Observed Polarization Effects Research Articles

Switching behavior induced by different substituents of group in single molecular device

We investigate the electronic transport properties of photochromic azobenzene-based molecular devices with Au electrodes using non-equilibrium Green’s function and density functional theory. A reversible switching behavior between cis and trans isomerization is found in the device. In addition, the substituent of −NH2 on the right end hydrogen atom of azobenzene molecule reduces the switching ratio of current, consequently the disappearance of switching behavior, while the substituent of −NO2 improves the switching ratio of current. We discuss the different electronic transport induced by different substituents through the transmission spectra, localized density of states, molecular projected self-consistent Hamiltonian and transmission pathways. The observed polarization effect under bias is explained by the evolution of molecular projected self-consistent Hamiltonian of LUMO level. The results indicate that the electron-withdrawing group −NO2 substituting right terminal hydrogen of azobenzene molecule becomes a candidate for improving the performance of molecular device.

Role of the reaction intermediates in determining PHIP (parahydrogen induced polarization) effect in the hydrogenation of acetylene dicarboxylic acid with the complex [Rh (dppb)]+ (dppb: 1,4-bis(diphenylphosphino)butane)

This study deals with the parahydrogenation of the symmetric substrate acetylene dicarboxylic acid catalyzed by a Rh(I) complex bearing the chelating diphosphine dppb (1,4-bis(diphenylphosphino)butane). The two magnetically equivalent protons of the product yield a hyperpolarized emission signal in the (1)H-NMR spectrum. Their polarization intensity varies upon changing the reaction solvent from methanol to acetone. A detailed analysis of the hydrogenation pathway is carried out by means of density functional theory calculations to assess the structure of hydrogenation intermediates and their stability in the two solvents. The observed polarization effects have been accounted on the basis of the obtained structures. Insights into the lifetime of a short-lived reaction intermediate are also obtained.

Changes in alcohol consumption after a natural disaster: a study of Norwegian survivors after the 2004 Southeast Asia tsunami

BackgroundMany studies suggest that disaster exposure is related to a subsequent increase in alcohol consumption. Most of these studies have relied on retrospective self-reports to measure changes in alcohol use. The aim of the present study was to examine the association between disaster exposure and drinking behaviors more closely, analyzing data on both self-perceived changes in alcohol consumption and current drinking habits in groups with different extents of disaster exposure.MethodsA sample of Norwegian adults (≥ 18 years) who resided in areas affected by the 2004 Southeast Asia tsunami (N = 899) were assessed by a postal questionnaire 6 months after the disaster. Based on detailed questions about experiences with the tsunami, participants were grouped according to their extent of disaster exposure. The Impact of Event Scale-Revised was applied to measure the level of post-traumatic stress. Participants were asked whether they had increased or decreased their alcohol consumption after the disaster. Moreover, weekly alcohol consumption and frequency of intoxication during the past month were used as indicators of current drinking behaviors.ResultsSeverely exposed individuals more often reported changing their alcohol consumption compared with those who were less exposed. Severe exposure to the tsunami was associated with both a self-perceived increase (OR 21.38, 95% CI 2.91–157.28) and decrease in alcohol consumption (OR 7.41, 95% CI 1.74–31.51). The odds ratios decreased and were not significant when adjusting for post-traumatic stress symptoms. Weekly consumption and frequency of intoxication during the past month did not vary with extent of disaster exposure.ConclusionsOur findings indicate a polarization effect of severe disaster exposure on self-perceived changes in alcohol consumption; that is, disaster exposure was associated with self-perceived increases and decreases in drinking. However, the absence of associations between disaster exposure and indicators of current drinking behaviors suggests that the observed polarization effect may be overestimated because of attribution and recall bias.

