Atom in a femtosecond bichromatic laser field

The ionization of real hydrogen atom is studied under the action of a two-color pulsed laser field of duration 2.12 fs consisting of a fundamental frequencyω and one of its harmonics. We study the effect of phase difference (φ betweenω and its harmonic on the ionization probability. It is found that ionization can be switched on and off by varying the relative phaseφ betweenω and its harmonic.     

On the nature of the BOLD fMRI contrast mechanism

Since its development about 15 years ago, functional magnetic resonance imaging (fMRI) has become the leading research tool for mapping brain activity. The technique works by detecting the levels of oxygen in the blood, point by point, throughout the brain. In other words, it relies on a surrogate signal, resulting from changes in oxygenation, blood volume and flow, and does not directly measure neural activity. Although a relationship between changes in brain activity and blood flow has long been speculated, indirectly examined and suggested and surely anticipated and expected, the neural basis of the fMRI signal was only recently demonstrated directly in experiments using combined imaging and intracortical recordings. In the present paper, we discuss the results obtained from such combined experiments. We also discuss our current knowledge of the extracellularly measured signals of the neural processes that they represent and of the structural and functional neurovascular coupling, which links such processes with the hemodynamic changes that offer the surrogate signal that we use to map brain activity. We conclude by considering applications of invasive MRI, including injections of paramagnetic tracers for the study of connectivity in the living animal and simultaneous imaging and electrical microstimulation.     

Analysis of the Abraham-Minkowski Controversy by Means of Two Simple Examples

Using a very simple example, H. M. Lai [1] argued in favor of Abraham's proposal for defining electromagnetic field momentum density. Later, using another simple device, Johnson et al. [2] argued in favor of Minkowski's proposal. This indicates that the Abraham-Minkowski controversy remains open. Both models consider nonmagnetic media ( = 1). In this work we analyze both models pointing out some weak points and extend the analysis to magnetic media ( 1). We show that in this case Minkowski's proposal is better than Abraham's. Our analysis throws some light on this very old controversy.     

Continuous medium theory for nonequilibrium solvation: II. Interaction energy between solute charge and reaction field and single-sphere model for spectral shift

On the basis of continuous medium theory, a model for evaluation of spectral shifts in solution has been developed in this work. The interaction energy between solute dipole and reaction field and the self-energy of the reaction field have been formulated through derivations. Applying the interaction energy expression together with the point dipole approximation to the case of spherical cavity produces new formulations of spectral shifts. The same expression of electrostatic free energy of the nonequilibrium state is achieved by integrating the change of the electrostatic free energy for a charging process. Moreover, generalized formulations evaluating spectral shifts have been established in the charge-potential notation, and the reduction of them to the point dipole case consistently leads to the same formulations of spectral shifts as those by interaction energy approach. Mutual supports provide convincing evidences for the reliability of the present results. In this work, attentions are particularly paid to the conclusion of zero self-energy of the reaction field, which is different from the previous theory. Reasoning and arguments are given on this point. From the present derivations, it is concluded that the spectral shifts of light absorption and emission were theoretically exaggerated in the past, in particular, by a factor of 2 for the spectral shift sum.     

Simulation of the Field Emission Arrays with Parallel Wedge Emitter, Gates and Co-Planar Lens

Using the particle-in-cell (PIC) simulation code MAGIC, the wedge emitter, parallel gate and anode with and without co-planar lens have been simulated. The electron beam streamlines and the X-V_x phase space have been evaluated, which show that the co-planar lens focuses the electron. Simulation results are presented for a single FEA's cell with the anode biased at 500V and spaced 100m from the cathode, which indicated that the electron beam width at anode is about 60.2m for without lens FEAs, after add the lens with –70V, the electron beam width will be decrease to 8.1m, meanwhile the maximum transverse velocity will decrease from ±1×10⁶m/s to ±1.46×10⁵m/s.     

Nuclear hyperfine coupling interactions in the rotational spectra of the linear and bent isomers of HF-N2O

The rotational spectra of six isotopomers of the linear and bent isomers of HF-N₂O have been collected in the 7-18 GHz region with a Fourier transform microwave spectrometer. The nuclear hyperfine structure in the spectra produced by HF spin-spin coupling interaction and nuclear quadrupole coupling interactions due to the D nucleus of DF and the nuclei of N₂O have been resolved and analyzed. In the linear isomer, H in HF is bonded to the terminal N in N₂O. The NF bond lengths are 2.9808(2) Å for the HF-containing isotopomers and 2.9732(2) Å for the DF-containing isotopomers. The zero point angles are 23.1° for HF and 31-34° for N₂O. The hyperfine constants suggest that the HF bond is lengthened by 0.0105 Å upon complexation and that the electric field gradients of the two nitrogen nuclei in N₂O are perturbed differently in the complex. In the bent isomer, the hydrogen bond is formed between HF and O in N₂O. The intermolecular distances are 3.4942(2) Å for the HF-containing isotopomers and 3.4436(2) Å for the DF-containing isotopomers, with HF and N₂O forming angles of 34° and 46°, respectively, with the intermolecular axis. The nuclear quadrupole coupling constants of the two nitrogen nuclei do not indicate electric field gradient perturbation in this isomer.     

Problems of Dust Grains Charging to Negative Potentials

The presence of dust grains is a common phenomenon in the space environment. Grains can be charged by many different processes (e.g., photoemission, attachment of electrons/ions, the secondary emission, etc.). If the grain's surface potential becomes high enough, one can observe field emission of ions or electrons. We are trapping a single dust grain in a Paul trap, expose it to a low-energy electron beam, and investigate the evolution of its charge-to-mass ratio with respect to the energy of primary electron beam. We use micron-sized (D = 2–11 m) glass grains and charge them up to -300 V of surface potentials; it corresponds to the electric field strength of about 10⁸ V/m. Analysis of the charging/discharging processes has shown that (1) the effect of the field enhanced secondary emission is negligible in the case of insulators and (2) the effective work function for electron field emission from charged insulators is as low as 1 eV.     

Hermiticity of Quantum Observables Versus Commutation Relations

In order to obtain sum rules and spectral representations the Hermiticity property , A = A, of observables is used. It is shown that for certain and the property turns out to be inconsistent with the commutation relations that contain A. The known Schwinger paradox is explained by this inconsistency.     

Predictions of pulsed field gradient NMR echo-decays for molecules diffusing in various restrictive geometries. Simulations of diffusion propagators based on a finite element method

Pulsed field gradient NMR diffusometry is a promising tool for investigating structures of porous material through determinations of dynamic displacements of molecules in porous systems. A problem with this approach is the lack of closed analytical expressions for echo-decays in anything but idealized pore geometries. We present here an approach based on calculating the appropriate diffusion propagator by means of finite element calculations. The suggested method is quite general, and can be applied to arbitrary porous systems. The protocol for the calculations is outlined and we show results from some different cases: diffusion in confined geometries and in systems that are spatially inhomogeneous with respect to concentration.     

NMR-microscopy with TrueFISP at 11.75T

The purpose of this paper is to demonstrate that a fully balanced gradient echo technique (TrueFISP) can be used for microscopic experiments at high static magnetic field strengths. TrueFISP experiments were successfully performed on homogeneous and inhomogeneous objects at 11.75T. High-resolution TrueFISP images were obtained from phantoms, plants, formalin-fixed samples, and from an isolated beating rat heart with an in-plane resolution of 78 micro m and a slice thickness of 500 micro m. The signal-to-noise ratio (SNR) gain of TrueFISP compared to conventional gradient echo or spin echo sequences will allow faster acquisition times or an improvement in spatial resolution for microscopic experiments.