Study Of Field Effects Research Articles

The detection and identification of transient free radicals in solution

The various Electron Spin Resonance methods that have been used for studying transient free radicals are described, and their limitations and advantages discussed. Continuous wave and Fourier Transform time-resolved methods are compared and shown to be complementary in studies of radicals produced using flash photolysis. The role of Chemically Induced Dynamic, Electron Polarization (CIDEP) in determining reaction pathways and in linking the photochemistry and photophysics of molecules is described as is the role played by Reaction Yield Detected Magnetic Resonance (RYDMR) and magnetic field effect (MFE or MARY) studies in understanding the reactions of radicals.

An X-ray dichroism study of magnetic and crystal field effects in thin rare earth overlayers

We have measured linear and circular X-ray dichroism at the M 4,5 edges of several rare earths (RE) deposited on magnetic and non-magnetic transition metal substrates. A comparison of the data shows that for RE overlayers the surface crystal field is the main parameter in determining the RE ground state splitting. This should be taken into account in the interpretation of magnetic measurements on RE overlayers.

Self-field effects on flux flow in two-dimensional arrays of Nb Josephson junctions.

Measurements and numerical studies of the self-induced magnetic field effects on flux flow in two-dimensional arrays of niobium Josephson junctions have been performed. It was found that the flux-flow resistance becomes larger as the penetration depth of the array decreases. A phenomenological model, which agrees qualitatively with the experiments and simulations, is presented to explain the self-field effects on flux flow. Due to the smaller spatial extent of supercurrents around a vortex when self-fields are important, both the mass of the vortex and the array viscosity decrease. The decreased mass and viscosity lead to an increase in flux-flow resistance. The effects of spin-wave damping are also discussed for underdamped arrays. Measurements and simulations on the spatial dependence of flux flow indicate that more complex dynamics is involved in the flux-flow regime than a simple linear flow of the vortices.

A numerical study of the wind field effect on the growth and observability of equatorial spread F

The neutral wind field effects on the development of the equatorial plasma bubbles have been simulated by a two‐dimensional time‐dependent model similar to that developed by Zalessak and Ossakow. The results indicate that when there exists no neutral wind, any perturbation on the bottom‐side of the ionosphere density profile will be amplified by the gravitational Rayleigh‐Taylor (GRT) instability and an upwelling bubble will form as expected. When a zonal neutral wind field exists, the uplift velocity of the bubble will be enhanced by a uniform neutral wind, but suppressed by vertical shear of the wind field. Secondary structures called plumes will grow out from the side walls of the primary bubble if some secondary perturbation signal with vertical structure is seeded. We notice that patches and multiple plume structures have been observed in the mid‐latitude and low‐latitude ionosphere. So we have developed a two‐dimensional local theory of the generalized GRT instability to calculate the growth rate of a small‐scale perturbation signal in the vicinity of the primary bubble and found that the growth rate increases with time along with the growth of the bubble. It is proved that the irregularities generated by some of our simulations should be able to cascade into observable (e.g., by radar) turbulence in a reasonable period of time after their generation. Similar to the effect on the growth of the primary bubble, a strong uniform wind field may enhance the growth rate of a perturbation signal in the vicinity of the bubble, while a strong vertical wind shear will suppress the growth rate. The main point of this theory is that the growth rate of the GRT instability is controlled by the density gradients and the magnitudes of the velocity of the plasma motion relative to the neutral wind in both directions. In addition, our simulations have proved that a seeding wave with phase velocity matching the background wind speed will generate the fastest growing bubble, disapproving the theory of spatial resonance.

The Origin of Magnetic Field Dependent Recombination in Alkylcobalamin Radical Pairs

Magnetic field effect studies of alkylcobalamin photolysis provide evidence for the formation of a reactive radical pair that is born in the singlet spin state. The radical pair recombination process that is responsible for the magnetic field dependence of the continuous-wave (CW) quantum yield is limited to the diffusive radical pair. Although the geminate radical pair of adenosylcob(III)alamin also undergoes magnetic field dependent recombination (A.M. Chagovetz and C. B. Grissom, J. Am. Chem. soc. 115, 12152-12157, 1993), this process does not account for the magnetic field dependence of the CW quantum yield that is only observed in viscous solvents. Glycerol and ethylene glycol increase the microviscosity of the solution and thereby increase the lifetime of the spin-correlated diffusive radical pair. This enables magnetic field dependent recombination among spin-correlated diffusive radical pairs in the solvent cage. Magnetic field dependent recombination is not observed in the presence of nonviscosigenic alcohols such as isopropanol, thereby indicating the importance of the increased microviscosity of the medium. Paramagnetic radical scavengers that trap alkyl radicals that escape the solvent cage do not diminish the magnetic field effect on the CW quantum yield, thereby ruling out radical pair recombination among randomly diffusing radical pairs, as well as excluding the involvement of solvent-derived radicals. Magnetic field dependent recombination among alkylcobalamin radical pairs has been simulated by a semi-classical model of radical pair dynamics and recombination. These calculations support the existence of a singlet radical pair precursor.

