Audibility thresholds for a pure tone in the presence of a pulse jam in the bottlenose dolphin

The effect of a pulse jam on the audibility of pure tones in the bottlenose dolphin (Tursiops truncatus) is investigated. The pulse jam consists of a sequence of pairs of identical short pulses with a pulse spacing of 50 µs in each pair and with a pair repetition rate of 300 s^?1. The test signal is represented by pure tones in the frequency range 20–100 kHz. The audibility thresholds for the test signals are measured at 10-kHz steps, both in the presence of the pulse jam and in its absence, on the basis of the conditioned-reflex method with food reinforcement. The resulting dependence of the threshold shifts (TS) due to the pulse jam on the frequency of the test signal has a complex form. This dependence can be separated into three components: (1) the oscillations of the TS curve that correlate with the extrema of the spectral density of the jam, so that the peaks and dips of the TS curve correspond to the maxima and minima of the spectral density, respectively; (2) the component monotonically decreasing as the frequency grows up to 80 kHz, which distinguishes the TS curve under consideration from the rising curve obtained for masking by random noise; and (3) the frequency-independent component of the TS curve. The following auditory features are associated with these components: component 1 determines the timbre of the pulse jam; component 2 is presumably related to the pitch corresponding to the frequency 1/τ; and component 3 exhibits a rather strong auditory feature of random noise due to the random neural activity caused by the pulse jam in the whole auditory filter band.     

Coherence time measurement of picosecond pulses by a light-induced grating method

The temporal coherence function |Γ(τ)| of picosecond pulses from a Nd: YAG laser has been measured by diffraction at a transient grating. The coherence timet _c =7 ps is small compared to the pulse widtht _p =22 ps but is in correspondence with the spectral bandwidth Δv=12·10¹⁰ Hz. The new method for measuring the coherence function |Γ(τ)| is discussed and compared with a previous experiment.     

Controlling the width of picosecond laser pulses

The relaxation time τ_R of the saturable dye used to mode-lock a Nd: YAG laser has been changed using different dyes or dye solvent mixtures and the laser bandwidth Δω changed by the insertion of an etalon. The pulse duration τ_p was approximately transform-limited for τ_R<2π/Δω but increased to about twice this value when 2π/Δω<τ_p<τ_R. No significant increase in pulse duration was observed for τ_R?2π/Δω but multiple pulses were generated within each round-trip-transit-time.     

Dislocation dynamics in pulse-loaded NaCl crystals

The mean path l of freshly introduced dislocations in NaCl crystals under the effect of triangular loading pulses τ(t) is shown to depend only on the pulse amplitude τ_m and be insensitive to the rate of stress growth. The replacement of triangular pulses with trapezoidal ones with a constant-load plateau (τ=const) extension of up to 60 min only insignificantly changes the l(τ_m) dependence. The data obtained are interpreted on the basis of the concept of quasi-static relaxation in a nonequilibrium system of dislocations subject to a combined effect of time-dependent applied stresses τ(t), coordinate-dependent internal stresses τ_i(x), and “dry friction” τ_p due to the pinning of dislocations by point defects. In such a model, the l(τ_m) dependence should saturate at τ_m<2τ_p; this is in fact observed in the range of 0.2τ_m<τ_m<0.3τ_y (here, τ_y is the yield stress), which gives an estimate for the pinning stress τ_p≈0.1τ_y. Based on the model suggested, a series of experimentally confirmed predictions were obtained, e.g., a recipe of “switching-off” of anomalies. Thus, a preliminary treatment of the samples by a series of stress pulses or holding in a magnetic field, which transforms the system of fresh dislocations into a more equilibrium state, sharply decreases the density of mobile dislocations quasi-statically responding to a pulsed load. It is shown that the discussed anomalies of dislocation mobility should be observed only in sufficiently pure crystals, where , and should be absent in contaminated crystals, where τ_p ～ τ_y.     

Quasi-two-dimensional metallic hydrogen in diphosphide at a high pressure

The average optical reflectivity of bismuth as a function of time t after irradiation by a short laser pulse has been calculated. The amplitude A of photoinduced oscillations in the average optical reflectivity is shown to have extrema under certain conditions. The time τ_j (j is a natural number) at which the amplitude A reaches the jth extremum has been calculated. The calculated dependences of the times τ₁ and τ₂ at which, respectively, the first and second extrema (the first minimum and the first maximum) of the amplitude A are reached on the maximum laser pulse energy density Q are consistent with the experimental data from [8].     

