Thermoluminescence and thermally stimulated conductivity in polymers

Thermoluminescence (TL) and thermally stimulated conductivity (TSC) glow curves in poly(vinyl chloride), polyethylene, polystyrene, polytetrafluoroethylene, and polyimide have been compared, and many similarities have been observed. Comparison with available NMR, dynamic mechanical loss, and dielectric loss, molecular mobility data shows that most TL and TSC peaks occur at temperatures similar to those assigned to the onset of specific molecular motions, suggesting that the peaks are due to the liberation of electrons from traps formed by the polymer chains themselves, e.g., potential wells or cavities due to chain entanglement in amorphous regions, or main-chain branching points. Peaks for which correlation with molecular motion is not apparent are tentatively assigned to liberation of electrons from traps centered on impurities. The TSC peak temperatures in PVC were not affected in any consistent fashion by the application of high-strength electric fields during the warming process, indicating that the electron traps are electrically neutral when empty and charged when filled; the direction of the TSC currents appears to be determined by temperature gradients existing within the samples. The TL glow-curves are generally in good agreement with the results of other workers. The dark dc conductivity of PVC not exposed to ionizing radiation rises sharply in the temperature region assigned to the β-relaxation process, suggesting that the electron mobility in that polymer is dominated by molecular chain motion, i.e., the interchain charge transport process is probably best described in terms of a hopping process.     

Design and performance of an on-line atomic fluorescence monitor for magnesium

On-line flame atomic fluorescence spectrometry is used to measure the magnesium content of boiler feedwater in the range 0.01–2.00 mg l^-1. Details of the design, operation and performance of the spectrometer are given. Lanthanum is added to eliminate interference from silicon.     

Solvent influence on photochemical isomerizations: Photophysics of DODCI

We have studied the photophysics of DODCI dissolved in a series of polar solvents. Through measurements of the temperature dependence of the photoisomer quantum yield, fluorescence lifetime and fluorescence quantum yields we clarify the kinetic mechanism for nonradiative decay. By measuring the isomerization rate as a function of temperature at constant viscosity we are able to separate the innuence of internal barriers and solvent viscous drag. The apparent activation energy observed in solutions is less than the sum of the internal and viscosity activation energies. This is shown to be consistent with the full Kramer's rate expression for diffusive barrier crossing. We also establish the temperature dependence of a second “direct” internal conversion process which does not lead to photoisomer formation and dominates the nonradiative decay of DODCI at low temperatures or in a rigid matrix.     

Evidence of distinct water species in cellulosic environments from broad-line NMR

Broad-line NMR measurements were made on water in cellulosic samples. The spectra for oriented rayon at 65% relative humidity and room temperature consisted of a visually apparent doublet (S_D) and a narrow singlet (S_N) which was shifted upfield about 7 ppm from the center of the doublet. The doublet separation varied as A(3 cos²θ—1), where θ is the angle between the fiber axis and the magnetic field; the maximum doublet separation was 350 mG at both 15.1 and 56.4 MHz, indicating the doublet is dipolar in origin. The peak-to-peak linewidth of the narrow singlet was orientation and field dependent. The upfield shift of the narrow singlet from the doublet center was field dependent. Spectra for vacuum-treated rayon consisted of only the narrow singlet, which was orientation dependent. The doublet separation decreases with increasing moisture content and is essentially zero for oriented rayon samples at 100% relative humidity; the resulting spectra due to the collapsed doublet and S_N singlet was an asymmetric line. Temperature-dependent measurements were made on oriented rayon at 65% relative humidity with the fiber axis parallel to the magnetic field; when the temperature was increased from about 0° the peak-to-peak linewidth of the doublet halves and the doublet separation decreased. As the temperature was increased from about 30 to about 65°C, a previously unobserved singlet (S_E) became visible. The relative amplitudes of the three lines varied with temperature as follows: The S_E singlet increased, the S_D doublet decreased, and the amplitude of the S_N singlet remained constant. Measurements using oriented cotton samples and randomly packed rayon samples indicated that the NMR line shape of the water spectra depends upon the physical properties of the macromolecular substrate. The doublet component of the spectra is attributed to a water species (S_D) which is highly bonded to the macromolecular substrate. The S_E singlet is attributed to an energetic water species (S_E) which is rapidly tumbling in the macromolecular environment. The S_N singlet is not due to free water.     