Impulse breakdown of water with different conductivities

The complexity of the impulse breakdown of liquid water is reflected by the dependency of pre-breakdown processes on the polarity, rise-time and wave-shape of the applied impulses as well as on physical properties, such as the electrical conductivity of water itself. Further understanding of the mechanisms of formation and propagation of impulse discharges in water and water solutions is therefore required to enable the development of pulsed power and plasma technologies. This paper presents a study of the dielectric behaviour of water stressed with positive and negative high voltage impulses in a point-plane electrode topology. Water with different conductivities including distilled water and tap water was investigated. The volt-time breakdown characteristic of water is discussed and the total pre-breakdown time has been obtained for both positive and negative polarity impulses for 2 mm, 5 mm and 10 mm inter-electrode distances. The Laue statistical analysis has been used in order to calculate the statistical and formative time lags. Using the formative time data the nominal breakdown velocity in water for positive and negative impulses has been obtained. It has been shown that the anode discharges propagate with supersonic velocities, up to ~20×105 cm/s, and the cathode discharges are sub-sonic with a maximal velocity of ~c1x10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm/s. Discharge velocities are slightly higher in distilled water as compared with tap water. Potential breakdown mechanisms which can explain the observed polarity effects in the transient prebreakdown processes in liquid water stressed with high voltage impulses are discussed.

Surface states and photo-induced charge transfer on oxygen-terminated chemical vapor deposition diamond

Time-of-Flight (ToF) experiments were carried out to study the electrical transport and charge trapping properties of oxidized freestanding single crystalline (100) CVD diamond crystals. It is shown that the diamond surface and properties of the electrical contacts are of extreme importance, influencing measured charge carrier mobility values. While sputtered Al contacts yield textbook ToF spectra and electron mobilities μe exceeding 1860 cm2/Vs, thermally deposited contacts suffer from enhanced trapping. We propose a model in which the observed polarization effects can be explained by charge transfer to oxygen-related surface states, which interact with surface adsorbates, according to the well-known transfer-doping model.

Linear Dipole Behavior in Single CdSe-Oligo(phenylene vinylene) Nanostructures

We report on linearly polarized absorption and emission from individual (4.3 nm) CdSe quantum dots whose surfaces are coordinated with monodisperse oligo-phenylene vinylene ligands. Shown previously to suppress quantum dot blinking, we demonstrate here that the electronic interaction of photoexcited ligands with the quantum dot core is manifested as a strong polarization anisotropy in absorption (M = 0.5), as well as distinct linear dipole emission patterns from the quantum dot core. Further, there is a correlation between the quantum dot emission moment and polarization orientation corresponding to the absorption maxima that is manifested as fluctuations in emission moment orientation in the X-Y plane. The observed polarization effects can be switched off by tuning the excitation away from the ligand absorption band. We propose a mechanism based on exciton dissociation from the photoexcited ligand, followed by the pinning of electrons at the quantum dot surface. The resulting Stark interaction is sufficiently strong to break the 2D degeneracy of the emission moment within the dot, and may therefore account for the linear dipole emission character.

Effect of surface plasmon resonance on the optical activity of chiral metal nanogratings

We examine the mechanism responsible for the optical activity of a two-dimensional array of gold nanostructures with no mirror symmetry on a dielectric substrate. Measurements with different incident angles, polarizations and sample orientations allow us to reveal that observed polarization effect is enhanced by surface plasmon resonance. By performing numerical simulation with rigorous diffraction theory we also show that the grating chirality can be described in terms of the non-coplanarity of the electric field vectors at the front (air-metal) and back (substrate-metal) sides of the grating layer.

Two-photon spectroscopy of aligned acetaldehyde

We reinvestigate the two-photon absorption of the (3s-n) transition in acetaldehyde and its implications for the observation of collision-induced alignment. The two-photon spectroscopy of an aligned ensemble of acetaldehyde produced in collisions with He provides clear evidence for the quantum interference of tensors of different rank. The observed polarization effect can be understood in terms of the non-vanishing tensor components Z 0 (0)=1.0, Z 0 (2)=0.4±0.1 and | Z ±2 (2)|=0.3±0.1. For a collision energy of 916 cm −1, the alignment is largely consistent with the predictions of the kinematic apse model.

Quantum interference in the REMPI detection of aligned NO

We study the detection of collision induced alignment in ensembles of NO molecules scattered from He through (2 + 1) resonance enhanced multiphoton ionization (REMPI) of NO via the Rydberg states: H2Σ,H′2Π. The two-photon spectroscopy of the H–X transition is complicated by the state mixing due to L-uncoupling and the simultaneous presence of zeroth and second rank tensor components for this strong transition. The creation of an aligned ensemble of NO molecules in a well-defined collision process enables us to study in detail the polarization effect for different branches of the H–X two-photon spectrum. The polarization effect results from the interference of the second rank tensor components with the zeroth rank tensor of the two-photon absorption tensor. Variations in the polarization effect for perturbed levels of the excited state reflect directly the changes in the expansion coefficients for the involved electronic wavefunctions. The experimentally observed polarization effects are consistent with the non-vanishing components (Z(0)0:Z(2)0:Z(2)2)=(1.0:−0.2:0.6) of the two-photon absorption tensors.