Field-effect studies of C 60 thin films before and after implantation with potassium

We report measurements of the field-effect mobility in thin films of the fullerene C 60, before and after implantation with potassium ions at 30 keV. Potassium ion implantation is known to produce reductive doping in C 60 at low implantation doses, and was used to raise the bulk conductivity of the C 60 layer. Devices were fabricated on doped silicon wafers, using thermally grown oxide to provide the insulator layer, and gold source and drain contacts. C 60 was deposited by vacuum sublimation. Field-effect mobilities prior to implantation of about 7 × 10 −7 cm 2 V −1 s −1 were obtained, and the lowest implantation dose (10 10 ions cm −2), this was raised by about 50%. We attribute this improved mobility to the presence of extra bulk charge carriers. At higher implantation levels, however, the mobility decreased, falling to 1.4 × 10 −7 cm 2 V −1 s −1 at a dose of 2 × 10 11 ions cm −2. We propose that this is due to an increase in disorder in the C 60 lattice caused by the implantation process.

Study of magnetic ordering and crystal field effects of Sm in a single crystal of Sm2CuO4 by specific heat and susceptibility measurements

Heat capacity and magnetic susceptibility measurements were performed on a single crystal of Sm2CuO4 in different magnetic fields up to 7 Tesla applied both in parallel and perpendicular to the c-axis, in the temperature range from 1.5K to 150 K. Sm ions order antiferromagnetically atTN=5.96K and exhibits a sharp magnetic anomaly reminiscent to three dimensional ordering in the heat capacity and as a kink in the susceptibility. The magnetic anomaly shows a meagre magnetic field dependence ofTN indicating a strong magnetic coupling of Sm ions.

Does the Skrunda Radio Location Station diminish the radial growth of pine trees?

The Skrunda Radio Location Station (RLS), which has operated continuously for more than 20 years, has created a unique area for the study of pulse radio-frequency electromagnetic field effects. Trees were chosen to assess these effects. Since 1990, permanent plots have been established in pine forest stands around the Skrunda RLS and in control areas. The dynamics of tree growth changes were analysed using retrospective tree ring data. There is a statistically significant ( P < 0.01) negative correlation between the relative additional increment in tree growth and the intensity of the electric field. The radial growth of pine trees has diminished in all plots that received electromagnetic radiation. This decrease in growth began after 1970, which coincided with the start of operation of the Skrunda RLS, and was subsequently observed throughout the period of study. The effects of many other environmental and anthropogenic factors were evaluated, but no significant effects on tree growth were observed.

A study of magnetic field effects on fibroblast cultures Part 1. The evaluation of the effects of static and extremely low frequency (ELF) magnetic fields on vital functions of fibroblasts

Cultured fibroblasts isolated from murine livers by tissue trypsinization were exposed to a static magnetic field (0.49 T) and an extremely low frequency (ELF) magnetic field (50 Hz, 0.020 T). The cultures were exposed to magnetic fields on four consecutive days for exposure times of 2, 4, 8, 16, 32 and 64 min. After exposure the following parameters of the fibroblast cultures were determined: the dynamics of culture growth, the protein content and 14C-thymidine incorporation. The cytometric parameters of the fibroblasts were also assessed. The ELF magnetic field compromised vital functions of the fibroblasts (inhibition of culture growth, decreased cellular protein, lowered cytometric parameters) and caused a slowdown in the rate of 14C-thymidine incorporation which indicates altered DNA synthesis. Ongoing experiments with a static magnetic field have shown no effect on the vital functions of fibroblasts.

