NMRON spin echo in a slowly varying magnetic field

Nuclear spin-spin relaxation of⁶⁰Co and⁵⁶Co in iron single crystals has been studied, using the three-pulse NMRON spin echo. A previously reported rapidT ₂ in⁶⁰CoFe is shown to have arisen from a modulation of the echo amplitude, caused by variations in the phase of the Larmor precession relative to the applied rf field. A lower limit ofT ₂∼0.2s is found in⁵⁶Co⁵⁶ Fe. Extension of this result to other CoFe samples is discussed.     

Fusion reactions with the one-neutron halo nucleus (15)C

The structure of (15)C, with an s(1/2) neutron weakly bound to a closed-neutron shell nucleus (14)C, makes it a prime candidate for a one-neutron halo nucleus. We have for the first time studied the cross section for the fusion-fission reaction (15)C+(232)Th at energies in the vicinity of the Coulomb barrier and compared it to the yield of the neighboring (14)C+(232)Th system measured in the same experiment. At sub-barrier energies, an enhancement of the fusion yield by factors of 2-5 was observed for (15)C, while the cross sections for (14)C match the trends measured for (12,13)C.     

Local structure of multiferroic RMn2O5: Important role of the R site

The temperature and magnetic field dependent local structure of RMn₂O₅ systems was examined. While no significant displacements of the Mn ions are observed, it is found that the R-O distribution exhibits changes at low temperature which are possibly related to the changes in the electric polarization. Density functional computations are used to explore the system dynamics and to link the local structural measurements with anomalous changes in the infrared absorption spectra. The anomalous R-O distribution and observed coupling to magnetic fields point to the need to properly treat the 4f electrons on the R sites in these systems.     

Photoinitiation of acrylamide polymerization using the dianions of rhodizonic and croconic acids

Polymerization of acrylamide has been initiated by the photochemical reactions of the dianions of croconic acid (1,2-dihydroxycyclopentenetrione) and rhodizonic acid (1,2-dihydroxycyclohexenetetrone) in the presence of various acceptor molecules. Photoinitiation was most efficient with croconic acid where a maximum quantum yield for polymerization of 2 x 10³ was observed in neutral solution. Kinetic studies using this initiator showed that the rate of polymerization was a linear function of the concentration of monomer and proportional to the square root of the light absorbed by the croconate dianion. The rate of initiation of radicals calculated from the rate of polymerization was compared with the rate of production of radicals calculated from previous studies of the photochemistry of the croconate dianion and the mechanism of photoinitiation is discussed. © 1992 John Wiley & Sons, Inc.