Finite Rise Research Articles

Ion sheath expansion for a target voltage with a finite risetime

Properties of the ion sheath expansion in plasma are investigated for a target voltage with a finite rise time. A theoretical model for ion sheath expansion is developed when the negative target voltage increases linearly. The theory predicts that the sheath expansion is proportional to the square root of time at the beginning and is proportional to the 56 power of time later on. An experimental measurement has been carried out and the measured data are compared with theoretical results. It is shown that the sheath front propagates very fast at the beginning and slows down later, even for continuously rising negative voltage on target.

Linear and quasi-linear viscoelastic characterization of ankle ligaments.

The objective of this study was to produce linear and nonlinear viscoelastic models of eight major ligaments in the human ankle/foot complex for use in computer models of the lower extremity. The ligaments included in this study were the anterior talofibular (ATaF), anterior tibiofibular (ATiF), anterior tibiotalar (ATT), calcaneofibular (CF), posterior talofibular (PTaF), posterior tibiofibular (PTiF), posterior tibiotalar (PTT), and tibiocalcaneal (TiC) ligaments. Step relaxation and ramp tests were performed. Back-extrapolation was used to correct for vibration effects and the error introduced by the finite rise time in step relaxation tests. Ligament behavior was found to be nonlinear viscoelastic, but could be adequately modeled up to 15 percent strain using Fung's quasilinear viscoelastic (QLV) model. Failure properties and the effects of preconditioning were also examined.

Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the Snand S1Fluorescence of Tetracene

The electronic and vibrational relaxation of tetracene have been studied in solution by femtosecond time-resolved fluorescence spectroscopy. Tetracene was initially photoexcited to the highly excited singlet (Sn) state, 1Bb, and the dynamics of the fluorescence from the 1Bb state and the 1La state (S1) were investigated by fluorescence up-conversion. The fluorescence from the 1Bb state was observed in the ultraviolet region, and its lifetime was determined as ∼120 fs. The anisotropy of the 1Bb fluorescence was close to 0.4, which assured that the fluorescence is emitted from the excited state that was prepared by photoexcitation. The visible fluorescence from the 1La state showed a finite rise that agreed well with the decay of the 1Bb fluorescence. Negative anisotropy was observed for the 1La fluorescence, reflecting that the 1La transition moment is parallel to the short axis of the molecule and hence perpendicular to the 1Bb transition moment. The anisotropy of the 1La fluorescence, however, showed a v...

Analysis of the effect of write head field rise time on signal, noise, and nonlinear transition shift

The effect of finite flux rise time on the recording process has been examined by a comparison of theoretical analysis and experimental measurement. It was shown that a finite flux rise time increases the transition width, transition noise, and nonlinear transition shift. The effect of rise time on these recording phenomena was shown to scale with the product of disk velocity and rise time divided by the total magnetic spacing (ντ/y). Comparison of the experiment and theory yields reasonable flux rise time.

Origin of initial rise process of ultrafast exponential response in liquids

Ultrafast optical Kerr effect (OKE) measurement was performed for various liquids. It was found that any response function obtained from OKE measurement shows a slow initial rise early after excitation even when the material response contains an exponential response at a later time which indicates that the Debye relaxation model breaks down in such a ultrafast time range. It is shown that the finite rise process leading to an exponential response is closely related with the crossover from the dynamical to statistical regime and comes from the anharmonic couplings among numerous microscopic oscillators.

Shock jump equations for unsteady wave fronts of finite rise time

First, generalized Rankine–Hugoniot equations for unsteady wave fronts of finite width are derived. It is clarified from these jump equations for particle velocity, stress, and specific internal energy that shock jump conditions involve the effects of rise time of the front or the change in its velocity with time, in addition to that of strain rate and acceleration, and that the treatment in the previous study [Sano, J. Appl. Phys. 82, 5382 (1997)] where the rise time is reduced to an infinitesimal eliminates the terms of this change in the jump equations. Furthermore, the jump equations of the general form derived here support this elimination. Next, the influence of the rise time on the three jumps at the impacted surface of a lithium fluoride single crystal is calculated based on its experimental data and shown to be negligibly small. However, its influence, calculated for sandstone in a similar manner, is great. Finally, a parametric investigation of the influence is carried out for specific strain waves.