A study of magnetic field effects on fibroblast cultures part 3. The evaluation of the effects of static and extremely low frequency (ELF) magnetic fields on glycosaminoglycan metabolism in fibroblasts, cell coats and culture medium

Cultures of fibroblasts isolated from murine liver by the method of tissue trypsinization were exposed to a static magnetic field (0.49 T) and an extremely low frequency (ELF) magnetic field (50 Hz, 0.020 T). The cultures were exposed to magnetic fields for exposure periods of 2, 4, 8, 16, 32 and 64 min on four consecutive days. During the experiment we investigated the glycosaminoglycans isolated from the fibroblast, their coats and the culture medium. The investigations concerned heparan sulphate (DS) and chondroitin sulphates (CS). The changes observed in the fibroblast cultures exposed to ELF magnetic field suggest an increase in sulphate ion content in the glycosaminoglycans investigated, i.e. increased synthesis of the compounds. The ELF magnetic field also affects the degree of glycosaminoglycan sulphatization. Some changes in the quantitative relations between HS, DS and CS were also noted. The static magnetic field had no effect on glycosaminoglycan metabolism, i.e. there were no alterations in incorporation of labeled sulphur into sulphate glycosaminoglycans.

Theoretical study of the external electric field effect on the HeH2 (21A?) metastable state

Theoretical study of the external electric field effect on the HeH2 (21A?) metastable state

Theoretical study of the external electric field effect on the HeH2 (2 1A′) metastable state

A possibility of stabilization in an external homogeneous electric field on the HeH{sub 2}(2 {sup 1}A{prime}) metastable state relative to its nonadiabatic decay into the a {sup 1}A{prime} state is investigated theoretically. If the lowest vibrational levels of the HeH{sub 2}(2 {sup 1}A{prime}) state might be stabilized, one would try to detect the radiative transition 1 {sup 1}A{sup {prime}} {l_arrow} 2 {sup 1}A{prime}) from these levels and thus to verify experimentally the existence of the HeH{sub 2}(2 {sup 1}A{sup {prime}} from these levels and thus to verify experimentally the existence of the HeH{sub 2}(w {sup 1}A{prime}) state. It is shown that the separation between the potential energy surfaces (PESs) of the states in question and the energy of their avoided crossing point do increase in the field. Moreover, it is shown that in the fields with strength up to 10{sup 7} V/cm the nonadiabatic decay of the w {sup 1}A{prime} state along one of the vibrational modes may be closed. However, along two other modes such decay still may occur. It is noted nevertheless that the question of such stabilization possibility in inhomogeneous fields (i.e., in ionic crystal or on its surface) is still open. The effect of the fieldmore » was taken into account within an analytic two-level model. The unperturbed PESs were calculated by the CASSCF method.« less

Exploratory laser flash photolysis study of free radical reactions and magnetic field effects in melatonin chemistry.

Melatonin is an excellent free radical scavenger, reacting with tert-butoxyl and cumyloxyl radicals with rate constants of 3.4 x 10(7) and 6.7 x 10(7) M-1s-1, respectively. Reaction with benzophenone triplet occurs with a near-diffusion-controlled rate constant of 7.6 x 10(9) M-1s-1 in acetonitrile and probably involves charge transfer. When the radical pair formed by reaction of benzophenone triplet and melatonin is sequestered in a micelle, it is subject to extensive magnetic field effects that can be readily interpreted by the radical pair model.

Systematic study of field and concentration effects in capillary electrophoresis of DNA in polymer solutions

A systematic study of the separation of double-stranded DNA in hydroxypropylcellulose (HPC) with a molecular mass of 10 6 was undertaken, using a variety of concentrations (from 0.1 to 1%) and different electric fields (from 6 to 540 V/cm). The data show that a high polymer concentrations (⩾0.4%) and low fields, the separation mechanism is similar to that occurring in gels. The results are in good agreement with theoretical models, and in particular with a recently proposed theory for gels with a pore size smaller than the persistence length of DNA. For more dilute solutions and high fields, however, the separation pattern cannot be explained by existing theories. The existence of an original mechanism was confirmed by the direct observation of the conformation of double-stranded DNA molecules in the polymer solution by fluorescence videomicroscopy. Practical conclusions for the capillary electrophoretic separation of duplex DNA are drawn.