Polarization rotation in parametric scattering of polaritons in semiconductor microcavities

Polarization of light emitted by a semiconductor microcavity in the regime of a resonant parametric scattering of the exciton polaritons shows extremely strong and unusual dependence on the polarization of pumping light. This dependence is interpreted here using the pseudospin model and in the framework of a quasiclassical formalism where the parametric scattering is described as resonant four-wave mixing. We show that the optically induced splitting of the exciton-polariton eigenstate, both in linear and circular polarizations, is responsible for the observed polarization effects. The splitting in circular polarizations, achieving 0.5 meV, has been detected experimentally, while the splitting in linear polarizations, which is much weaker, only manifests itself in the pseudospin dynamics of the exciton polaritons.

Measuring amplitude and phase distribution of fields generated by gratings with sub-wavelength resolution

In this paper, we intend to gain an understanding of the interaction of light with microstructures. Measurements of amplitude and phase in the diffracted field close to gratings using a heterodyne scanning probe are presented. Coherent light diffracted by microstructures produces periodic features and can give birth to phase dislocations, also called phase singularities. Phase singularities are isolated points where the amplitude of the field is zero. We present measurements of such phase singularities with 10 nm spatial sampling and compare them with theoretical results obtained from rigorous diffraction calculations. The observed polarization effects reveal also important information about the vectorial field conversion by the fiber tip.

Intramultiplet mixing in the electron-volt energy range for Ne**{(2p)5(3p)}+He.

We present measurements on fine-structure-changing ${\mathrm{Ne}}^{\mathrm{*}\mathrm{*}}${(2p${)}^{5}$(3p)}+He collisions at superthermal energies with an average value of 〈E〉=470 meV. For an extensive set of transitions absolute values of polarized-atom cross sections are given. A wide range of cross section magnitudes and polarization effects is observed. Even higher order polarization effects are evidently present. A suitable set of anisotropy parameters is introduced which has large advantages for a direct comparison of experimental and calculated cross sections. Although the experimentally observed polarization effects are reproduced by quantum calculations, the calculated magnitude of the cross section is, in general, larger than the experimental value. Moreover, in contrast with the thermal energy range, the current semiclassical model fails in describing the most prominent cross sections in terms of the ``visible'' avoided crossings in the adiabatic potentials. In the quantum calculations, clear indications are found for an additional transition mechanism of radial matching. In this mechanism a change in the rotational quantum number of the nuclear motion matches the asymptotic energy gap \ensuremath{\Delta}${\mathit{E}}_{\mathit{k}\mathit{l}}$ between the initial and final state k and l, respectively. This process is most efficient at large internuclear distances, thus enhancing the magnitude of the cross section considerably. Radial matching is most effective when 〈E〉/\ensuremath{\Delta}${\mathit{E}}_{\mathit{k}\mathit{l}}$\ensuremath{\gg}1. The calculated magnitude of the cross section is thus sensitive to details of the long-range part of the potentials used as input. At thermal energies this sensitivity is masked, which explains the good agreement of the calculations with the model potentials of Hennecart and Masnou-Seeuws [J. Phys. B 18, 657 (1985)] with the experiments in the thermal range.

Spin exchange between ion probes and localized moments in ferromagnets as the origin of transient fields.

The transient field phenomenon has been ascribed to a polarization transfer between the electrons of the ionic projectiles and the surplus of majority spin electrons of the ferromagnetic host over the minority spin electrons. Earlier attempts to explain this crucial process failed to account for the order of magnitude of the experimentally observed transient field strengths. A recent model which proposes spin exchange scattering between bound projectile electrons and quasifree host electrons as the mechanism of polarization transfer arrives at the correct orders of magnitude but is in conflict with the weak velocity dependence of the experimental polarization, exhibiting a strongly decreasing behavior with increasing velocity. The new model presented here proposes spin exchange between the ionic shell and localized electrons of the ferromagnet as a more adequate approach to the problem. It is shown that calculations involving hydrogenlike ions explain the size of the experimentally observed polarization effects as well as their velocity dependence for various ion probes traversing thin iron foils.

