Pulse Excitation Method Research Articles

Photoconductivity in Tl4S3 Layered Single Crystals.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Characteristics of photoconductivity in Tl2S layered single crystals

AbstractIn this work the photoconductivity measurements are carried out for single crystals of the Tl2S compound by using both pulsed excitation (a.c) and steady state (d.c) methods in order to elucidate the nature of photoconductivity (PC) in this compound. Results are reported in the temperature range from 77 K to 300 K, excitation intensity range from 1800 V to 5200 V Lux, applied voltage range from 2 V to 14 V, and wavelength range from 840 nm to 1450 nm. Both of the ac‐photoconductivity (ac‐PC) and the spectral distribution of the photocurrent are studied at different values of light intensity, applied voltage, and temperature. Dependencies of the carrier lifetime on the light intensity, the applied voltage, and the temperature have also been investigated as results of the ac‐PC measurements. The temperature dependence of the energy gap width has been described as a result of studying the dc‐photoconductivity (dc‐PC). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoconductivity in Tl4S3 layered single crystals

AbstractPhotoconductivity measurements are carried out in this work for single crystals of Tl4S3 compound by using both pulsed excitation (a.c) and steady state (a.c) methods in order to elucidate the nature of photoconductivity (PC) in this compound. Results are reported in the temperature range from 77 to 300 K, excitation intensity range from 1800 to 3300 Lux, applied voltage range from 8 to 14 V, and wavelength range from 840 to 1450 nm. Both of the ac‐photoconductivity (ac‐PC) and the spectral distribution of the photocurrent are studied at different values of light intensity, applied voltage and temperature. Dependencies of carrier lifetime on light intensity, applied voltage and temperature are also investigated as results of the ac‐PC measurements. By using the results of the dc‐photoconductivity (dc‐PC) measurements, the temperature dependence of the energy gap width is described. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Shock Response of Ruby Crystal Studied by Pulsed Excitation Luminescence Spectroscopy

Time-resolved luminescence spectroscopy with a pulsed excitation method has been applied to investigate the elastoplastic deformation in shocked ruby single crystal. The ruby luminescence is induced by a nanosecond laser pulse, and before the luminescence decays significantly, a shock wave generated by plate impact is introduced into the sample. The elastoplastic deformation and its effect on the wave propagation have been recorded as streak images.

Preferential suppression of Auger energy backflow by separation of Er ions from carriers with a thin oxide interlayer in Er-doped porous silicon

Strong enhancement of the Er-related 1.54μm emission was obtained at room temperature from Er-doped porous silicon (PSi), when host PSi was slightly preoxidized at 900°C before Er incorporation. It was speculated that the formation of the oxide interlayer played an important role. Separate measurements of the energy transfer and the Auger deexcitation between carriers in Si crystallites and Er ions were carried out using a two-beam (cw and pulse) excitation method for various preoxidation time which was supposed to change the oxide interlayer thicknesses from about 1 to 10nm. It was found that a very thin SiO2 interlayer between Si crystallites and Er ions suppressed preferentially the Auger deexcitation to the carrier-mediated Er excitation. A thin SiO2 interlayer was also effective to suppress the phonon-assisted energy backtransfer at high temperatures (so-called temperature quenching). This preferential suppression of the energy backflow (both Auger deexcitation and temperature quenching) by a thin oxide interlayer led to a strong room temperature Er-related emission at 1.54μm in Er-doped porous silicon. The Er/SiO2/Si structure was also formed on a flat Si surface and quite the same result was obtained. The oxide interlayer thickness of ∼2nm was found optimum to suppress the energy backflow sufficiently with only a slight decrease in the carrier-mediated excitation of Er ions.