Study of the crystal field effect in REOF:Eu 3+ (RELa, Gd and Y) using optical spectroscopy and X-ray powder diffraction

The luminescence spectra of the Eu 3+-doped rare earth oxyfluorides REOF:Eu 3+ (RELa, Gd and Y) were measured at room and liquid nitrogen temperatures. The interpretation of the spectra and the simulation of the 7F J ( J = 0–5) Stark levels were carried out with the aid of phenomenological crystal field (CF) theory for the C 3v symmetry of the RE 3+ site. The best-fit set of the six non-zero CF parameters yielded the r.m.s. deviations 8, 11 and 10 cm −1 between the experimental and calculated energy levels for YOF, GdOF and LaOF respectively. The strength of the CF effect increases from the LaOF to the YOF host. The X-ray powder diffraction data for LaOF and GdOF were analysed by the Rietveld profile refinement method. From these results the CF parameters were calculated with a modified point charge model. The parameters of ranks four and six were reproduced quite close to the experimental values. The calculated | B 0 2| values were too large, evidently owing to short-range covalency effects.

SIMION study of the fringing field effects in deflector-type electrostatic electron energy analyzers: A new flexible Jost-based correction scheme

The common use of physical stops (such as slits and apertures) at the entrance and exit planes of a deflector-type electrostatic electron energy analyzer often introduces unwanted distortions in the ‘‘ideal’’ equipotential distribution and reduces the deflection angle. Since Herzog’s classic work on the fringing field correction, a variety of correction schemes have been devised. Most of these correction methods are not entirely satisfactory particularly for applications that require the use of position sensitive detectors (PSDs). In the present work, we investigate the effects of physical stops and Jost plates on the fringing field in a standard hemispherical energy analyzer by using the electron ray-tracing simulation program SIMION. By balancing the fringing field effect due to the physical stops with that of the Jost plates, we extend the correction scheme proposed earlier by Jost. The present correction scheme has the advantages of ease of construction and compatibility with PSDs.

Biological effects of environmental electromagnetic fields: molecular mechanisms

In recent years, epidemiological studies have shown an increased risk of leukemia in children living near electric power distribution lines. Also, medical studies have shown that low frequency electromagnetic (EM) fields accelerate the healing of bone fractures. The stimulation of biosynthesis suggested by both epidemiological and clinical studies is observed in vitro, and indicates that EM fields stimulate the biosynthetic ‘stress response’ in cells. Studies of EM field effects on enzyme (Na,K-ATPase) function suggest that these effects should be considered in the design of molecular electronic devices.

Theoretical Study of the Crystal Field Effects on the Transition Dipole Moments in Methylated Adenines

and rotation of the transition dipole moment directions. The interactions between the excited states in the crystal were evaluated by a perturbation treatment. Excited-state mixing leads to extensive transfer of intensities and a slight blue shift in the energy of the transitions. The predicted transition moment directions are in general agreement with experiment, although relative intensities differ in some cases, notably in the two lowest energy transitions.

Comparative study of the crystal field effects in rare earth oxynitrates

The photoluminescence spectra of the lanthanum and gadolinium oxynitrates doped with Eu 3+, REONO 3:Eu 3+ (RE  La and Gd) were measured at 77 and 300 K. The 7 F J ( J0–5) energy level schemes for the 4f 6 electron configuration were simulated by a C 2 ν phenomenological crystal field (CF). The nine non-zero CF parameters for the C 2 ν site symmetry reproduce the experimental energy level schemes with rms deviations of 7 and 5 cm −1 for the LaONO 3 and GdONO 3 hosts, respectively. The CF effect is stronger in the lanthanum host although otherwise the CF parameters for the two oxynitrate hosts differ only slightly. The B 2 k ( k = 2, 4 and 6) values are low indicating only a slight deviation from a higher C 4 ν symmetry. The B 0 2, B 0 4 and B 4 4 parameters assume high values which are similar to those obtained previously for the other RE oxy compounds, i.e. oxyhalides, oxysulfates, oxymolybdates, and oxytungstates. The sixth rank parameters deviate considerably from those obtained for the other RE oxy compounds.

A laser flash photolysis study of magnetic field effects on the hydrogen abstraction reaction of xanthone from xanthene in micellar solution

Abstract Excited triplet xanthone abstracts hydrogen from xanthene in sodium dodecyl sulphate micellar solution. The absorption intensity of the generated xanthenyl radicals increases anomalously in the presence of a magnetic field (less than 0.6 T)). The magnetic field effect is interpreted in terms of the radical pair mechanism. This anomaly can be attributed ti bimolecular reaction processes peculiar to micellar solution.