The phonon-induced line shape of the dynamical Stark effect in Cu2O

The authors study the influence of electron-phonon coupling on the resonant dynamical Stark effect in Cu2O. They start from a multi-band density matrix theory involving directly forbidden dipole transitions and directly allowed quadrupole transitions. The use of the multi-band formalism is necessary for the inclusion of the observed polarization effects. From a microscopic treatment of the electron-phonon interaction they derive a frequency-dependent transverse relaxation for the excitons and a renormalization of the optical transition matrix elements resulting in a Fano effect. They derive an expression for the differential transmission, taking into account a parametric sum frequency generation. Theoretical and experimental data are compared, showing satisfactory agreement.

Polarization effects in the ionization cross section of Ar, Kr, and Xe by laser-excited Ne**

In a crossed-beam experiment the total ionization cross section for the title systems has been investigated in the range 0.1\ensuremath{\leqslant}E (eV) \ensuremath{\leqslant}4 of collision energies. The population of the short-lived Ne**[(3p);J=3] state is produced by saturated optical pumping of the Ne*[(3s);J=2]\ensuremath{\leftrightarrow}Ne**[(3p);J=3] two-level system with a polarized laser beam, resulting in a well-determined distribution of the magnetic substrates |J,M〉 with respect to the relative velocity g. By measuring the ion yield in the scattering center at five different orientations of the laser polarization (linear and circular) with respect to g, the data can be analyzed in terms of pure-state total ionization cross sections Q3|M| corresponding to a single asymptotic state |J,M〉. The observed polarization effect at E=0.1 eV is Q3|M|=0,1/Q3|M|=3=2.5, which is in good agreement with the data of Bussert, T. Bregel, R. J. Allan, M. W. Ruf, and H. Hotop [Z. Phys. A 320, 105 (1985)] in the thermal energy range as obtained by analyzing the Penning electrons. This polarization effect decreases to a value of 1.4 for E&gt;2 eV. The results are discussed in terms of semiclassical scattering calculations with an optical potential as input, using a model-potential approach for calculating both the real and the imaginary parts. For the autoionization width this results in a two-state \ensuremath{\Gamma}\ensuremath{\sigma}\ensuremath{'} and \ensuremath{\Gamma}\ensuremath{\pi}\ensuremath{'} basis for the \ensuremath{\sigma}\ensuremath{'} and \ensuremath{\pi}\ensuremath{'} orientations of the (2p)-1 hole, calculated in a one-electron orbital overlap approximation. The preference for the \ensuremath{\Omega}=0,1 states at E=0.1 eV indicates the correct relative scaling of these two ionization widths, leading to \ensuremath{\Gamma}\ensuremath{\sigma}\ensuremath{'}=79\ensuremath{\Gamma}\ensuremath{\pi}\ensuremath{'} at R=4.5a0. The observed energy dependence is due to the decrease of ``locking'' of the total angular momentum J to the internuclear axis R with increasing angular velocity \ensuremath{\varphi}\ifmmode \dot{}\else \.{}\fi{}, leading to the dynamical criterion \ensuremath{\omega}prec=4\ensuremath{\varphi}\ifmmode \dot{}\else \.{}\fi{} for the transition of a space-fixed to a body-fixed description of J. The semiclassical precession frequency \ensuremath{\omega}prec of J around R is related to the average \ensuremath{\Omega} splitting of the real part of the optical potential by \ensuremath{\omega}prec=〈\ensuremath{\Delta}V\ensuremath{\Omega},\ensuremath{\Omega}\ifmmode\pm\else\textpm\fi{}1〉/\ensuremath{\Elzxh}. With these assumptions we observe a good agreement between the experimental results and the semiclassical calculations. Finally, we discuss the validity of a semiclassical locking picture, with emphasis on the difference between locking of the angular momentum versus locking of the electron orbitals involved.

