Time-resolved optical spectroscopy of the pulsating DA white dwarf HS0507+0434B - New constraints on mode identification and pulsation properties

We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, l, of the modes, most of which have l = 1. The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of 2.6 +/- 1.0 kms(-1), which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective.

Time-resolved resonance Raman spectroscopy

Vibrational Raman spectroscopy is now widely recognized as a useful technique for chemical analysis. It has become increasingly popular for the characterization of stable species since the technology which underpins Raman measurements has matured. Time-resolved Raman spectroscopy has also become established as an excellent method for the characterization of transient chemical species but it is not so widely applied. However, the technical advances which have reduced the cost and increased the reliability of conventional: Raman systems can also be exploited in studies of transient species. In some cases it is just as straightforward to record the Raman-spectra of a short-lived transient species as it is to monitor a more stable sample. This raises the possibility of routinely adding time-domain Raman measurements to more conventional Raman techniques, increasing the selectivity of the analysis while retaining its ability to provide spectral information which is characteristic of the species under investigation.

TIME-RESOLVED RESONANCE RAMAN-SPECTROSCOPY AND RAMAN SPECTROELECTROCHEMISTRY OF (CO)(5)W[4,4'-BPY]W(CO)(5), PROBED IN THE VISIBLE AND NEAR-INFRARED

Time-resolved resonance Raman spectroscopy of the lowest energy excited state of the 4,4'-bipyridyl ligand-bridged complex, [(CO)(5)W(L)W(CO5] (1), and Raman spectroscopy of electrochemically reduced 1, both give bands characteristic of the the L(.-) species. This confirms that the ligand L is negatively charged in the lowest energy exicited state which is therefore metal-ligand charge transfer (MLCT) in character. Raman spectra of the radical anion of 1 excited in the far red (800 nm) exhibited a band near 2050 cm(-1) due to a vco symmetric CO stretching mode, compared to the corresponding band at 2070 cm(-1) in the spectrum of the parent, uncharged complex. The lower vco in the reduced complex supports the recent finding by time-resolved IR spectroscopy of a similar frequency decrease for nu(CO) in the longest lived (MLCT) excited state of 1 which was attributed to electron/hole localisation in this state on the IR time scale.

TIME-RESOLVED RESONANCE RAMAN-SPECTROSCOPY OF EXCITED SINGLET AND TRIPLET-STATES OF FREE-BASE MESO-TETRAPHENYLPORPHYRIN

Two-color time-resolved resonance Raman spectroscopy has been used to probe the lowest excited singlet (S1) and triplet (T1) states of free-base meso-tetraphenylporphyrin and meso-tetrakis(4-sulphonatophenyl)porphyrin in solution at room temperature. The spectra were recorded using 532-nm excitation pulses and time-delayed probe pulses (DELTAT = 0-30 ns, 447 and 460 nm) near lambda(max) of the S1 and T1 states. Significant shifts in frequency of the porphyrin core vibrations were observed upon excitation to either the S1 or T1 state. Several of the strongest polarized bands in the spectra of both excited states, including nu1, nu2, nu4, nu6, and phi4, are assigned, and the information they give on the differences in electron distribution in the ground, S1, and T1 states is discussed.

Functional analysis of the glycero-manno-heptose 7-phosphate kinase domain from the bifunctional HldE protein, which is involved in ADP-L-glycero-D-manno-heptose biosynthesis

The core oligosaccharide component of the lipopolysaccharide can be subdivided into inner and outer core regions. In Escherichia coli, the inner core consists of two 3-deoxy-d-manno-octulosonic acid and three glycero-manno-heptose residues. The HldE protein participates in the biosynthesis of ADP-glycero-manno-heptose precursors used in the assembly of the inner core. HldE comprises two functional domains: an N-terminal region with homology to the ribokinase superfamily (HldE1 domain) and a C-terminal region with homology to the cytidylyltransferase superfamily (HldE2 domain). We have employed the structure of the E. coli ribokinase as a template to model the HldE1 domain and predict critical amino acids required for enzyme activity. Mutation of these residues renders the protein inactive as determined in vivo by functional complementation analysis. However, these mutations did not affect the secondary or tertiary structure of purified HldE1, as judged by fluorescence spectroscopy and circular dichroism. Furthermore, in vivo coexpression of wild-type, chromosomally encoded HldE and mutant HldE1 proteins with amino acid substitutions in the predicted ATP binding site caused a dominant negative phenotype as revealed by increased bacterial sensitivity to novobiocin. Copurification experiments demonstrated that HldE and HldE1 form a complex in vivo. Gel filtration chromatography resulted in the detection of a dimer as the predominant form of the native HldE1 protein. Altogether, our data support the notions that the HldE functional unit is a dimer and that structural components present in each HldE1 monomer are required for enzymatic activity.

Ixaru's Extended Frequency Dependent Quadrature Rules for Slater Integrals: Parallel Computation Issues

We describe recent progress of an ongoing research programme aimed at producing computational science software that can exploit high performance architectures in the atomic physics application domain. We examine the computational bottleneck of matrix construction in a suite of two-dimensional R-matrix propagation programs, 2DRMP, that are aimed at creating virtual electron collision experiments on HPC architectures. We build on Ixaru's extended frequency dependent quadrature rules (EFDQR) for Slater integrals and examine the challenge of constructing Hamiltonian matrices in parallel across an m-processor compute node in a block cyclic distribution for subsequent diagonalization by ScaLAPACK.

