236 resultados para Laser resonance ionization


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Analysis of gas-particle nozzle flow is carried out with attention to the effect of dust particles on the vibrational relaxation phenomena and consequent effects on the gain of a gasdynamic laser. The phase nonequilibrium between the gas mixture and the particles during the nozzle expansion process is taken into account simultaneously. The governing equations of the two-phase nozzle flow have been transformed into similar form, and general correlating parameters have been obtained. It is shown from the present analysis that the particles present in the mixture affect the optimum gain obtainable from a gasdynamic laser adversely, and the effect depends on the size and loading of the particles in the mixture.

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Theoretical analysis of internal frequency doubling in actively mode locked broadband solid state lasers is presented. The analysis is used to study the dependence of mode locked pulsewidth on the second harmonic conversion efficiency, the modulation depth, and the tuning element bandwidth in an AM mode locked Ti: sapphire laser. The results are presented in the form of graphs.  

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A molecule having a ketone group between two thiophene groups was synthesized. Presence of alternating electron donating and accepting moieties gives this material a donor-acceptor-donor (DAD) architecture. PolyDAD was synthesized from DAD monomer by oxidative polymerization. Device quality films of polyDAD were fabricated using pulsed laser deposition technique. X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectra (FTIR) data of both as synthesized and film indicate the material does not degrade during ablation. Optical band gap was determined to be about 1.45 eV. Four orders of magnitude increase in conductivity was observed from as synthesized to pulsed laser deposition (PLD) fabricated film of polyDAD. Annealing of polyDAD films increase conductivity, indicating better ordering of the molecules upon heating. Rectifying devices were fabricated from polyDAD, and preliminary results are discussed.

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Sequence specific resonance assignment constitutes an important step towards high-resolution structure determination of proteins by NMR and is aided by selective identification and assignment of amino acid types. The traditional approach to selective labeling yields only the chemical shifts of the particular amino acid being selected and does not help in establishing a link between adjacent residues along the polypeptide chain, which is important for sequential assignments. An alternative approach is the method of amino acid selective `unlabeling' or reverse labeling, which involves selective unlabeling of specific amino acid types against a uniformly C-13/N-15 labeled background. Based on this method, we present a novel approach for sequential assignments in proteins. The method involves a new NMR experiment named, {(CO)-C-12 (i) -N-15 (i+1)}-filtered HSQC, which aids in linking the H-1(N)/N-15 resonances of the selectively unlabeled residue, i, and its C-terminal neighbor, i + 1, in HN-detected double and triple resonance spectra. This leads to the assignment of a tri-peptide segment from the knowledge of the amino acid types of residues: i - 1, i and i + 1, thereby speeding up the sequential assignment process. The method has the advantage of being relatively inexpensive, applicable to H-2 labeled protein and can be coupled with cell-free synthesis and/or automated assignment approaches. A detailed survey involving unlabeling of different amino acid types individually or in pairs reveals that the proposed approach is also robust to misincorporation of N-14 at undesired sites. Taken together, this study represents the first application of selective unlabeling for sequence specific resonance assignments and opens up new avenues to using this methodology in protein structural studies.

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We report on the X-band (similar to 9.43 GHz) electron paramagnetic resonance (EPR) investigations carried out on polycrystalline Ga1-xMnxSb (x=0.02). A strong EPR signal with an effective g factor (g(eff)) close to 2.00 was observed, suggesting that the ionic state of Mn which replaces Ga ion in the lattice, is Mn2+ attributable to Delta M=1 transition of the ionized Mn acceptor A(-), Mn (3d(5)). The apparent absence of EPR signal, typical for neutral Mn acceptor at g=2.7 suggests either no such centers are present or the signal broadens beyond detection limit. The temperature dependent EPR studies combined with dc magnetization data suggest the possible coexistence of antiferromagnetic and ferromagnetic phases at very low temperatures. (C) 2011 American Institute of Physics. doi:10.1063/1.3543983]

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Single crystals of calcium hydrazine carboxylate, monohydrate have been studied by ESR of Mn2+ doped in the calcium sites. X-band ESR indicated a large crystal field splitting necessitating experiments at Q band. The analysis shows two magnetically inequivalent (but chemically equivalent) sites with g(xx) = 2.0042+/-0.0038, g(yy) = 2.0076 +/-00029, g(zz) =2.0314+/-0.001, A(zz) = 0.0099+/-0.0002 cm(-1), A(xx) = 0.0099+/-0.0002 cm(-1), A(yy) = 0.0082+/-0.0002cm(-1), D = 3/2D(zz) = 0.0558+/-0.0006cm(-1), and E = 1/2(D-xx-D-yy) = 0.0127+/-0.0002 cm(-1).One of the principal components of the crystal field, (D-zz), is found to be along the Ca<->Ca direction in the structure and a second one, (D-xx), along the perpendicular to the plane of the triangle formed by three neighbouring calciums. The A tensor is found to have an orientation different from that of the g and D tensors reflecting the low symmetry of the Ca2+ sites.

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New vibrational Raman features characteristic to the conductive form of polyaniline have been observed with the near-infrared excitation at 1047 nm. Based on an analogy with the resonance Raman spectrum of Michler's ketone in the lowest excited triplet (T-1) state, we consider these features as due to a dynamic structure of a diimino-1,4-phenylene unit in the polyaniline chain exchanging a positive charge very rapidly. This consideration directly leads to a conducting mechanism in which a positive charge migrates from one nitrogen to the other through the conjugated chain of polyaniline.

