Zero-field NMR and NQR line shapes

A general density operator formalism is given for the NMR and NQR experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field.     

Extended range Zeeman atomic absorption spectrometry based on a 3-field a.c. magnet

The dynamic range of a.c. Zeeman atomic absorption (ZAA) analytical curves can be extended 10 times when intensity measurements are performed at three different field strengths, i.e. zero, maximum and intermediate field strength during each modulation cycle of the magnet, rather than at zero and maximum field strength only.The conventional ZAA analytical curve and the extended range curve are obtained simultaneously from one series of standards.The power supply of the magnet is described. In the extended range system the background correction frequency is 50 Hz, whereas the system operates at 100 Hz in the conventional ZAA mode.     

Correction for background absorption and stray radiation in a.c. modulated Zeeman atomic absorption spectrometry

Analytical results are presented obtained with Zeeman atomic absorption (ZAA) using a modified sine wave magnetic field. The measurement at zero field strength is lengthened to 0.5 ms for a 50 Hz magnetic field of 10 kG.The a.c. Zeeman system is extended with an additional intensity measurement performed at an intermediate field strength. The three field Zeeman system (3FZAA) permits simultaneous correction for background absorption and stray radiation at the expense of halved analytical sensitivity. The background correction capabilities of ZAA and 3FZAA are the same. However, the 3FZAA signals show increased noise in comparison to ZAA.In the three field system the roll-over problem, inherent in existing Zeeman systems, is shifted to higher concentration and to higher absorbance. The height and the position of the maximum in ZAA and 3FZAA analytical curves do not depend on the amount of background absorption.A method for the extension of analytical curves in AA is presented. Utilizing an a.c. modulated magnetic field any sensitivity between zero and ordinary AA sensitivity can be obtained.     

Diffusion-influenced reversible geminate recombination in one dimension. III. Field effect on the excited-state reaction

We obtain exact analytic solutions of the diffusion-influenced excited-state reversible geminate recombination reaction, A* + B<-->(AB)*, with two different lifetimes and quenching under the influence of a constant external field in one dimension. These fundamental solutions generalize two previous results [Kim et al., J. Chem. Phys. 111, 3791 (1999); 114, 3905 (2001)] and provide us with the insight necessary to analyze their specific relations and asymptotic kinetic transition behaviors. We find that the number of kinetic transitions can be changed due to interplay between the field strength and lifetimes. Unlike the previous works, the number of lifetime dependent transitions is found to be one or zero. On the other hand, the number of the field dependent transitions becomes two, one, or zero. We find a new pattern of kinetic transition e(t)-->t(-1/2)-->e(t) when there is only one field dependent transition.     

Pulsed-electric-field crossflow ultrafiltration of bovine serum albumin

A conventional crossflow ultrafiltration (CUF) apparatus was modified by the inclusion of electrodes which permitted a pulsed electric field to be produced across the ultrafiltration membrane (PEF-UF process). Using this apparatus, a discontinuous electrophoretic velocity was imposed upon the proteins being concentrated, opposing their convective movement toward the CUF membrane. This resulted in a lower concentration of rejected solute protein in the fluid boundary layer adjacent to the high-pressure side of the membrane and, hence, in a lower solute-related filtration resistance than in the case of conventional ultrafiltration (zero electric field). Studies of the PEF-UF process with bovine serum albumin (BSA) in the range of 0.5–5% w/v demonstrated a 25–40% decrease in the solute-related resistance to the permeate flux compared to the case of a zero electric field. Accordingly, higher permeate fluxes and, therefore, higher rates of concentration of the protein solution were obtained than for conventional crossflow ultrafiltration. When the electric field was reimposed following a period of operation under conventional CUF conditions, the permeate flux could be restored to nearly the same higher value observed initially for the PEF-UF process.     

Prediction of Dielectric Properties of Air Plasma for Circuit Breaker Application Based on a Chemically Non-equilibrium Model

Dielectric properties of air plasma in a model circuit break were investigated. A chemically non-equilibrium (non-CE) model was used to simulate the arc dynamic behavior inside the nozzle during current zero period. Distribution of the critical breakdown electric field E_cr of hot air was derived from the electron energy distribution function by solving the Boltzmann transport equation, using calculated temperature, pressure and species composition. Then, the electric field at applied recovery voltage (E_a) was calculated. The probability of dielectric breakdown inside the nozzle can be predicted by comparing E_a and E_cr. The results show that neglect of departure from chemical equilibrium may lead to the overestimation of the dielectric recovery strength of circuit breaker arc during the first several hundred microseconds after current zero.     

