Single-shot two-dimensional spectral interferometry for ultrafast laser-produced plasmas

Coherent white light was used as a light source for spectral interferometry of ultrafast laser-produced plasmas. Using a narrowband filter, two-dimensional images of field ionization in helium were obtained with a 14 fs time resolution.     

Mössbauer spectroscopy and magnetic studies of orientated textured Fe3O4 ferrofluid

Nano scale magnetite based ferrofluid is synthesized by chemical co pre cipitation technique and stabilized with oleic acid. Magnetization and viscosity measurements were used to optimize for texturing purpose. The freeze-textured ferrofluid in two configurations, namely, (1) field texture system (FTS) and (2) zero field texture system (ZTS) are investigated by magnetization measurements at 298 K and Mössbauer spectroscopy measurements at 77 and 298 K. These results are analysed on the basis of the contributions from collective superparamagnetic reversal and the strength of the inter particle interactions.     

Chaos in the solar wind flow near Earth

We have done a time series analysis of daily average data of solar wind velocity, density and temperature at 1 AU measured by ACE spacecraft for a period of nine years. We have used the raw data without filtering to give a faithful representation of the nonlinear behaviour of the solar wind flow which is a novel one. The sensitivity of the results on filtering is highlighted. The attractor dimension is estimated for every parameter of the solar wind and it is found that they differ substantially. Hence a chaotic picture for the problem from different angles have been obtained. The calculated Kolmogorov entropies and Lyapunov exponents are positive showing evidences that the complex solar wind near the Earth is most likely a deterministic chaotic system.      

Stability of electrostatic ion cyclotron waves in a multi-ion plasma

We have studied the stability of the electrostatic ion cyclotron wave in a plasma consisting of isotropic hydrogen ions (H⁺) and temperature-anisotropic positively (O⁺) and negatively (O⁻) charged oxygen ions, with the electrons drifting parallel to the magnetic field. Analytical expressions have been derived for the frequency and growth/damping rate of ion cyclotron waves around the first harmonic of both hydrogen and oxygen ion gyrofrequencies. We find that the frequencies and growth/damping rates are dependent on the densities and temperatures of all species of ions. A detailed numerical study, for parameters relevant to comet Halley, shows that the growth rate is dependent on the magnitude of the frequency. The ion cyclotron waves are driven by the electron drift parallel to the magnetic field; the temperature anisotropy of the oxygen ions only slightly enhance the growth rates for small values of temperature anisotropies. A simple explanation, in terms of wave exponentiation times, is offered for the absence of electrostatic ion cyclotron waves in the multi-ion plasma of comet Halley.     

Stochastic heating and acceleration of electrons in colliding laser fields in plasma

We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.     

Laser reflection method for determination of shear stress in low density transitional flows

The details of laser reflection method (LRM) for the determination of shear stress in low density transitional flows are presented. The method is employed to determine the shear stress due to impingement of a low density supersonic free jet issuing out from a convergent divergent nozzle on a flat plate. The plate is smeared with a thin oil film and kept parallel to the nozzle axis. For a thin oil film moving under the action of aerodynamic boundary layer, the shear stress at the air–oil interface is equal to the shear stress between the surface and air. A direct and dynamic measurement of the oil film slope generated by the shear force is done using a position sensing detector (PSD). The thinning rate of the oil film is directly measured which is the major advantage of the LRM. From the oil film slope history, calculation of the shear stress is done using a three-point formula. The range of Knudsen numbers investigated is from 0.028 to 0.516. Pressure ratio across the nozzle varied from 3,500 to 8,500 giving highly under expanded free jets. The measured values of shear, in the overlapping region of experimental parameters, show fair agreement with those obtained by force balance method and laser interferometric method.     

Identifying sites of protein modification by using matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry and on-line membrane preconcentration-capillary electrophoresis-tandem mass spectrometry

A strategy for rapidly identifying the number and sites of chemical or posttranslational modification of proteins is described. The use of matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry to determine the molecular weight of the adducted protein as well as map the proteolytic digest of peptides offers a rapid method to screen for the possible site of adduction. To unequivocally determine the amino acid sequence of the peptide bearing the adduct as well as structurally characteize the covalent modification, the peptide mixture is subjected to membrane preconcentration-capillary electrophoresis-mass spectrometry and tandem mass spectrometry (mPC-CE-MS/MS). The high resolving separation capability of capillary electrophoresis (CE) afford a chromatograhic step that lends itself to separation of complex mixtures of peptides with minimal sample loss. The membrane preconcentration-CE cartridge allows sample loading volumes 10,000-fold greater than conventional CE. In this work the binding site of the fluorescent label acrylodan to the intestinal fatty binding protein is characterized and shown to be covalently bound at lysine-27, by using mPC-CE-MS/MS.     

Synthesis and characterization of novel amphiphilic block copolymers di‐, tri‐, multi‐, and star blocks of PEG and PIB

A simple but efficient strategy has been developed for the synthesis of novel di‐, tri‐, multi‐, and star‐block copolymers comprising poly(ethylene glycol) (PEG) and polyisobutylene (PIB) blocks. The synthesis principle involves the coupling of appropriately terminally functionalized PEG and PIB sequences, specifically the hydrosilation of mono‐, di‐, and tetra‐allyl‐telechelic PEGs (PEG‐allyl, allyl‐PEG‐allyl, and C(‐PEG‐allyl)₄ by mono‐ and di‐Si(CH₃)₂H telechelic PIBs (PIB‐SiH and HiS‐PIB‐SiH). Representative block copolymers, for example, PEG‐PIB, PIB‐PEG‐PIB, (‐PIB‐PEG‐)_n, and C(‐PEG‐PIB)₄ have been assembled and their structures determined by ¹H and ¹³C NMR spectroscopy. The bulk and surface morphology of select triblocks have been investigated by DSC and AFM and the findings interpreted in terms of phase‐separated PEG and PIB microdomains. The swelling behavior in water of various block copolymers also has been studied. Block copolymers containing 50–70 wt % PIB produce hydrogels, the integrity of which is maintained by physical crosslinks by PIB segments. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3200–3209, 2000