Far-infrared absorption spectra of water vapor H216O and isotopic modifications

Far-infrared absorption spectra of H₂¹⁶O, H₂¹⁸O, H¹⁶OD, H¹⁸OD, D₂¹⁶O, D₂¹⁸O have been observed between 10 and 40 cm^?1 at a resolution of 0.07 cm^?1. Experimental and calculated line positions agree within the accuracy of the experiment (±0.003 cm^?1). The relative intensities of type a and type b transitions of H¹⁶OD and H¹⁸OD are used to estimate the ratio $\text{μ}{}_{\mathrm{a}}\text{μ}{}_{\mathrm{b}}$.     

Femtosecond spectroscopy of optical excitations in single-walled carbon nanotubes: evidence for exciton-exciton annihilation

Frequency-resolved femtosecond transient absorption spectra and kinetics measured by optical excitation of the second and first electronic transitions of the (8,3) single-walled carbon nanotube species reveal a unique mutual response between these transitions. Based on the analysis of the spectra, kinetics, and their distinct amplitude dependence on the pump intensity observed at these transitions, we conclude that these observations originate from both the excitonic origin of the spectrum and nonlinear exciton annihilation.     

Tunable two-dimensional femtosecond spectroscopy

We have developed a two-dimensional (2D) Fourier-transform femtosecond spectroscopy technique for the visible spectral region. Three-pulse photon echo signals are generated in a phase-matched noncollinear four-wave mixing box geometry that employs a 3-kHz repetition-rate laser system and optical parametric amplification. Nonlinear signals are fully characterized in amplitude and phase by spectral interferometry. Unlike for previous setups, we achieve long-term phase stability by employing diffractive optics and interferometric accuracy of excitation-pulse time delays by using movable glass wedges. As an example of this technique, 2D correlation and relaxation spectra at 600 nm are shown for a solution of Nile Blue dye in acetonitrile.     

Long-period-grating-assisted optical add-drop filter based on mismatched twin-core photosensitive-cladding fiber

A noninterferometric optical fiber add-drop channel filter based on a mismatched twin-core photosensitive-cladding fiber and long-period fiber grating is demonstrated. With assistance from the fiber grating, co-directional spectrally selective cross coupling between the two cores can be realized with an efficiency of 90%.     

Bragg fiber design for linear polarization

A new design is presented for Bragg fibers that allows low-loss propagation for linearly polarized light. Predictions based on a simple ray model show that approximately doubling the thickness of the first wall layer results in low losses at TM-like boundaries while keeping TE-like boundary losses manageable. This contrasts sharply with conventional quarter-wave designs that are extremely low loss for TE01 modes but very high loss for linear polarization. We fabricate Bragg fibers based on this design concept in a Si/SiO2 system and verify experimentally that they propagate linearly polarized light with losses less than 6 dB/cm over a 60-nm spectral range.     

Up quark mass in lattice QCD with three light dynamical quarks and implications for strong CP invariance

A standing mystery in the standard model is the unnatural smallness of the strong CP violating phase. A massless up quark has long been proposed as one potential solution. A lattice calculation of the constants of the chiral Lagrangian essential for the determination of the up quark mass, 2alpha(8)-alpha(5), is presented. We find 2alpha(8)-alpha(5)=0.29+/-0.18, which corresponds to m(u)/m(d)=0.410+/-0.036. This is the first such calculation using a physical number of dynamical light quarks, N(f)=3.