About the secondary electron yield and the sign of charging of electron irradiated insulators

Following the total yield approach (used to predict the sign of charging of electron irradiated insulators) a positive charging is expected when the primary beam energy Ep is situated in the energy range where the number of outgoing electrons, , is larger than the number of incoming electrons, I0. But a negative charging is often experimentally observed when a positive charging is predicted. The present paper is an attempt to elucidate this experimental fact. The arguments being developed are based on the use of a dynamic double layer model (+ for the secondary electron mission, SEE; − for the incident electron implantation) which explains a negative charging via the influence of an evolving S-shape potential function, V(z), which induces a partial freezing of the nominal (uncharged) SEE, δ°, combined to a progressive compression of the negative space charge below the surface. The observed large difference in the measurements of the yield by using pulse excitation methods, or by permanent irradiation methods, δ° or δ is then explained. Furthermore, the influence of an oblique incidence is also deduced.

A metastable state in self-electro-optic effect devices using Stark ladder transitions

A self-electro-optic effect device (SEED) based on Wannier-Stark localization in GaAs/AlAs superlattices, can show multiple negative differential resistance (NDR) regions in the photocurrent vs reverse bias voltage curve. This phenomenon comes from the multiple Stark ladder transitions, which successively come into resonance with the optical input to the SEED, when the external electric field intensity is varied. Using these multiple NDR regions in a symmetric SEED configuration, we have found, for the first time, a metastable point isolated inside the bistable hysteresis curve of the optical switching characteristics, in the sense that it cannot be reached by conventional CW light quasistatic intensity variations. Optical tri-state switching via this intermediate state has been observed in transmission by an optical pulse excitation method, and can be explained clearly with a novel global stability analysis of differential equations for photocurrents in the symmetric SEED system.

AN EFFECTIVE METHOD FOR SIGNAL ENHA-NCEMENT IN SOLID-STATE MAGNETIC RESONANCE SPECTROSCOPY

That the multiple single pulse exci tation method proposed in the present articlecan largely increase the signal-to- noise ratio of the solid-state magnetic resonancespectroscopy is analyzed and confirmed experimentally.In addition,the applicationof this technique to the measurements of the central lines of half- integer quadrupo- le nuclei and ￣14N remote nuclear quadrupole resonance spectra is also discussed indetail.

Thermal equilibration between the M and N intermediates in the photocycle of bacteriorhodopsin

The stages in the photocycle of bacteriorhodopsin (BR) involving the M and N intermediates are investigated using a double pulse excitation method. A first (cycling) pulse at 532 nm is followed, with an appropriate time delay, by a second pulse (337, 406, 446, or 470 nm) which induces the M-->BR back-photoreaction. After depletion by the second pulse a repopulation of M in the millisecond range is observed which is interpreted in terms of a thermal N-->M relaxation. It is thus concluded that a (thermal) M<-->N equilibrium accounts for the biphasic decay of M in the BR photocycle. Other models for this stage of the light-driven proton-pump are therefore unnecessary.

Pulse excitation method for measurement of high frequency magnetic properties of large cores (abstract)

It is very important to obtain saturated magnetic properties from reverse saturation (full B-H curve) of ferromagnetic cores to design magnetic switches which are used in high power pulse generators. The magnetic switch is excited in the high frequency range (∼MHz). But, it is extremely difficult to measure full B-H curve of large toroidal cores of which diameter is some hundreds of mm, using the conventional ac excitation method at high frequency. The main reason is poor output ability of power source for core excitation. Therefore we have developed pulse excitation method to get high frequency magnetic properties. The measurement circuit has two sections. One is excitation part composed by charge transfer circuit. The others is reset part for adjustment initial point on direct B-H curve. The sample core is excited by sinusoidal voltage pulse expressed as 1−cos(2π ft). Excitation frequency f is decided by the constants of the elements of the charge transfer circuit. The change of magnetic flux density ΔB and magnetic field H are calculated, respectively, by measuring the induced voltage of search coil and magnetizing current. ΔB-H characteristics from reverse saturation of four different kinds of large cores were measured in frequency range from 50 kHz to 1 MHz. Core loss increases in proportion to Nth powers of the frequency, where the index N depends on each of cores. N is about 0.5 in case of winding ribbon cores, such as Fe-based amorphous, Co-based amorphous, and Finemet, but N is about 0.2 in case of the Ni-Zn ferrite.