Spin-free quantum chemistry. XIV. The infinite interaction range model for ferromagnetism

The infinite interaction range model (IIRM ) for ferromagnetic systems is presented in its spin-free formulation. In this formulation the states are labelled by partitions which provide a natural variable for thermodynamic computation. We have extended the calculations of Kittel and Shore by computing to a practical thermodynamic limit (N ～ 100,000). The heat capacity, magnetic susceptibility and the magnetization of the first two functions exhibit a critical temperature while the magnetization is zero at zero field for all temperatures. Spontaneous magnetization is obtained by linear extrapolation from high field or equivalently by a polarized partition function. Relationships are explored among IIRM , the Heisenberg model and the mean field model. Application to IIRM of the Yang-Lee condition for a phase transition yields a critical temperature identical to that obtained by the direct calculation.     

Zero electric field gradient mixtures for the elucidation of orientational mechanisms

A novel zero electric field gradient nematic liquid crystal made up from two nematic liquid crystal components is employed as a solvent for a series of molecules ranging from small molecules to mesogens themselves. Nuclear magnetic resonance is used to determine the degree of order of the solute and solvent molecules. Results are compared to those obtained for two completely different zero electric field gradient nematic mixtures. The comparison strongly indicates that for a variety of molecules largely differing in size, shape and flexibility their degree of order can be described by a single orientation mechanism. This mechanism can be adequately modelled by a simple phenomenological mean field model based on the size and shape anisotropy of the dissolved species. The use of zero electric field gradient mixtures in combination with this mean field model allows the prediction of solute order parameters at approximately the 10 per cent level.     

The influence of finite dimensions on the static ordering of the S*C phase in an electric field

A ferroelectric liquid-crystalline sample of a finite length along the helical axis is studied in an external electric field applied perpendicular to the helical axis. By taking into account the linear coupling to the field, the equilibrium state is found for the case of free boundary conditions. Just below the critical field which induces the transition into the homogeneous S*_C phase the domain-like structure appears as in the case of an infinite sample. The helical period in a finite sample is not a continuous function of the field, but it increases in finite jumps. We show that the finiteness of the sample has also an influence on the dielectric response at zero field.     

The influence of finite dimensions on the static ordering of the S*C phase in an electric field

Abstract

A ferroelectric liquid-crystalline sample of a finite length along the helical axis is studied in an external electric field applied perpendicular to the helical axis. By taking into account the linear coupling to the field, the equilibrium state is found for the case of free boundary conditions. Just below the critical field which induces the transition into the homogeneous S*_C phase the domain-like structure appears as in the case of an infinite sample. The helical period in a finite sample is not a continuous function of the field, but it increases in finite jumps. We show that the finiteness of the sample has also an influence on the dielectric response at zero field.     

Perdeuterated Conjugated Polymers for Ultralow‐Frequency Magnetic Resonance of OLEDs

The formation of excitons in OLEDs is spin dependent and can be controlled by electron‐paramagnetic resonance, affecting device resistance and electroluminescence yield. We explore electrically detected magnetic resonance in the regime of very low magnetic fields (<1 mT). A pronounced feature emerges at zero field in addition to the conventional spin‐ Zeeman resonance for which the Larmor frequency matches that of the incident radiation. By comparing a conventional π‐conjugated polymer as the active material to a perdeuterated analogue, we demonstrate the interplay between the zero‐field feature and local hyperfine fields. The zero‐field peak results from a quasistatic magnetic‐field effect of the RF radiation for periods comparable to the carrier‐pair lifetime. Zeeman resonances are resolved down to 3.2 MHz, approximately twice the Larmor frequency of an electron in Earth's field. However, since reducing hyperfine fields sharpens the Zeeman peak at the cost of an increased zero‐field peak, we suggest that this result may constitute a fundamental low‐field limit of magnetic resonance in carrier‐pair‐based systems. OLEDs offer an alternative solid‐state platform to investigate the radical‐pair mechanism of magnetic‐field effects in photochemical reactions, allowing models of biological magnetoreception to be tested by measuring spin decoherence directly in the time domain by pulsed experiments.     

Polarized triplet-minus-singlet absorbance difference spectra measured by absorbance-detected magnetic resonance. An application to photosynthetic reaction centres

A method is presented to measure linear dichroic triplet-minus-singlet absorbance difference spectra using absorbance-detected magnetic resonance of the triplet state in zero magnetic field. Orientational selection is achieved by using a linearly-polarised microwave field. The microwave-induced change in the absorbance of light with the electric vector parallel or perpendicular to the microwave field vector is monitored using a Morvue photoelastic modulator and an analyzing polarizer. The method is applied to reaction centres of the photosynthetic bacterium Rhodopseudomonas viridis.     

