5,697 research outputs found
Photosensitized oxidation of unsaturated polymers
The photosensitized oxidation or singlet oxygenation of unsaturated hydrocarbon polymers and of their model compounds was reviewed. Emphasis was on cis and trans forms of 1,4-polyisoprene, 1,4-polybutadiene and 1,2-poly(1,4-hexadiene), and on 1,4-poly(2,3-dimethyl-1,3-butadiene). The microstructural changes which occur in these polymers on reaction with O2-1 in solution were investigated by infrared H-1 and C-13 NMR spectroscopy. The polymers were shown to yield allylic hydroperoxides with shifted double bonds according to the ene mechanism established for simple olefins. The photosensitized oxidation of the above unsaturated polymer exhibited zero order kinetics, the relative rates paralleling the reactivities of the corresponding simple olefins towards O2-1
Vacuum ultraviolet irradiation of polybutadiene final report
Vacuum ultraviolet irradiation of polybutadien
Photochemistry of unsaturated polymers
Radiation induced photochemical reactions in polyisoprene and polybutadiene thin films including microstructural change
Waste streams in a typical crewed space habitat: An update
A compilation of generation rates and chemical compositions of potential waste streams in a typical crewed space habitat, reported in a prior NASA Technical Memorandum and a related journal article, was updated. This report augments that compilation by the inclusion of the following new data: those data uncovered since completion of the prior report; those obtained from Soviet literature relevant to life support issues; and those for various minor human body wastes not presented previously (saliva, flatus, hair, finger- and toenails, dried skin and skin secretions, tears, and semen), but included here for purposes of completeness. These waste streams complement those discussed previously: toilet waste (urine, feces, etc.), hygiene water (laundry, shower/handwash, dishwasher water and cleansing agents), trash, humidity condensate, perspiration and respiration water, trace contaminants, and dust generation. This report also reproduces the latest information on the environmental control and life support system design parameters for Space Station Freedom
Frequency shifts and relaxation rates for spin 1/2 particles moving in electromagnetic fields
We discuss the behaviour of the Larmor frequency shift and the longitudinal
relaxation rate due to non-uniform electromagnetic fields on an assembly of
spin 1/2 particles, in adiabatic and nonadiabatic regimes. We also show some
general relations between the various frequency shifts and between the
frequency shifts and relaxation rates. The remarkable feature of all our
results is that they were obtained without any specific assumptions on the
explicit form of the correlation functions of the fields. Hence, we expect that
our results are valid both for diffusive and ballistic regime of motion and
arbitrary cell shapes and surface scattering. These results can then be applied
to a wide variety of realistic systems
Spin and transport effects in quantum microcavities with polarization splitting
Transport properties of exciton-polaritons in anisotropic quantum
microcavities are considered theoretically. Microscopic symmetry of the
structure is taken into account by allowing for both the
longitudinal-transverse (TE-TM) and anisotropic splitting of polariton states.
The splitting is equivalent to an effective magnetic field acting on polariton
pseudospin, and polarization conversion in microcavities is shown to be caused
by an interplay of exciton-polariton spin precession and elastic scattering. In
addition, we considered the spin-dependent interference of polaritons leading
to weak localization and calculated coherent backscattering intensities in
different polarizations. Our findings are in a very good agreement with the
recent experimental data.Comment: 8 pages, 6 figure
Structure-activity correlations for organophosphorus ester anticholinesterases. Part 2: CNDO/2 calculations applied to ester hydrolysis rates
Quantitative structure-activity relationships are presented for the hydrolysis of organophosphorus esters, RR'P(O)X, where R and R' are alkyl and/or alkoxy groups and X is fluorine, chlorine or a phenoxy group. CNDO/2 calculations provide values for molecular parameters that correlate with alkaline hydrolysis rates. For each subset of esters with the same leaving group, X, the CNDO-derived net atomic charge at the central phosphorus atom correlates well with the alkaline hydrolysis rate constants. For the whole set of esters with different leaving groups, equations are derived that relate charge, orbital energy and bond order to the hydrolysis rate constants
Calculation of geometric phases in electric dipole searches with trapped spin-1/2 particles based on direct solution of the Schr\"odinger equation
Pendlebury [Phys. Rev. A , 032102 (2004)] were
the first to investigate the role of geometric phases in searches for an
electric dipole moment (EDM) of elementary particles based on Ramsey-separated
oscillatory field magnetic resonance with trapped ultracold neutrons and
comagnetometer atoms. Their work was based on the Bloch equation and later work
using the density matrix corroborated the results and extended the scope to
describe the dynamics of spins in general fields and in bounded geometries. We
solve the Schr\"odinger equation directly for cylindrical trap geometry and
obtain a full description of EDM-relevant spin behavior in general fields,
including the short-time transients and vertical spin oscillation in the entire
range of particle velocities. We apply this method to general macroscopic
fields and to the field of a microscopic magnetic dipole.Comment: 11 pages, 4 figure
Deformed Wigner crystal in a one-dimensional quantum dot
The spatial Fourier spectrum of the electron density distribution in a finite
1D system and the distribution function of electrons over single-particle
states are studied in detail to show that there are two universal features in
their behavior, which characterize the electron ordering and the deformation of
Wigner crystal by boundaries. The distribution function has a -like
singularity at the Fermi momentum . The Fourier spectrum of the density
has a step-like form at the wavevector , with the harmonics being absent
or vanishing above this threshold. These features are found by calculations
using exact diagonalization method. They are shown to be caused by Wigner
ordering of electrons, affected by the boundaries. However the common Luttinger
liquid model with open boundaries fails to capture these features, because it
overestimates the deformation of the Wigner crystal. An improvement of the
Luttinger liquid model is proposed which allows one to describe the above
features correctly. It is based on the corrected form of the density operator
conserving the particle number.Comment: 10 pages, 11 figures. Misprints fixe
Weak antilocalization in quantum wells in tilted magnetic fields
Weak antilocalization is studied in an InGaAs quantum well. Anomalous
magnetoresistance is measured and described theoretically in fields
perpendicular, tilted and parallel to the quantum well plane. Spin and phase
relaxation times are found as functions of temperature and parallel field. It
is demonstrated that spin dephasing is due to the Dresselhaus spin-orbit
interaction. The values of electron spin splittings and spin relaxation times
are found in the wide range of 2D density. Application of in-plane field is
shown to destroy weak antilocalization due to competition of Zeeman and
microroughness effects. Their relative contributions are separated, and the
values of the in-plane electron g-factor and characteristic size of interface
imperfections are found.Comment: 8 pages, 8 figure
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