Ground-State Reverse Double Proton Transfer of 7-Azaindole

Dynamics of the ground-state reverse proton transfer in 7-azaindole (7AI(N)) have been investigated by two-step laser-induced fluorescence (TSLIF) in various nonpolar solvents. Comprehensive analyses reveal a previously unrecognized finite rise kinetics for the long-lived transient species. Furthermore, the time-dependent spectral evolution indicates that the TSLIF spectrum obtained at the rise component is different from that of the decay component, while both spectra are red shifted relative to the prompt tautomer emission. The results lead us to propose that the transient species originates from the monomer of the 7AI proton-transfer tautomer (7AI(T)) produced by a minor dissociation channel (∼4%) of the excited 7AI(T) dimer, which subsequently undergoes a slow reverse proton transfer via the formation of a 7AI(T)/7AI(N) hydrogen-bonded complex. This proposed mechanism rationalizes the recent thermal lensing experiment which concluded that the 7AI(T) dimer is only 0.97 kcal/mol higher in energy than the 7AI(N) dimer,35 while theoretical approaches,38,39 in contrast, predict an energy difference of >20 kcal/mol.

Time course of transmembrane voltage induced by time-varying electric fields—a method for theoretical analysis and its application

The paper describes a general method for analysis of time courses of transmembrane voltage induced by time-varying electric fields. Using this method, a response to a wide variety of time-varying fields can be studied. We apply it to different field shapes used for electroporation and electrofusion: rectangular pulses, trapezoidal pulses (approximating rectangular pulses with finite rise time), exponential pulses, and sine(RF)-modulated pulses. Using the described method, the course of induced transmembrane voltage is investigated for each selected pulse shape. All the studies are performed at different pulse durations, each for both the normal physiological and the low-conductivity medium. For all the pulse shapes investigated, it is shown that as the conductivity of extracellular medium is reduced, this slows down the process of transmembrane voltage inducement. Thus, longer pulses have to be used to attain the desired voltage amplitude, as the influence of the fast, short-lived phenomena on the induced voltage is diminished. Due to this reason, RF-modulation in such a medium is ineffective. The appendix gives a complete set of derived expressions and a discussion about possible simplifications.

Delay-induced multistable synchronization of biological oscillators

We analyze the dynamics of pulse coupled oscillators depending on strength and delay of the interaction. For two oscillators, we derive return maps for subsequent phase differences, and construct phase diagrams for a broad range of parameters. In-phase synchronization proves stable for inhibitory coupling and unstable for excitatory coupling if the delay is not zero. If the coupling strength is high, additional regimes with marginally stable synchronization are found. Simulations with $N\ensuremath{\gg}2$ oscillators reveal a complex dynamics including spontaneous synchronization and desynchronization with excitatory coupling, and multistable phase clustering with inhibitory coupling. We simulate a continuous description of the system for $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty}$ oscillators and demonstrate that these phenomena are independent of the size of the system. Phase clustering is shown to relate to stability and basins of attraction of fixed points in the return map of two oscillators. Our findings are generic in the sense that they qualitatively are robust with respect to modeling details. We demonstrate this using also pulses of finite rise time and the more realistic model by Hodgkin and Huxley which exhibits multistable synchronization as predicted from our analysis as well.

Properties of the YAP : Ce scintillator

Light yield, light pulse shape due to γ-rays and α-particles, energy and time resolutions for a 10 × 10 × 5 mm 3 YAB crystal coupled to the XP2020Q photomultiplier were studied. The measured light output of 17000 ± 850 photons/MeV includes a correction for the calibrated quantum efficiency of the XP2020Q. The fast component of the light pulse with the decay time constant of 26.7 ± 0.12 ns also shows a finite rise time described approximately by the time constant of 380 ± 45 ps. The YAP crystal which was studied exhibited an energy resolution of 5.7% for 662 keV γ-rays fron a 137Cs source. This very good energy resolution is due to a low intrinsic energy resolution of 3.4%. These characteristics, together with a good time resolution for 60Co γ-rays of 160 ps measured at the threshold of 1 MeV, suggest a broad range of applications for the YAP scintillator.