Crack detection of steel girders using Brillouin optical time domain analysis

This article presents the results from an experimental program designed to evaluate the performance of a system consisting of a readout unit and a ribbon type Fiber Optic Sensor (FOS) based on Brillouin Optical Time Domain Analysis (BOTDA). The system is intended for the detection of cracks as well as the monitoring of long-term performance for steel bridge girders. The program consisted of introducing a crack at the center of a 3-m-long steel beam and monitoring its progression using static loading tests performed at ambient and sub-zero temperatures. For sensor lengths similar to those used in the field, the resonant frequency shifts per unit increase in crack width were found to decrease from 114 MHz/mm at ambient temperature (~25C) to 65 MHz/mm at -10C. Results also revealed nonlinearity and variability, which can be attributed to an incompatibility between the settings of the laser pump in the readout unit and the sensor length. Significant losses were detected along the bonded segments of the sensor and were attributed to the presence of ripples along the sensor. These undulations worsen with a reduction in temperature and are induced by the bonding procedure as well as the slack provided in the plastic sleeves containing the splices.

Substrate Clamping Effects on Irreversible Domain Wall Dynamics in Lead Zirconate Titanate Thin Films

The role of long-range strain interactions on domain wall dynamics is explored through macroscopic and local measurements of nonlinear behavior in mechanically clamped and released polycrystalline lead zirconate-titanate (PZT) films. Released films show a dramatic change in the global dielectric nonlinearity and its frequency dependence as a function of mechanical clamping. Furthermore, we observe a transition from strong clustering of the nonlinear response for the clamped case to almost uniform nonlinearity for the released film. This behavior is ascribed to increased mobility of domain walls. These results suggest the dominant role of collective strain interactions mediated by the local and global mechanical boundary conditions on the domain wall dynamics. The work presented in this Letter demonstrates that measurements on clamped films may considerably underestimate the piezoelectric coefficients and coupling constants of released structures used in microelectromechanical systems, energy harvesting systems, and microrobots.

Real-space mapping of dynamic phenomena during hysteresis loop measurements: Dynamic switching spectroscopy piezoresponse force microscopy

Dynamic switching spectroscopy piezoresponse force microscopy is developed to separate thermodynamic and kinetic effects in local bias-induced phase transitions. The approaches for visualization and analysis of five-dimensional data are discussed. The spatial and voltage variability of relaxation behavior of the a-c domain lead zirconate-titanate surface suggest the interpretation in terms of surface charge dynamics. This approach is applicable to local studies of dynamic behavior in any system with reversible bias-induced phase transitions ranging from ferroelectrics and multiferroics to ionic systems such as batteries, fuel cells, and electroresistive materials. (C) 2011 American Institute of Physics. [doi:10.1063/1.3590919]

Tip-enhanced Raman microscopy: practicalities and limitations

The feasibility of apertureless scanning near-field Raman microscopy, exploiting the local enhancement in Raman scattering in the vicinity of a silver or gold tip, was investigated. Using the finite difference time domain method we calculated the enhancement of electric field strength, and hence Raman scattering, achieved through the resonant excitation of local modes in the tip. By modelling the frequency-dependent dielectric response of the metal tip we were able to highlight the resonant nature of the tip-enhancement and determine the excitation wavelength required for the strongest electric field enhancement, and hence Raman scattering intensity, which occurs for the excitation of modes localized at the tip apex. It is demonstrated that a peak Raman enhancement of 10(7)-fold should be achievable with <5 nm spatial resolution. We show that surface-enhanced Raman scattering from carbon contamination on a silver or gold tip can be significant. However, we find for a tip of radius of curvature 20 nm that the Raman enhancement should decay totally within 20 nm from the tip. Hence withdrawal of the tip by this distance should lead to the disappearance of the tip-enhanced signal, leaving only that from carbon contamination on the tip itself and the intrinsic signal from the sample. Copyright (C) 2003 John Wiley Sons, Ltd.

Comparison of Wavelet Transform and Time-Domain Analysis of Second Trimester Uterine Artery Doppler Waveforms in Screening for Pre-Eclampsia

The aim of this study was to compare time-domain waveform analysis of second-trimester uterine artery Doppler using the resistance index (RI) with waveform analysis using a mathematical tool known as wavelet transform for the prediction of pre-eclampsia (PE). This was a retrospective, nested case-cohort study of 336 women, 37 of whom subsequently developed PE. Uterine artery Doppler waveforms were analysed using both RI and waveform analysis. The utility of these indices in screening for PE was then evaluated using receiver operating characteristic curves. There were significant differences in uterine artery RI between the PE women and those with normal pregnancy outcome. After wavelet analysis, significant difference in the mean amplitude in wavelet frequency band 4 was noted between the 2 groups. The sensitivity for both Doppler RI and frequency band 4 for the detection of PE at a 10% false-positive rate was 45%. This small study demonstrates the application of wavelet transform analysis of uterine artery Doppler waveforms in screening for PE. Further prospective studies are needed in order to clearly define if this analytical approach to waveform analysis may have the potential to improve the detection of PE by uterine artery Doppler screening.