Metal-centered radical-pair mechanism for alkyne hydrogenation with a binuclear rhodium hydride complex. CIDNP without organic radicals

Abstract : The binuclear complex Rh2H2(CO)2(dppm)2, 1, reacts with alkynes under H2 to form the corresponding olefins with predominantly cis addition, and an alkyne-bridged A-frame complex. Specific alkynes studied include PhC triple bond CH, MeC triple bond CH, t-Buc triple bond CH, EtOC triple bond CH, HC triple bond CCOOMe, HC triple bond CCH2OH, and CH3C triple bond CCH2OH. In all of the hydrogenation reactions, CIDNP is observed with major polarizations occurring as multiplet effects in thr trans and geminal proton resonnances of the product olefins. Specific labelling studies using 1 + PhC triple bond 13CH, Ph13C triple bond CH, and PhC triple bond CD, and 1-d sub 2 + PhC triple bond CH show no polarization due to 13C hyperfine coupling, and major polarizations only in those protons originally on the binuclear complex 1. These studies, together with radical trapping experiments, lead to the conclusion that the radical pair responsible for the observed CIDNP is metal-based and does not involve alkenyl radicals. The basis of the observed CIDNP thus differs from the previously described mechanism of H-atom transfer between a transition metal hydride and an organic substrate. The present results are consistent with a new mechanism in which a metal-centered biradical acts as the radical pair leading to the observed polarization effects.

Organization of collagen fibers in the intestine.

The characteristic extinction pattern which is observed when the submucosa is viewed in the optical polarizing microscope has been analyzed in terms of the configuration and orientation of the 4 micron diameter collagen fibers. It is shown that the observed polarization effects are produced by periodic variations in orientation of fully birefringent fibers. The fiber configuration required to produce the observed polarization effects is a tilted wave configuration with a crimp period of approximately 20 micron. In the model, the tilted waveform fibers are crimped in register and form parallel arrays. The arrays are oriented in layers at approximately +30 degrees and -30 degrees to the longitudinal direction and are mirror images of each other. Analysis of the extinction pattern shows that the model satisfactorily accounts for the observed polarization effects at several different angles of the crossed polaroids. The calculated strain necessary to straighten the wavy fibers of the model correlates well with the observed strain to uncrimp the collagen fibers in the intestine. This suggests that the initial response to stress is gradual uncrimping of the collagen fibers, and concurrently, a decrease in the angle between biaxially oriented fibers, rather than extension of the straight fibers.

A theory and model for wave propagation through foliage

A theory is presented and a model is developed to predict the effects of foliage on a line‐of‐sight propagating field. In view of the sparse concentration of foliage, the Foldy‐Twersky theory for wave propagation through discrete random media is used to obtain the complex propagation constant for the coherent field. On the basis of the measurements of Stutzman et al. (1979), the dominant foliage components are taken to be Rayleigh‐like in their scattering and absorbing characteristics. The effective fractional volume of foliage comprising the Rayleigh components is constructed as a function of frequency using forest stand table data. Trunks and branches are modeled as Rayleigh cylinders; leaves are ignored because of their small total volume. The model fails above the frequency for which the effective fractional volume goes to zero. The model provides reasonably good agreement with propagation measurements and is capable of explaining some of the observed polarization effects.

A semiclassical model for the polarization on inclusively produced Λ 0-particles at high energies

We present a semiclassical model for inclusive Λ 0-production which is able to explain the observed polarization effect in proton-proton collisions. It is well known that hard scattering processes, when calculated in perturbative QCD, do not give rise to large polarization effects. The mechanism producing the polarization in this model is basically a soft process, where quark-antiquark pairs are produced by a tunneling process in the colour field and where perturbatice QCD is not applicable.

Magnetic fields in galaxies as revealed by the polarization of light

Some new results on optical polarization in galaxies are reported. These results as well as some other available data indicate the presence of large-scale magnetic fields in galaxies. Spiral galaxies seen nearly edge-on show polarization of light in the dark band across the center, indicating that the large-scale magnetic fields in these galaxies are mainly parallel to the symmetry plane of each galaxy, as has also been observed in the Milky Way. In more tilted galaxies, where the spiral structure can be seen more clearly, the observed polarization effects indicate large-scale magnetic fields along the arms. Observed polarization effects also indicate the presence of ordered magnetic fields in some peculiar galaxies. In NGC 2685 the fields seem to be parallel to the helical or arc-like filaments. In NGC 3718 the dark band which might be a ‘dust-bar’ through the nucleus shows strong optical polarization in the direction of the band, indicating a large-scale magnetic field in the same direction.