Field-free Value Research Articles

Hyperfine splitting and lifetime of the positronium lowest level in a strong electric field

When placed in an external electric field, positronium, which already has a finite lifetime due to annihilation, becomes additionally unstable due to field-induced ionization. Calculations of the ionization rate using ab initio methods show that, compared to annihilation, it becomes dominant in the range of field strengths belonging to over-the-barrier ionization (OBI) regime. On the other hand, the hyperfine splitting of the positronium lowest level decreases in the tunnelling domain, taking at the beginning of the OBI domain a value that is about 20% lower than the field-free value. As the field strength increases further, this splitting varies rather slowly, but here an additional splitting of triplet levels occurs, whose rate is comparable to this variation. Finally, it is demonstrated that the lowest energy levels and ionization rates for the hydrogen atom and positronium, determined within the gross structure, are related to scaling transformations.

Fraunhofer and refractive scattering of heavy ions in strong laser fields

Until recently the potential scattering of a charged particle in a laser field received attention exclusively in atomic physics. The differential cross-section of laser-assisted electron-atom collisions for n emitted or absorbed photons is provided by a simple law which casts the result as a product between the field-free value and the square of the Bessel function of order n with its argument containing the effect of the laser in a non-perturbative way. From the experimental standpoint, laser-assisted electron-atom collisions are important because they allow the observation of multiphoton effects even at moderate laser intensities. The aim of this study is to calculate the nucleus-nucleus differential cross section in the field of a strong laser with wavelengths in the optical domain such that the low-frequency approximation is fulfilled. We investigate the dependence of the n-photon differential cross-section on the intensity, photon energy and shape of the pulse for a projectile/target combination at a fixed collision energy which exhibits a superposition of Fraunhofer and refractive behavior. We also discuss the role of the laser perturbation on the near and farside decomposition in the angular distribution, an issue never discussed before in the literature. We apply a standard optical model approach to explain the experimental differential cross-section of the elastic scattering of 4He on 58Ni at a laboratory energy E = 139 MeV and resolve the corresponding farside/nearside (F/N) decomposition in the field-free case. We give an example of reaction in which Fraunhofer diffraction and refractive rainbow hump effects are easily recognized in the elastic angular distribution. Next, we apply the Kroll-Watson theorem, in order to determine the n -photon contributions to the cross-section for continuous-wave (cw) and modulated pulses. In the elastic scattering of heavy ions in a radiation field of low intensity, the amplitude drops by orders of magnitude with respect to the unperturbed case once the exchange of photons is initiated. For intensities approaching $I=10^{17}$ W/cm2 multiphoton effects become important. In the case of short laser pulses we conclude that the strength of n-photon contribution increases with the pulse duration.

Stopping of a relativistic electron beam in a plasma irradiated by an intense laser field

AbstractThe effects of a radiation field (RF) on the interaction process of a relativistic electron beam (REB) with an electron plasma are investigated. The stopping power of the test electron averaged with a period of the RF has been calculated assuming an underdense plasma, ω0 > ωp, where ω0is the frequency of the RF and ωpis the plasma frequency. In order to highlight the effect of the radiation field we present a comparison of our analytical and numerical results obtained for nonzero RF with those for vanishing RF. In particular, it has been shown that the weak RF increases the mean energy loss for small angles between the velocity of the REB and the direction of polarization of the RF while decreasing it at large angles. Furthermore, the relative deviation of the energy loss from the field-free value is strongly reduced with increasing the beam energy. Special case of the parallel orientation of the polarization of the RF with respect to the beam velocity has been also considered. At high-intensities of the RF two extreme regimes have been distinguished when the excited harmonics cancel effectively each other reducing strongly the energy loss or increasing it due to the constructive interference. Moreover, it has been demonstrated that the energy loss of the ultrarelativistic electron beam increases systematically with the intensity of the RF exceeding essentially the field-free value.