一维复合势垒中的Larmor钟

利用Larmor钟方法计算了一维复合势垒系统中的隧穿时间．从计算结果，对由Larmor钟所定义的Larmor时间在复合势垒中的有效性、可加性，以及它们与透射时间τ_T、反射时间τ_R、逗留时间τ_D的关系作了讨论．6个Larmor时间中，τ_xT,τ_xR,τ_yT和τ_yR都不是有效的时间量，它们不能为隧穿时间提供有效的描述，只有τ_yT和τ_yR满足可加性和判据τ_D＝Tτ_yT+Rτ_yR，它们最有可能给出透射时间τ_T、反射时间τ_R．τ_yT,τ_yR与势垒的对称性有关，等式τ_yT＝τ_yR是否成立与势垒是否对称有关． 关键词：     

Progressive formation of fine and coarse ripples on SiC surface by repeated irradiation of femtosecond laser pulses

In this work, we report the progressive formation of first nanoparticles, next fine ripples, and eventually coarse ripples during the irradiation of single-crystal 6H-SiC surfaces with increasing number of femtosecond laser pulses (λ = 515 nm, τ = 250 fs, repetition rate = 100 kHz). At laser fluences greater than the single-pulse ablation threshold, nanoparticles were produced on the surface by the first few pulses over which fine ripple patterns overlapped at increased pulse numbers. As the pulse number was further increased over ten, the surface was gradually transformed into a coarse ripple–covered one. At laser fluence below the threshold, however, only fine ripples were formed nonuniformly.     

Effect of the duration of the front edge of the voltage pulse on the electric breakdown of ammonium perchlorate

The results of a study of the effect of the front edge duration of a voltage pulse (τ) on the electric breakdown of ammonium perchlorate single crystals are submitted. Experiments have shown that at τ<1.5 µs, the breakdown threshold field increases with reduction in τ and does not depend on τ at τ>1.5 µs. It is shown that the electric breakdown of ammonium perchlorate results from an impact generation of electrons. The impact generation rate of electrons versus electric field and the diameter of the through channel being formed during electric breakdown in ammonium perchlorate are estimated.     

Dynamical study of triplet-triplet energy transfer in rigid solution

Triplet-triplet (T-T) energy transfer from acetophenone to naphthalene-d₈ has been studied in EPA rigid-glass solution at 77 K by masuring the decay of donor phosphorescence and the rise of acceptor T-T absorption over a wide range of time. The results were analyzed in terms of the theory proposed by Inokuti and Hirayama on the basis of a point molecular model. Two parameters, R_O and γ, were determined which are involved in their expression for the rate constant of T-T energy transfer, i.e. n(R) = (1/τ_D) exp[γ(1?R/R_O)], where R and τ_D denote the donor-acceptor distance and the lifetime of donor triplet in the absence of acceptor, respectively. It was found that, in contrast to the Inokuti-Hirayama theory in which γ is assumed to have a constant value, γ increases significantly as the time (t) after flash excitation of the donor becomes smaller: γ ≈ 25 for t/τ_D=10^-2?1, ≈ 35 for t/τ_D = 10^-3? 10^-2, and ≈ 45 for t/τ_D = 10^-4?10^-3. This finding suggests that the true rate constant increases with decreasing R value to a greater extent than the n(R) employed by Inokuti and Hirayama.     

Tau hadronic branching ratios

From 64492 selected τ-pair events, produced at the Z⁰ resonance, the measurement of the tau decays into hadrons from a global analysis using 1991, 1992 and 1993 ALEPH data is presented. Special emphasis is given to the reconstruction of photons and π⁰s, and the removal of fake photons. A detailed study of the systematics entering the π⁰ reconstruction is also given. A complete and consistent set of tau hadronic branching ratios is presented for 18 exclusive modes. Most measurements are more precise than the present world average. The new level of precision reached allows a stringent test τ-μ, universality in hadronic decays, ${cal G}_{?u}/{cal G}_{?}=1.0013pm 0.0095$, and the first measurement of the vector and axial-vector contributions to the non-strange hadronic τ decay width: R_τ,V = 1.788 ± 0 0.025 and R_τ,A = 1.694 ± 0.027. The ratio (R_τ,V ? R_τ,A)/(R_τ,V + R_τ,A), equal to (2.7 ± 1.3) %, is a measure of the importance of QCD non-perturbative contributions to the hadronic τ decay width.     

The measurement and verification of parameters of pulse electromagnetic radiation generated by a large-radius ring current

Experimental and analytic studies of the generation and propagation of electromagnetic radiation due to repetitive current pulses of a nanosecond duration (peak power to 1 MW, current slew rate of 3.5 A/ns) are presented. The radiation source was a fine-wire ring antenna of large radius (ρ_a=1.4 m). The antenna was driven along its full length instantaneously within the time τ shorter than the time of wave travel along the ring diameter (τ≤2ρ_a/c). Parameters of the emitted wave were measured. The experimental data are consistent with the calculated emitted-wave parameters that take into account radiation reflection from the conducting walls of the laboratory. The efficiency of transformation of drive pulse energy into ultra-wideband radiation was found to be approximately 15%. A ring antenna driven by repetitive current pulses (within the time τ≤2ρ_a/c) is suggested to be used as a reference ultra-wideband emitter.     