Pressure effect on vibrational frequency and dephasing of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids

Raman spectra in the range of the totally symmetric stretching mode of the [PF6]− anion, νs(PF6), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [CnC1im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C6C1im][PF6] and [C8C1im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C4C1im][PF6], whichcrystallizes above ∼0.5 GPa. Equations of state based either on a group contribution model or Carnahan-Starling-van der Waals model have been used to estimate the densities of the ionic liquids at high pressures. The shifts of the vibrational frequency of νs(PF6) with density observed in [C6C1im][PF6] and in [C8C1im][PF6] have been calculated by a hard-sphere model of a pseudo-diatomic solute under short-range repulsive interactions with the neighboring particles. The stochastic model of Kubo for vibrational dephasing has been used to obtain the amplitude of vibrational frequency fluctuation, ⟨Δω 2⟩, and the relaxation time of frequency fluctuation, τ c , as a function of density by Raman band shape analysis of the νs(PF6) mode of [C6C1im][PF6] and [C8C1im][PF6].

Tailoring Alphabetical Metamaterials in Optical Frequency: Plasmonic Coupling, Dispersion, and Sensing

Tailoring optical properties of artificial metamaterials, whose optical properties go beyond the limitations of conventional and naturally occurring materials, is of importance in fundamental research and has led to many important applications such as security imaging, invisible cloak, negative refraction, ultrasensitive sensing, transformable and switchable optics. Herein, by precisely controlling the size, symmetry and topology of alphabetical metamaterials with U, S, Y, H, U-bar and V shapes, we have obtained highly tunable optical response covering visible-to-infrared (Vis-NIR) optical frequency. In addition, we show a detailed study on the physical origin of resonance modes, plasmonic coupling, the dispersion of electronic and magnetic surface plasmon polaritons, and the possibility of negative refraction. We have found that all the electronic and magnetic modes follow the dispersion of surface plasmon polaritons thus essentially they are electronic- and magnetic-surface-plasmon-polaritons-like (ESPP-like and MSPP-like) modes resulted from diffraction coupling between localized surface plasmon and freely-propagating light. Based on the fill factor and formula of magnetism permeability, we predict that the alphabetical metamaterials should show the negative refraction capability in visible optical frequency. Furthermore, we have demonstrated the specific ultrasensitive surface enhanced Raman spectroscopy (SERS) sensing of monolayer molecules and femtomolar food contaminants by tuning their resonance to match the laser wavelength, or by tuning the laser wavelength to match the plasmon resonance of metamaterials. Our tunable alphabetical metamaterials provide a generic platform to study the electromagnetic properties of metamaterials and explore the novel applications in optical frequency.

Electron dynamics and frequency coupling in a radio-frequency capacitively biased planar coil inductively coupled plasma system

The electron dynamics in a planar coil inductively coupled plasma (ICP) system with a capacitively biased electrode is investigated using space and phase resolved optical emission spectroscopy. The two power source frequencies are exact multiple of each other and phase-locked. In this configuration, the system is investigated when the coil is operated in both E-mode and H-mode. The results show that in a phase synchronized RF biased ICP, the electrode bias power couples with the capacitive contribution of the coil, in both E-mode and H-modes, similar to dual-frequency capacitively coupled plasmas (2f-CCPs). It is also demonstrated that in H-mode, the phase between the electrode bias frequency and the ICP coil frequency influences the electron heating, similar to the electrical asymmetry effect in 2f-CCPs.

Investigation of tanpura string vibrations using a two-dimensional time-domain model incorporating coupling and bridge friction

Tanpura string vibrations have been investigated previously using numerical models based on energy conserving schemes derived from a Hamiltonian description in one-dimensional form. Such time-domain models have the property that, for the lossless case, the numerical Hamiltonian (representing total energy of the system) can be proven to be constant from one time step
to the next, irrespective of any of the system parameters; in practice the Hamiltonian can be shown to be conserved within machine precision. Models of this kind can reproduce a jvari effect, which results from the bridge-string interaction. However the one-dimensional formulation has recently been shown to fail to replicate the jvaris strong dependence on the thread placement. As a first step towards simulations which accurately emulate this sensitivity to the thread placement, a twodimensional model is proposed, incorporating coupling of controllable level between the two string polarisations at the string termination opposite from the barrier. In addition, a friction force acting when the string slides across the bridge in horizontal direction is introduced, thus effecting a further damping mechanism. In this preliminary study, the string is terminated at the position of the thread. As in the one-dimensional model, an implicit scheme has to be used to solve the system, employing Newton's method to calculate the updated positions and momentums of each string segment. The two-dimensional model is proven to be energy conserving when the loss parameters are set to zero, irrespective of the coupling constant. Both frequency-dependent and independent losses are then added to the string, so that the model can be compared to analogous instruments. The influence of coupling and the bridge friction are investigated.