Electronic properties of two isomeric charge transfer [2.2]paracyclophanes

The emission spectra and the zero field splitting parameters of the two diastereomeric 4,7-dicyano-12,15-dimethoxy-[2.2] paracyclophanes 3 and 4 in comparison to the corresponding monomers 1 and 2 were investigated in order to study the orientation dependence of charge transfer (CT) interactions. The general results in glasses (PMMA) are: broad structureless emission bands with large spectral overlap between fluorescence and phosphorescence; strong reduction of the zero field splitting parameters D and D* by a factor of two for the pseudo-ortho isomer 3 and by a factor of four for the pseudo-geminal isomer 4 showing the strong effect of the geometrical orientation. In single crystals of the same phanes the zero field parameters were found to be further reduced to about one fifth of the value of the monomers which indicates, in accordance with the emission spectra, an additional intermolecular interaction between adjacent phanes.     

Chemical reactions in microdroplets and microbubbles. Thermodynamic approach

In small dispersed systems, the standard chemical potential of the components present in the curved phases is different from the one in phases with a flat interface. Thermodynamic properties of reactions in such systems, at mechanical equilibrium, may be different from those for the same reactions at the same temperature, in a phase with zero curvature. Among these properties, we study here the dependence of the equilibrium constant on the curvature, on the surface tension and on the stoichiometry of the process. Reactions in an electric field are also discussed.     

Ambipolar diffusion in multicomponent plasmas

A recently described self-consistent effective binary diffusion approximation is applied to ambipolar diffusion in a neutral multicomponent plasma in zero magnetic field, where forced diffusion due to the electric field E plays an essential role. The field E is determined by the constraint that the net current flow produced by the diffusion fluxes must be zero. The resulting effective binary diffusion fluxes are the sum of those that would obtain for E=0 and ambipolar correction terms proportional to E. The formulation is .self=consistent with respect to both mass and charge, the net diffusional fluxes of which are both identically zero. The results may be further simplified due to the small mass of the electrons. The effective binary diffusivity D_e of the electrons no longer appears in the simplified expressions. They are therefore well suited to numerical calculations, where the large value of D_e, might otherwise have resulted in unacceptable stability or accuracy restrictions. The well-known effective doubling of ion diffusivities due to ambipolar diffusion occurs in simple situations but is not a general feature.     

Trace gas analysis for the evaluation of zero air generators

Trace amount of substance fractions of air pollutants in zero air are measured applying sensitive analytical methods in combination with the infrastructure to operate zero air generators under simulated field conditions at place. The uncertainties of the amount of substance fractions of a diluted standard gas mixture and the dilution offsets are calculated under conditions that consider the effect of trace amounts of analyte in the zero gas with increasing dilution of a gravimetric gas standard. The analytical methods and their calibration, the test procedure and results for various zero air generators are described. The results are compared with the specifications, the Swiss regulatory requirements and the European norms for monitoring ambient air pollutants. By knowing the residual amount of substance of the analyte in a zero gas, the instrument offsets by dilution can be eliminated and the uncertainties for the measurement values of the diluted standard gas mixtures be calculated.     

Experimentally measured permanent dipoles induced by hydrogen bonding. The Stark spectrum of indole-NH3

Hydrogen bond pairs involving the chromophore indole have been extensively studied in the gas phase. Here, we report high resolution electronic spectroscopy experiments on the indole-NH(3) hydrogen bond pair in the absence and presence of an electric field. The S(1)-S(0) origin band of this complex recorded in zero field at high resolution reveals two overlapping spectra; a consequence of NH(3) hindered internal rotation. The barrier to internal rotation is predicted by theory to be less than 20 cm(-1) in the ground state, therefore requiring a non-rigid rotor Hamiltonian to interpret the spectra. Conducting the experiment in the presence of an applied electric field further perturbs the already congested spectrum of the complex, but makes possible the measurement of the permanent electric dipole moments in its S(0) and S(1) states. These values reveal significant changes in electron distribution that arise from hydrogen bonding effects.     

1H and 13C endor on phenoxyl biradicals randomly oriented in frozen solution

ENDOR measurements have been performed on ¹³C labelled biradicals, namely Yang's biradical and p-phenylenebisgalvinoxyl, in frozen solutions. Besides intense ENDOR lines at the free nuclear Zeeman frequencies, hyperfine shifted ¹H and ¹³ ENDOR lines could be detected with excellent signal-to-noise ratios. From these spectra not only the magnitudes, but also the signs of the hyperfine splitting components relative to the zero field splitting parameter D could be derived. It is demonstrated that the sign of D in Yang's biradical must be negative. This may be due to a spin-orbit coupling contribution to the zero field splitting parameter.     

Self-assembly of magnetic nanostructures

We use Monte Carlo and quaternion molecular dynamics simulations to study the self-assembly of intriguing structures which form in colloidal suspensions of small magnetite particles. We show that the only stable isomers with few particles, a ring and a chain, can be efficiently interconverted using a magnetizable tip. We propose to use the oscillating dipole field of the tip to locally anneal the aggregates to either a ring in zero field or a chain in nonzero applied field.