Three dimensional simulation on binding efficiency of immunoassay for a biosensor with applying electrothermal effect

In this work, we perform three dimensional finite element simulations on the binding reaction kinetics of the commonly used analyte–ligand protein pairs, namely, C-reactive protein (CRP) and anti-CRP, in a reaction chamber (microchannel) of a biosensor. For the diffusion limited binding biomolecular pairs, due to the slower transport speed of the analyte and the faster reaction rate of analyte–ligand complex, diffusion boundary layers often develop on the reaction surface. To enhance the performance of a biosensor by accelerating the transport speed, a non-uniform AC electric field is applied to induce the electrothermal force to stir the flow field. The swirling flow in the fluid can accelerate the transport of the analyte to and from the reaction surface and hence enhance the association and dissociation of analyte–ligand complex. Four types of biosensors with different arrangements of the geometric locations of the electrode pair and the reaction surface are designed to study the effects of varying geometric configuration on the binding efficiency. The simulation results show that the performance of a biosensor can be better improved by placing the electrodes and the reaction surface on the same side of the microchannel against the opposite side. For the best case studied in this work, the maximum initial slope of the binding curve can be raised up to 6.94 times (with respect to the field-free value) in the association phase, under applying AC field of 15 Vrms and operating frequency of 100 kHz. Another important result with applying electrothermal effect is that it is feasible to use the slower sample flow in the microchannel to save a lot of sample consumption without sacrificing the performance of a biosensor. Several control factors not studied in our previous works such as the thermal boundary condition and the effect of electrical conductivity are also discussed.

Enhancement of On-chip Bioassay Efficiency With Electrothermal Effect

AbstractWe have performed the finite element simulations to study the binding reaction kinetics of the analyte-ligand protein pairs, C-reactive protein (CRP) and anti-CRP, in a reaction chamber of a biosensor. For diffusion limited reactions, diffusion boundary layers often develop on the reaction surface, thus hindering the reaction. To enhance the efficiency of a biosensor, a non-uniform AC electric field is applied to induce the electrothermal force which stirs the flow field. Biosensors with different arrangements of the electrode pairs and the reaction surface are designed to study the effects of geometric configurations on the binding efficiency. The maximum initial slope of the binding curve can be 6.94 times of the field-free value in the association phase, under an AC field of 15 rms and an operating frequency of 100 kHz. With the electrothermal effect, it is possible to use a slower flow and save much sample consumption without sacrificing the performance of a biosensor. Several design factors not studied in our previous works such as the thermal boundary conditions are discussed.

Spherical accretion with anisotropic thermal conduction

We study the effects of anisotropic thermal conduction on magnetized spherical accretion flows using global axisymmetric MHD simulations. In low collisionality plasmas, the Bondi spherical accretion solution is unstable to the magnetothermal instability (MTI). The MTI grows rapidly at large radii where the inflow is subsonic. For a weak initial field, the MTI saturates by creating a primarily radial magnetic field, i.e., by aligning the field lines with the background temperature gradient. The saturation is quasilinear in the sense that the magnetic field is amplified by a factor of $\sim 10-30$ independent of the initial field strength (for weak fields). In the saturated state, the conductive heat flux is much larger than the convective heat flux, and is comparable to the field-free (Spitzer) value (since the field lines are largely radial). The MTI by itself does not appreciably change the accretion rate $\dot M$ relative to the Bondi rate $\dot M_B$. However, the radial field lines created by the MTI are amplified by flux freezing as the plasma flows in to small radii. Oppositely directed field lines are brought together by the converging inflow, leading to significant resistive heating. When the magnetic energy density is comparable to the gravitational potential energy density, the plasma is heated to roughly the virial temperature; the mean inflow is highly subsonic; most of the energy released by accretion is transported to large radii by thermal conduction; and the accretion rate $\dot M \ll \dot M_B$. The predominantly radial magnetic field created by the MTI at large radii in spherical accretion flows may account for the stable Faraday rotation measure towards Sgr A* in the Galactic Center.

Nonlinear Simulations of the Heat-Flux-driven Buoyancy Instability and Its Implications for Galaxy Clusters

In low collisionality plasmas heat flows almost exclusively along magnetic field lines, and the condition for stability to convection is modified from the standard Schwarzschild criterion. We present local two and three-dimensional simulations of a new heat flux driven buoyancy instability (the HBI) that occurs when the temperature in a plasma decreases in the direction of gravity. We find that the HBI drives a convective dynamo that amplifies an initially weak magnetic field by a factor of ~20. In simulations that begin with the magnetic field aligned with the temperature gradient, the HBI saturates by rearranging the magnetic field lines to be almost purely perpendicular to the initial temperature gradient. This magnetic field reorientation results in a net heat flux through the plasma that is less than 1% of the field-free (Spitzer) value. We show that the HBI is likely to be present in the cool cores of clusters of galaxies between ~0.1-100 kpc, where the temperature increases outwards. The saturated state of the HBI suggests that inward thermal conduction from large radii in clusters is unlikely to solve the cooling flow problem. Finally, we also suggest that the HBI may contribute to suppressing conduction across cold fronts in galaxy clusters.