Photoresponse of Langmuir-Blodgett film containing bacteriorhodopsin

The photoresponse of the bacteriorhodopsin (BR) molecule in a purple membrane (PM) fragment LB film was investigated by pulsed laser excitation method. The initial results of laser flash photolysis carried out at room temperature indicated that the intermediate of the photochemical reaction is much the same as that of BR in the aqueous suspension of PM fragments. However the span of the appearance and dissappearance of the intermediates for multilayers is different from that for the aqueous suspension.

Study of volume and surface processes in low pressure radio frequency plasma reactors by pulsed excitation methods. I. Hydrogen–argon plasma

In low pressure reactors, for which the mean free paths of reactive particles are of the order of plasma chamber dimensions, the chemical equilibrium of the gas phase is strongly dependent on surface chemistry. Unfortunately data on elementary surface processes are often not available, particularly for surfaces exposed to plasma conditions. In this work such data are obtained through a time resolved analysis of species concentration in a reactor where a square wave modulated rf power is used for excitation. A very short pulse excitation technique, acting as a time resolved ‘‘in situ chemical probe,’’ has been developed in order to follow the radical concentrations in the afterglow where surface processes are dominant. Results for hydrogen–argon plasmas are reported. The surface reactions of hydrogen atoms with reactor walls and silicon surface are studied. The recombination probability on reactor walls is shown to be very sensitive to plasma conditions. These surface properties evolve on time scales as long as few tens minutes, in connection with bulk diffusion process of atomic Hydrogen. For Silicon surface, the measured recombination probability is very high (near unity) while etching appears as an added small effect. In a companion paper determination of F atom reactivity for surfaces submitted to fluorinated plasmas will be described.

Nuclear magnetic resonance in solid ethylene/α-olefin copolymers: Relaxation, spin-diffusion and lamellar widths

1H and 13C-n.m.r. spectra and spin-lattice relaxation behaviour (laboratory frame, T1 (13C and 1H), and rotating frame, T1ρ (1H)) are reported for a range of solid ethylene copolymers with α-olefins having different types and concentrations of branches. The spin-diffusion model for relaxation in semicrystalline polymers is summarised and some new theoretical results given. The 13C high-resolution n.m.r. spectra obtained using both cross-polarisation (c.p.) and single pulse excitation (s.p.e.) methods associate side chain resonances mainly with the mobile, short T1ρ protons and, hence, the more disordered region of the solids, consistent with the short 13C T1 components. 1H laboratory-frame spin-lattice relaxation is single component whilst those for 13C and the 1H on-resonance rotating-frame relaxation are both multiexponential processes, requiring a minimum of three components to describe them. Annealing of samples, which is known to increase the lamellar thicknesses of the crystalline region, causes large increases in the longer relaxation time components. The 1H and 13C spin-lattice relaxation data for a set of annealed and quenched ethyl-branched materials having different branch contents are compared in detail with the predictions of the spin-diffusion relaxation model. The results are internally and semi-quantitatively consistent with this theory and it is concluded that for the 1H spin-lattice relaxation the model is clearly appropriate and for 13C it is consistent with observations. Questions concerning the relevance of spin-diffusion for the magnetically dilute 13C nucleus at natural abundance are mentioned and possible alternative explanations for the relationships observed are referred to briefly.

Measurement and Analysis of the Atomic Velocity Distribution and the Second-Order Doppler Shift of the Cesium Beam Frequency Standard, NRLM-II

The atomic velocity distribution of NRLM-II was measured by the pulse-excitation method and the second-order Doppler shift was estimated (Δf=-4.7±0.3 mHz). The results were consistent with the previous results measured by the truncated Maxwellian method within the experimental errors. An analysis of the experimental errors is also reported.

Laser-based measurement of transition probabilities of neon 2p 53s-2p 53p transitions

By using the magic-angle, pulsed-excitation method in the presence of a magnetic field, the authors have measured the branching ratios for 2p 53s-2p 53p transitions in neon. By combining values for the lifetime of the upper levels with the branching ratios, they have determined the transition probabilities of 31 transitions. The results are in good agreement with those from emission spectroscopy of a high-pressure are plasma by Bridges and Wiese.

