EXAFS methods. In a solution containing only molybdenum ions, we found only a poly-molybdate ion consisting of seven edgesharing MoO 6 octahedra. In a solution containing both Mo and Ni ions, there exists another polymolybdate ion consisting of 6 edge-sharing MoO 6 surrounding an Ni ion. The total coordination number of Mo and Ni ions around a molybdenum ion is reduced by half when introducing citric ions into the Ni and Mo solution. This indicates that the large poly-molybdate ion is decomposed to smaller molybdate ions when forming citric complexes. Since molybdenum-nickel alloys can not be electrodeposited from solutions without citric ions, we propose a view that the structural change of the poly-molybdate ions in the solutions is closely related with the mechanism of induced codeposition of molybdenum and nickel alloy. The results of small-angle X-ray scattering (SAXS) measurements also support this conclusion.

The dynamics of intercalated water molecules in Li-saponite was studied by measurement of solid state 2 H and 7 Li NMR spectra and of the 2 H spin-lattice relaxation time at 175 ~ 350 K. Only a single component was observed in the 2 H spectra above 260 K, suggesting that the water hydrogens rapidly exchange their positions between various distinct environments. Analysis of the observed spectra suggests that the water molecules possess C 2 rotational freedom at around 260 K and that the hydration shell around Li + cations is highly symmetrical in the same temperature region

A system of Class Q q is a polygonal system consisting of one central q-gon circumscribed by q other polygons, which may possess different sizes. As chemical graphs, these systems represent polycyclic conjugated hydrocarbons, which include homologous series of C 20 H 10 corannulene and C 24 H 12 coronene along with many other molecules of interest in organic chemistry. The problem of isomer enumeration for the Qq systems is solved completely in terms of generating functions by means of Póya's theorem

We investigated the nuclear quadrupole coupling of the 11 B and 10 B nuclei in fluorodihydroxy borane, BF(OH) 2 . An analysis of the hyperfine splittings resulted in the coupling constants % aa = -1.414(11) MHz, % bb = - 1.206(11) MHz, % cc = 2.620(11) MHz for 11 B and % aa = -2.872(67) MHz, y M = -2.525(69) MHz, %cc = 5.397(69) MHz for 10 B. From these constants the ratio r of the quadrupole moments r = Q( 10 B)/Q( 11 B) = 2.051 (44) could be derived and compared to data taken from the literature

This paper reviews calculations of the electromagnetic properties of baryons using the constituent quark model. We start with a short discussion of spontaneous chiral symmetry breaking, which is essential in understanding the transition from QCD to the constituent quark model. We then discuss a chiral version of the constituent quark model, which simulates the symmetries and dynamical content of the underlying field theory in terms of gluon, pion and sigma exchange between constituent quarks. We show that the electromagnetic current charge and current operators, usually approximated by one-body operators (impulse approximation), must be supplemented by appropriate two-body terms (exchange currents). The latter represent the gluon and pion exchange degrees of freedom in the electromagnetic current operator. These exchange currents must be included for reasons of completeness and consistency. Most importantly, however, they are needed in order for the electromagnetic current to be conserved. We also study the effect of scalar exchange currents connected with the confinement and sigma exchange potentials. By including these twobody exchange currents we go beyond the single-quark impulse approximation, which has mainly been used up to now. The inclusion of gluon- pion-, and scalar-exchange currents in the quark potential model is the new point of the present work. We show that for some observables, such as the magnetic moments, charge, and magnetic radii of the proton and charged Δ (1232) states, exchange currents contribute at the level of some 10%. The same holds true for the magnetic moments of the entire baryon octet, with the exception of the Ξ - magnetic moment. On the other hand, the neutron charge radius, the quadrupole moments of the Δ, and the N → Δ transition quadrupole moment, are dominated by pion and gluon exchange contributions to the charge density operator. The inclusion of the pion and gluon exchange currents leads to a neutron charge radius of the correct size and sign. Based on the gluon and pion exchange current diagrams, we derive parameter-free relations between the neutron charge radius, the quadrupole moment of the Δ, and the N → Δ transition quadrupole moment. Neglecting configuration mixing, we find that the neutron charge radius and the N → Δ transition quadrupole moment are simply related as Q N→Δ = r 2 n √2. The implications of Siegert's theorem for the calculation of the E2 form factor in the N → Δ transition are studied. Finally, we discuss the axial coupling constant of the nucleon. We show that the inclusion of axial pair exchange currents does not significantly alter the NRQM prediction.