Energy levels in even actinide isotopes from (t,p) reactions

Energy levels in^{232, 234}Th,^{236, 238, 240}U and in²⁵⁰Cm have been measured using the (t, p) reaction. Angular distributions were obtained for the^{234, 238}U targets and evidence for second order effects in the direct reaction mechanism was found.     

Kinetics of the reaction: 2C2H4 → C2H5 + C2H3· heat of formation of the vinyl radical

The initial rates of formation of the major products in the thermal reactions of ethylene at temperatures in the neighborhood of 800 K have been measured in the presence and absence of the additives neopentane and ethane. It has been shown that in the absence of the additive the main initiation process is (1) while in the presence of neopentane and ethane the following additional initiation processes occur: (2) From the ratios of the rates of formation of the major products in the presence and absence of the additive the ratios k_N/k₁ and k_E/k₁ were measured over the temperature range of 750–820 K. Taking values from the literature for k_N and k_E, the following value was obtained for k₁: Previous results using butene-1 as additive were rexamined and shown to be consistent with this measurement. From this measurement the following values were derived: ΔH_f(C₂H₃) = 63.4 ± 2 kcal/mol and D(C₂H₃? H) = 103 kcal/mol.     

Dielectric relaxation studies of dipolar aromatics in polyethylene. II. Oriented low-density polyethylene

Stretched polyethylene has been used for several years by organic spectroscopists as a means of orienting isolated aromatic molecules. Dielectric relaxation studies are reported which consider dipolar aromatic molecules dissolved in stretched polyethylene in order to learn more about the environment of these oriented molecules. The research builds on earlier studies of the dielectric relaxation behavior of dipolar aromatic molecules dissolved in unoriented low density polyethylene. Studies demonstrate that molecules in the amorphous phase are oriented at temperatures below the glass transition, both the β and γ relaxations being orientation dependent. It is shown through studies of oriented rods that large numbers of the orientable molecules are immobilized by the oriented polyethylene and cannot relax. An essential criterion for immobilization to occur is that molecules exhibit geometrical symmetry.     

Photophysics of C<Subscript>60</Subscript> and C<Subscript><Emphasis Type="Bold">60</Emphasis></Subscript><Superscript><Emphasis Type="Bold">-</Emphasis></Superscript> in faujasite zeolites

While the intercalation of C₆₀ and the formation of C ₆₀ ^- in the supercages of NaX and NaY are confirmed by using ¹²⁹Xe NMR and ESR, the photophysical properties of C₆₀ and C ₆₀ ^- are characterized by monitoring transient reflectance spectra and kinetics, fluorescence kinetics, and diffuse reflectance spectra. C ₆₀ ^- is considerably more abundant in NaY than in NaX. This difference is explained in terms of polarity difference between two zeolites. Both C₆₀ and C ₆₀ ^- have remarkably elongated excited-state lifetimes due to their collision-free environment in zeolitic nanocavities although C ₆₀ ^- has much shorter lifetimes than C₆₀. C ₆₀ ^- , in particular, shows intense absorption and emission due to its reduced symmetry in zeolites. Received 13 July 2001 and Received in final form 8 October 2001     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.     

Synthesis and characterization of poly(epoxy‐imide) crosslinked networks

Poly(epoxy imide)s were prepared by a reaction between a hydroxyl‐group‐containing soluble copolyimide and commercial epoxy resins at 220 °C for 2 h. Poly(epoxy imide) thin films exhibited higher thermal stability and lower dielectric constants than a commercial flip‐chip package material (U300). The thermal stabilities of the poly(epoxy imide)s were 1.4–2.0 times higher than that of U300. The thermal stability increased with increasing crosslink density and with decreasing bulky CF₃ groups (which were easily decomposable). The dielectric constants of the poly(epoxy imide)s were 1.1–1.3 times lower than that of U300, and this is highly desirable for the microelectronic packaging industry. The dielectric constant dramatically decreased when bulky CF₃ groups were added and when the functionalities of epoxy resins decreased. The residual stresses, slopes in the cooling curves, and glass‐transition temperatures of the poly(epoxy imide)s were measured with a thin‐film stress analyzer. Low residual stresses and slopes in the cooling curves were achieved with a higher crosslink density. However, in the presence of bulky CF₃ groups, the copolyimide backbone structure did not affect the residual stress values. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4293–4302, 2004     

Analysis of the diffracted wave from a volume hologram stored with an irregular pattern

The theoretical analysis for the diffracted wave from a volume hologram stored with an irregular pattern is presented. The relative diffraction efficiencies with respect to position and wavelength of readout wave are calculated by applying the Born approximation for light diffraction. The results are compared with experimental results and the effects of irregular pattern are discussed.     

Test of scaling in two dimensions

The purpose of this paper is to report on the first comprehensive experimental test of the scaling hypothesis in two-dimensional physics. This hypothesis predicts that the equation of state near a phase transition of a system in thermodynamic equilibrium obeys a simple scaling law. Our experimental data, obtained on a truly two-dimensional magnetic system consisting of a subnanometer thick Fe films grown on top of a non-magnetic surface, explicitly display scaling. The experimental evidence suggests that this system is an almost perfect realization of a 2d Ising model.     

Neutron activation analysis of high purity nickel by solvent extraction using 2-benzylpyridin/benzene

A radiochemical neutron activation analysis using solvent extraction has been applied for the determination of trace impurities in high purity nickel. Because of the high activity of⁵⁸Co produced by the nuclear reaction,⁵⁸Ni(n,p)⁵⁸Co, cobalt should be separated from the impurities. Removal of cobalt from the other trace elements in the aqueous acidic solution containing 1M thiocyanate ion (KSCN) was achieved by extraction with 1M2-benzylpyridin (BPy) in benzene. From the result of tracer experiments, cobalt was completely separated from most other elements except Fe, Mo and Zn. To determine the experimental accuracy, NIST SRM 673 nickel oxide was analyzed and the results agreed well within 10% deviation. This established radiochemical method was applied to the analysis of high purity nickel samples.     

Irreducible tensor operators and selective/hard single and double resonance experiments in insulators: Applications to NMRON and MQ-NMR

Recently, pulsed NMRON experiments have been carried out on trace amounts of radioactive⁵⁴Mn in the antiferromagnet MnCl₂4H₂O at 500 MHz (Le Gros et al. [1]). In this compound, the quadrupole splitting between the two lowest NMR transitions is ≈3 MHz, which precludes the use of non-selective (hard) rf pulses. Yet within the restricted 2*2 manifold, associated with a given transition, the nuclear rotation is “hard”. In this paper, the theory of “selective-hard” NMRON and MQ-NMR experiments is developed within the framework of irreducible tensor operators. In essence, the theory extends the early work of Jaynes [4] to deal with the higher-order multipolar states created during the course of a given NMR experiment. Several new pulsed NMRON and MQ-NMR experiments are proposed. For example, it is demonstrated how “ouble resonance”, “selective-hard” experiments on the pseudo spin-1 manifold spanned by |±1> and |0> Zeeman states of any integer spinI could be used to extract small chemical shifts in the face of very large quadrupole splittings.