The Effect of an Electric Field on Reaction Fronts in Autocatalytic Systems

The effects of applying a constant electric field E to an ionic autocatalytic reaction with a general power p (p≥1) are discussed through a consideration of the equations for the corresponding travelling waves. It is shown that, when the ratio of diffusion coefficients D of autocatalyst and substrate is less than some value D0 (which depends on p), the effect of a negative electric field is to increase the wave speed of the reaction front over its field-free value. This is in contrast to previous results for cubic autocatalysis (p=2) [J.H. Merkin and H. Sevcikova. J. Math. Chem. 25 (1999) 111.] for D>D0, where the effect is to decrease the wave speed. This feature, seen in the numerical solutions of the travelling wave equations, is confirmed by an expansion for small E and in an asymptotic analysis for p large

Discrete variable method for non-integrable quantum systems

The states of an atom in external electric fields become quasi-bound since the electron can ionize by tunneling through the potential barrier into the continuum. Due to the external electric field the ionization threshold of the atom is lowered from the field-free value. This process becomes important for states close to the classical ionization energy or above. These resonance states can be studied using the complex coordinate method. In this method the Hamiltonian of the system is continued into the complex plane by a complex dilatation, therefore the Hamiltonian is no longer Hermitian and can support complex eigenenergies associated with decaying states. Resonances are uncovered by the rotated continuum spectra with complex eigenvalue and square-integrable (complex rotated) eigenfunctions. The basic idea is to combine this complex coordinate rotation method with the finite element method, and the discrete variable technique. These two methods have been successfully used to compute atomic data for the hydrogen atom in external magnetic and electric fields. We obtain a complex symmetric Hamiltonian matrix, which we solve using the implicitly restarted Arnoldi method (ARPACK). These methods have been extended to alkali atoms in external strong magnetic and electric fields by including model potentials and have also been successfully used in studying various effective one-particle problems.

Light-induced excited states in H-

We present a nonperturbative, fully correlated calculation of the structure of H - in an intense laser field, based on the high-frequency Floquet theory. We show that, as the intensity increases at sufficiently high frequencies, the electron affinity manifests a peculiar behavior and can become substantially higher than its field-free value. Besides, H - acquires excited states, some of which are doubly excited and nonautodetaching. At high intensities H - undergoes stabilization, similarly to the H atom

Chemical shifts in proteins: a shielding trajectory analysis of the fluorine nuclear magnetic resonance spectrum of the Escherichia coli galactose binding protein using a multipole shielding polarizability-local reaction field-molecular dynamics approach

We report the first successful calculation of the fluorine nuclear magnetic resonance spectrum of a protein, the galactose binding protein from Escherichia coli, labeled with ( 5-'9F) tryptophan. Our results indicate that the experimental l9F chemical shifts are dominated by weak, or long-range, electrical interactions, which can be calculated by using the responses of the tensor elements to the uniform field components (&~d/dE,) and the nonuniform or gradient terms (t3uap/W,,), together with the average values of the fields (Ex) and field gradients (V,,) obtained from molecular dynamics (MD) trajectories. A series of shielding Au(Ex,Vxx,Vyy,Vzz)f(7), are obtained, and the mean values, A;, correlate well with the actual shift pattern and overall shift range observed experimentally (Luck, L. A.; Falke, J. J. Biochemistry 1991,30,42484256). The computed 19F NMR of the pentapeptide Gly- Gly-(5-F)Trp-Gly-Gly in HzO is close to, but somewhat more shielded than, that of the denatured protein. Almost all computed 19F chemical shifts are upfield of the field-free value, in accord with the results of ab initio calculations. The chemical shifts calculated are sensitive to the charge chosen for F in the LRF-MD trajectories, and best agreement with experiment is obtained with qF = -0.25. Calculations on the Salmonella typhimurium galactose binding protein yield extremely similar chemical shift spectra, consistent with the -7% difference in amino acid composition. The uniform field components make the largest contributions to the patterns observed, presumably because the gradient terms fall off more rapidly with distance. The exposed residue, Trp 284, has the largest amplitude of fluctuation associated with its trajectory, possibly due to the rapid and random movement of neighboring water molecules. Trp 284 is highly shielded due to interaction with water, although other buried residues (e.g. Trp 133) may also be highly shielded, due to electric field effects within the protein. Our results imply that van der Waals interactions do not play a major role for fluorine nonequivalence in proteins, since the experimental results can be reproduced by using solely the computed field and field gradient terms. The ability to predict protein NMR patterns and ranges offers promise for structural analysis and also provides a way of validating different methods of computing protein electrostatics. In this respect, it is instructive to note that the charge fields from ionized surface groups are found to be largely shielded by the dielectric response of the solvent and the protein.