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Application of ultrafast lasers to chemistry and biology has been an active area of research in the international scene for over a decade for physical and biophysical chemists. Perhaps, ultrafast laser spectroscopy is one of the most versatile tools available today to experimentally study structure and dynamics in the time domain of nanoseconds (10(-9) sec) to femtoseconds (10(-15) sec). In this article we attempt to highlight some of the recent developments in ultrafast laser spectroscopy with particular reference to vibrational spectroscopy, viz. infrared and Raman spectroscopy, in the above time domain.

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Microstructural and superconducting properties of YBa2Cu3O7-x thin films grown in situ on bare sapphire by pulsed laser deposition using YBa2Cu3O7-x targets doped with 7 and 10 wt% Ag have been studied. Ag-doped films grown at 730 degrees C on sapphire have shown very significant improvement over the undoped YBa2Cu3O7-x films grown under identical condition. A zero resistance temperature of 90 K and a critical current density of 1.2 x 10(6) A/cm(2) at 77 K have been achieved on bare sapphire for the first time. Improved connectivity among grains and reduced reaction rate between the substrate and the film caused due to Ag in the film are suggested to be responsible for this greatly improved transport properties.

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Polycrystalline strontium titanate (SrTiO3) films were prepared by a pulsed laser deposition technique on p-type silicon and platinum-coated silicon substrates. The films exhibited good structural and dielectric properties which were sensitive to the processing conditions. The small signal dielectric constant and dissipation factor at a frequency of 100 kHz were about 225 and 0.03 respectively. The capacitance-voltage (C-V) characteristics in metal-insulator-semiconductor structures exhibited anomalous frequency dispersion behavior and a hysteresis effect. The hysteresis in the C-V curve was found to be about 1 V and of a charge injection type. The density of interface states was about 1.79 x 10(12) cm(-2). The charge storage density was found to be 40 fC mu m(-2) at an applied electric field of 200 kV cm(-1). Studies on current-voltage characteristics indicated an ohmic nature at lower voltages and space charge conduction at higher voltages. The films also exhibited excellent time-dependent dielectric breakdown behavior.

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The entry of the plant toxin ricin and its A- and B-subunits in model membranes in the presence as well as absence of monosialoganglioside (GM(1)) has been studied. Dioleoylphosphatidylcholine and 5-, 10-, and 12-doxyl- or 9,10-dibromophosphatidylcholines serve as quenchers of intrinsic tryptophan fluorescence of the proteins. The parallax method of Chattopadhyay and London [(1987) Biochemistry 26, 39-45] has been employed to measure the average membrane penetration depth of tryptophans of ricin and its B-chain and the actual depth of the sole Trp 211 in the A-chain. The results indicate that both of the chains as well as intact ricin penetrate the membrane deeply and the C-terminal end of the A-chain is well inside the bilayer, especially at pH 4.5. An extrinsic probe N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl) ethylenediamine (I-AEDANS) has been attached to Cys 259 of the A-chain, and the kinetics of penetration has been followed by monitoring the increase in AEDANS fluorescence at 480 nm. The insertion follows first-order kinetics, and the rate constant is higher at a lower pH. The energy transfer distance analysis between Trp 211 and AEDANS points out that the conformation of the A-chain changes as it inserts into the membrane. CD studies indicate that the helicity of the proteins increases after penetration, which implies that some of the unordered structure in the native protein is converted to the ordered form during this process. Hydrophobic forces seem to be responsible for stabilizing a particular protein conformation inside the membrane.

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Polycrystalline SrTiO3 films were prepared by pulsed excimer laser ablation on Si and Pt coated Si substrates. Several growth parameters were varied including ablation fluence, pressure, and substrate temperature. The structural studies indicated the presence of [100] and [110] oriented growth after annealing by rapid thermal annealing at 600-degrees-C for 60 s. Deposition at either lower pressures or at higher energy densities encouraged film growth with slightly preferred orientation. The scanning electron microscopy studies showed the absence of any significant particulates on the film surface. Dielectric studies indicated a dielectric constant of 225, a capacitance density of 3.2 fF/mum2, and a charge density of 40 fC/mum for films of 1000 nm thick. The dc conductivity studies on these films suggested a bulk limited space charge conduction in the high field regime, while the low electric fields induced an ohmic conduction. Brief time dependent dielectric breakdown studies on these films, under a field of 250 kV/cm for 2 h, did not exhibit any breakdown, indicating good dielectric strength.

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Ferromagnetic resonance spectra of La1-xCaxMnO3 powders (0.1 less than or equal to x 0.9) have been investigated over a range of temperatures. The spectra could be fitted to a sum of two Lorentzians for all the compositions. The intense line with a nearly constant g shows a linear decrease in linewidth with increase in temperature, while the weaker line with a variable g shows a maximum in linewidth in the T-c region. The latter is also associated with a g(eff) which depends on the composition. Copyright (C) 1996 Elsevier Science Ltd

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The binding affinity of the oligosaccharide moiety of a neutral glycosphingolipid, asialoGM1, towards Ricinus communis agglutinin (RCAI) was determined for the first time by fluorescence resonance energy transfer (RET). The asialoGM1 was incorporated into a phospholipid (DMPC) vesicle doped with dansylated DPPE and then titrated with an increasing amount of the galactose specific RCAI. The efficiency of RET was determined by a saturable increase in the quenching of 'donor' fluorescence, i.e. the 'trp' residue of RCAI, due to the energy transfer from the 'acceptor' dansyl group on the surface of the vesicle. The apparent binding constant was found to be in the range of 10(5)-10(6) M-1 at 27 degrees C.