Effect of finite rise time on the strength of the magnetic field generated in a switched plasma

The effect of a magnetized transient plasma on a circularly polarized source wave, when the rise time of the electron density profile is comparable to the source wave period, is considered. The H-formulation Green's function is developed to study the effect of a finite rise time on the generation of a wiggler magnetic field.

Coherent transient in photoluminescence of excitonic molecules in GaAs quantum wells

Time-resolved photoluminescence (PL) of excitonic molecules in GaAs quantum wells (QW's) reveals an initial transient characterized by a finite rise and an extremely fast nonexponential decay [H. Wang, J. Shah, and T. C. Damen et al., Solid State Commun. 98, 807 (1996)]. The transient is attributed to coherent quantum evolution towards the molecule ground state of two optically correlated excitons, ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}},$ which undergo the Coulombic attraction. In the present paper, we develop a theory of the transient PL and fit the experimental data. The radiative decay of a quasi-two-dimensional (2D) excitonic molecule is analyzed with the giant oscillator strength model adapted to QW's and with the bipolariton model. Within the 2D bipolariton model, the main ``hidden'' channel of the radiative decay of an excitonic molecule is the resonant dissociation into two outgoing interface (surface) polaritons rather than the observable decay into the bulk radiative modes. We conclude that quasi-2D molecules dissociate already within the initial transient of PL, while the following exponential decay of the molecule-mediated PL is due to the escape of secondary interface polaritons into the bulk modes. According to the 2D bipolariton model, the sequence ``two incoming $\mathrm{photons}\ensuremath{\rightarrow}\mathrm{two}$ virtual $\mathrm{excitons}\ensuremath{\rightarrow}2\mathrm{D}$ $\mathrm{molecule}\ensuremath{\rightarrow}\mathrm{two}$ outgoing interface polaritons'' is a completely coherent process. The fit of the time-resolved molecule-mediated PL provides us with numerical estimates of the exciton-photon coupling in GaAs QW's.

Spin–orbit effects in the formation of the Na–He excimer on the surface of He clusters

In this paper we describe an application of reversed time-correlated single photon counting to the time-resolved spectroscopy of impurity atoms and molecules bound to large quantum clusters. The photo-induced dynamics of Na atoms on the surface of He and H2 clusters have been studied by following the time dependence of their emission at selected excitation and emission wavelengths. Collection of atomic (16980±145 cm-1) fluorescence arising from Na atoms excited on the cluster surface and immediately desorbed from it yields a finite (ca. 70 ps) rise time and a decay time of 16.3±0.1 ns, equal to the known lifetime of the 3P→3S transition of atomic Na. The frequency distribution of the red emission due to atoms that do not leave the cluster immediately after excitation is shown to be due to a desorbed Na*–He exciplex by obtaining quantitative agreement with predictions derived from available abinitio Na–He potentials. Formation of this excimer can occur along either the 2Π1/2 or 2Π3/2 excited state surface. ‘Slow’ (700 ps) and ‘fast’ (ca. 70 ps) components of the rise time of the red emission (15800±125 cm-1) are assigned, respectively, to the two formation channels. Introducing spin–orbit coupling effects into the long range abinitio pair potential for an isolated Na*–He generates a small barrier on the 2Π1/2 potential curve, which is linked to the observed slow exciplex formation time.

Transient optical emission from excitonic molecules in GaAs quantum wells: Coherent quantum evolution in momentum space

Resonantly-excited photoluminescence (PL) of excitonic molecules with femtosecond resolution reveals a surprising initial transient characterized by a finite rise and an extremely fast non-exponential decay. We show that this initial transient stems from coherent quantum evolution of relative momentum of two excitons in the molecule, in sharp contrast to radiative dynamics of excitons. The initial transient also reflects a type of quantum beats that differ from the usual quantum beats in transient optical measurements.