Atoms in superintense laser fields

A new calculational ansatz for the quantum problem of a hydrogenlike atom in a nonresonant superintense laser field is presented. It is shown that the ground-state energy scales down from the field-free value by the ratio of the Bohr radius to the electron oscillation amplitude in the laser field.

Control of collisionless and collisional processes by nonresonant laser fields

The presence of coherent radiation fields can modify various atomic, ionic, and molecular processes. Cases were examined where the changes can be effected without actual absorption or emission of photons. The radiation fields cause optical Stark shifts of the energies and dress the nonadiabatic couplings between states. Rates of processes such as dissociation, predissociation, atomic and molecular collisions can be radiatively controlled. Formation of new avoided crossings due to radiative interaction gives rise to new phenomena. B exact numerical results from which the influences of the radiation field on arbitrary configurations can be calculated, and simple but accurate analytic results, from which these effects can be conveniently assessed are given. It is shown that the inelastic transition probability is decreased at a true crossing and is increased at an avoided crossing as a result of the presence of the nonresonant radiation field. Furthermore, for two parallel levels with constant nonadiabatic coupling, it is shown that the amplitudes of the nonadiabatic transition are invariant, but its flopping frequency is reduced by the field. Depending on the kinds of final measurements with the field on or off, general behaviors of the inelastic transition probability as a function of the field parameters andmore » the charge-system parameters are predicted and the possibility of inversion (from a field-free value of less than 1/2 to a value greater than 1/2) is demonstrated. Experimental investigation with iodine molecules is suggested. Comparison of nonperturbative results with those of stationary perturbation theory shows that the latter is inadequate in an important parameter region where the field modification of processes is greater than 1%.« less

Photoluminescence of EuSe and the magnetic polaron model

We report on extensive new measurements relating to the near-infrared photoluminescence of EuSe first reported by Busch and Wachter in 1966. The emission and excitation spectra are obtained as functions of temperature and external magnetic field. The total red shift is 0.13 eV in the excitation spectrum (in good agreement with the absorption-edge shift), and 0.25 eV in the emission spectrum. At 4.3°K an external magnetic field of 11 kOe quenches the yield to about 30% of its field-free value. The experimental results are discussed in relation to photoconductivity and optical absorption measurements. An intrinsic and localized model for the luminescence is proposed, in which the emission takes place by radiative recombination of a localized magnetic polaron with a hole in the europium 4f-shell.

Micrometer U-tube manometers for medium vacuum measurements: A M Thomas and J L Cross,J vac sci technol, 4 (1) Jan–Feb 1967, 1–5

The states of an atom in external electric fields become quasi-bound since the electron can ionize by tunneling through the potential barrier into the continuum. Due to the external electric field the ionization threshold of the atom is lowered from the field-free value. This process becomes important for states close to the classical ionization energy or above. These resonance states can be studied using the complex coordinate method. In this method the Hamiltonian of the system is continued into the complex plane by a complex dilatation, therefore the Hamiltonian is no longer Hermitian and can support complex eigenenergies associated with decaying states. Resonances are uncovered by the rotated continuum spectra with complex eigenvalue and square-integrable (complex rotated) eigenfunctions. The basic idea is to combine this complex coordinate rotation method with the finite element method, and the discrete variable technique. These two methods have been successfully used to compute atomic data for the hydrogen atom in external magnetic and electric fields. We obtain a complex symmetric Hamiltonian matrix, which we solve using the implicitly restarted Arnoldi method (ARPACK). These methods have been extended to alkali atoms in external strong magnetic and electric fields by including model potentials and have also been successfully used in studying various effective one-particle problems.

Effects of an electric field on the laser emission of ruby

It has been shown that an external electric field gives rise to a splitting of theR 1 emission lines of ruby (77°K). The resulting changes on the laser output were investigated withDC andAC electric fields. The application of an electric field modifies the amplification per path in a predictable way. Maser action is delayed or even suppressed depending upon the pumping level. At large fields (E≳130 KV/cm) the amplification is reduced by a factor of two; an excess inverted population of twice the field-free value is required to maintain maser action. The energy stored in this way can be released in an intense maser pulse when the electric field is switched off suddenly.

EFFECTS OF AN ELECTRIC FIELD ON THE LASER EMISSION OF RUBY

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