Interests and limits of continuous excitation photo-physical methods applied to cellular rheology.

It is possible to characterize the cohesion of the lipidic zones in cells or in membranes by using molecular emission spectroscopy techniques such as : fluorescence polarization, quenching reactions, intramolecular reactions. However, applying these techniques in biology can be improved by the use of pulsed excitations leading to the time-resolved evolutions of some properties of the probe, which can themselves be a function of their environment. The first and the last techniques, based on the use of diphenylhexatriene and dipyrenylpropane respectively are used in practice by steady state excitation. In the case of fluorescence polarization, it can be shown that the method can be applied, within certain limits, which have to be precised when studying the modifications in the membranes. This is also partly true for methods based on the use of intramolecular reactions. However these continuous excitation techniques usually only lead to a global result which is a function of several spectroscopic parameters and of the "cohesion" of the membrane. Then it can be shown that to go beyond this step of semi-quantitative analysis, it is necessary to use pulsed or modulated excitation methods. Moreover this more sophisticated technical approach implies a new theoretical effort to understand the molecular dynamics of the membranes.

Composite pulse excitation in three-level systems

It is demonstrated by theory and experiment that it is possible to excite coherence uniformly in three-level systems with a wide range of anharmonicities by using a composite pulse excitation method. The theory is based on a formal analogy between the anharmonic three-level case and the simpler system with equally spaced energy levels where composite pulse techniques are already well known to compensate for the effects of mismatch between photon energies and level separations. Computer simulations and solid-state NMR experiments on the three-level system of deuterium (spin 1) verify the uniformity of the excitation. The NMR powder spectrum of deuterated polymethylmethacrylate is faithfully reproduced even using a radio-frequency (rf) field strength weaker than the quadrupole interactions.

Dynamic error correction of thin-wire resistance thermometer

Presents an electrical pulse excitation method for determination of the dynamic error of a thin-wire resistance thermometer. If the system being measured works cyclically in the steady-state and sharp electrical pulses are applied to the thermometer, the dynamical error and the correction factors can be evaluated by determining the mean value of the thermometer signal. An experiment to verify the practical application of this method was carried out, and measurement results are presented.

Determination of triplet state spin-lattice relaxation rates by a pulsed excitation method: anthraquinone in shpolskii matrices

Spin-lattice relaxation rates in the T 1 triplet state of 9,10-anthraquinone dissolved in n-heptane and isooctane are measured in the temperature region 1.9 to 30 K using an excitation of phosphorescence by short flashes. The relaxation rates are determined from the tri-exponential decay curves measured with a 5 μs resolution. The relaxation is found to be anisotropic and the dominating rate to connect the two most short-lived sublevels. The temperature-dependence of the relaxation rates is close to a T 3 law.

Primary processes in photochemistry of rhodopsin at room temperature.

MOST of the information concerning the primary photochemical events in vision has been derived from low temperature work1–5. Recently, pulsed laser excitation methods with nanosecond6–8 or picosecond9 time resolution have been applied, in an attempt at clarifying the primary act at physiological temperatures. These studies showed that prelumirhodopsin PLR; or bathorhodopsin), which is the primary photoproduct at 77 K, is also formed at room temperature, with a rise time of less than 6×10−12s (ref. 9) and a half life of ∼10−7s (refs 6–9). The problem remained, however, as to whether other primary transients, in addition to PLR, are formed at room temperature8. It is also important to establish if PLR is the unique precursor of the visual sequence at physiological temperatures, ruling out direct routes to subsequent intermediates—for example, lumirhodopsin (LR)—circumventing PLR. The present pulsed Nd laser study has attempted to clarify the above points, essentially testing the applicability of the low temperature photochemical picture (refs 10 and 11, and T.R., B. Honig, M.O. and T. Ebrey, unpublished) in physiological conditions.