DETERMINATION OF THE ELASTIC CONSTANTS OF ANISOTROPIC SOLIDS FROM GROUP VELOCITIES MEASURED IN SYMMETRY DIRECTIONS

We present in this article a novel method of determining all three elastic constants of cubic crystals from a single broadband waveform propagating in the [001] direction. The method can be easily extended to media of orthorhombic symmetry whose symmetry plane quasitransverse (QT) group velocity sheet is folded across the symmetry axis. The usefulness of these formulas lies in providing a very convenient and easy method of determining all the elastic constants of a medium from the rays propagating in the principal symmetry directions of the medium. Particular emphasis is given to the so-called oblique-mode QT rays with group velocity lying along a symmetry axis but with wave normal lying away from that axis in a symmetry plane. Through the use of these oblique modes the need to make off-axis measurements to obtain a complete set of elastic constants is circumvented. Moreover, we describe a method of clarifying the ambiguity that arises; with which symmetry plane is the wave normal of an oblique-mode QT ray propagating in the symmetry axis associated. Further, we show how the effect of a finite rise time source can be corrected for in the determination of a mixed index elastic constant.

Theory of resonant rayleigh scattering of excitons in semiconductor quantum wells

Disorder-induced Rayleigh scattering of excitons in quantum wells is investigated theoretically. For classical excitions (long-range spatial correlation), the disorder average can be performed exactly. For the full problem, a kinetic equation with «weak memory» for the excition distribution is derived which coincides with the classical result in the appropriate limit. The theoretical results are compared with recent experimental data on femtosecond time-resolved luminescence explaining well the finite rise time and the heavy-light-hole beating.

Dynamics of structures: theory and applications to earthquake engineering

Dynamics of structures: theory and applications to earthquake engineering

Noninstantaneous, finite rise-time effects on the precursor field formation in linear dispersive pulse propagation

The effects of a noninstantaneous finite rise time on the transient field phenomena associated with dispersive pulse propagation are considered by use of the asymptotic description of the propagated plane-wave field in a single-resonance Lorentz model dielectric. The asymptotic description is presented for both an input hyperbolic tangent modulated signal with initial pulse envelope u(t) = [1 + tanh(βt)]/2 and an input raised cosine envelope signal with initial pulse envelope u(t) = 0 for t ≤ 0, u(t) = [1 − cos(βt)]/2 for 0 ≤ t ≤ π/β, and u(t) = 1 for t ≥ π/β. In both cases the parameter β is indicative of the rapidity of the initial rise time of the signal f(t) = u(t)sin(ωct) with input carrier frequency ωc. In the limit as β → ∞ both of these initial envelope functions approach the unit step function. The dynamical evolution of the propagated field is described by means of the dynamics of the saddle points in the complex ω plane that are associated with the complex phase function appearing in the integral representation of the propagated field and their interaction with the simple pole singularities of the spectrum ũ(ω − ωc) of the initial pulse envelope function. The analysis shows that the Sommerfeld and Brillouin precursor fields that are characteristic of the propagated field that is due to an input unit step function modulated signal will persist nearly unchanged for values of the rise-time parameter β of the order of δ or greater, where δ is the damping constant of the Lorentz model dielectric. As β is decreased below δ, the precursor fields become less important in the overall field structure and the field becomes quasi-monochromatic.

Pulse propagation in sea water

The propagation in sea water of a low-frequency electromagnetic pulse generated by an electric dipole is investigated analytically. The dipole is excited by a rectangular current pulse with a finite, nonzero rise and decay time. In order to obtain an explicit formula for the field in the equatorial plane of the dipole source that is uniformly valid in distance and time, Fourier-transform methods are applied. Certain limiting forms of the current pulse are studied separately. Simple analytic expressions of the field are obtained, compared to previous results, and examined thoroughly. The effect of the finite rise and decay time is discussed. It is noted that the present analysis may be used for studying pulse propagation in any highly conducting medium besides sea water.

Femtosecond dynamics of excitons in quantum wells and quantum well microcavities

AbstractInvestigations of femtosecond dynamics of excitons in two different regimes are reported: 1. perturbative or weak coupling regime where the exciton decay is irreversible (i.e. excitons in quantum wells), and 2. non‐perturbative or strong coupling regime (i.e. quantum well excitons in microcavities) where the coherent exchange of energy between the coupled cavity and exciton modes dominates the femtosecond dynamics. In the perturbative regime, the first femtosecond luminescence measurements are reported, which show (i) a finite rise time for the emission from resonantly‐excited excitons and (ii) femtosecond quantum beats of heavy‐hole and light‐hole excitons. In microcavities, the first nonlinear measurements are reported, which show quantum beats from the coupled cavity‐exciton modes, and provide information about the system not available from linear measurements.