Strongly positive semigroups and faithful invariant states

Let (?, τ, ω) denote aW*-algebra ?, a semigroupt>0?τ _t of linear maps of ? into ?, and a faithful τ-invariant normal state ω over ?. We assume that τ is strongly positive in the sense that $$\tau _t (A^ * A) \geqq \tau _t (A)^ * \tau _t (A)$$ for allA∈? andt>0. Therefore one can define a contraction semigroupT on ?= \(\overline {\mathcal{M}\Omega } \) by $$T_t A\Omega = \tau _t (A)\Omega ,{\rm A} \in \mathcal{M},$$ where Ω is the cyclic and separating vector associated with ω. We prove 1. the fixed points ?(τ) of τ are given by ?(τ)=?∩T′=?∩E′, whereE is the orthogonal projection onto the subspace ofT-invariant vectors, 2. the state ω has a unique decomposition into τ-ergodic states if, and only if, ?(τ) or {?υE}′ is abelian or, equivalently, if (?, τ, ω) is ?-abelian, 3. the state ω is τ-ergodic if, and only if, ?υE is irreducible or if $$\mathop {\inf }\limits_{\omega '' \in Co\omega 'o\tau } \left\| {\omega '' - \omega '} \right\| = 0$$ for all normal states ω′ where Coω′°τ denotes the convex hull of {ω′°τ _t } _t>0. Subsequently we assume that τ is 2-positive,T is normal, andT* _t ?₊Ω \( \subseteqq \overline {\mathcal{M}_ + \Omega } \) , and then prove 4. there exists a strongly positive semigroup |τ| which commutes with τ and is determined by $$\left| \tau \right|_t \left( A \right)\Omega = \left| {T_t } \right|A\Omega ,$$ 5. results similar to 1 and 2 apply to |τ| but the τ-invariant state ω is |τ|-ergodic if, and only if, $$\mathop {\lim }\limits_{t \to \infty } \left\| {\omega 'o\tau _t - \omega } \right\| = 0$$ for all normal states ω′.     

<Superscript>53</Superscript>Cr NMR study of CuCrO<Subscript>2</Subscript> multiferroic

The magnetically ordered phase of the CuCrO₂ single crystal has been studied by the nuclear magnetic resonance (NMR) method on ⁵³Cr nuclei in the absence of an external magnetic field. The ⁵³Cr NMR spectrum is observed in the frequency range ν_res = 61–66 MHz. The shape of the spectrum depends on the delay tdel between pulses in the pulse sequence τ_π/2–t _del–τ_π–t _del–echo. The spin–spin and spin–lattice relaxation times have been measured. Components of the electric field gradient, hyperfine fields, and the magnetic moment on chromium atoms have been estimated.     

Influence of the shape of the exciting laser pulse on fluorescence saturation in the quantitative analysis of dissolved trace organic substances

Theoretical investigations of the influence of the exciting laser pulse shape on fluorescence saturation in the quantitative analysis of a single-component fluorescing solution, when the Raman signal of the solvent is usedas a reference, are carried out. A quantity α is introduced, responsible for the influence of laser pulse shape on the precision of the analysis. The dependences of α on the exciting radiation photon flux density I_m in the range I_m = 10²² ? 10²⁸ photons/cm²?s and on the duration of the exciting pulses τ_p in the range τ_p = 0.6 ? 60 ns are computed for different pulse shapes in the case of rhodamine 6G dissolved in water. Conditions when α is, in practice, independent of τ_p and of the pulse shape are found. A general analytical expression is found for α, which produces errors not greater than several percent when compared to the computed values of α. An experimental verification of the theoretical results in the case of a rectangular-shaped pulse in the spatial domain and a Gaussian-shaped pulse in the temporal domain is realized.     

混响强度及其衰减规律与脉宽的关系

本文由短脉宽下混响强度的普遍表达式I_r(t,τ)=Kτt^-me^-βt出发,导出对应的可用于长脉宽的混响强度精确表达式,并给出了忽略脉宽尺度范围的指数衰减损失后相应的近似表达式。当指数衰减系数β足够小和脉宽大于有效脉宽τ_m=a_mt(a_m随m增大而减小,m是大于2的实数)时,近程混响强度趋向饱和且随时间t的(m-1)次幂衰减。实验结果与理论符合较好。     

Fast luminescence processes in KI : In caused by excitation in the a absorption band

The temperature dependence of the decay time (τ) of the luminescence pulses from the 440 nm emission band as well as the rise time (τ_R) of luminescence pulses from the 575 nm emission band has been investigated for KI : In by pulsed light excitation in the A absorption band. From the agreement of τ and τ_R independent of the temperature it can be concluded that the X minimum on the ³T₁₄ adiabatic potential energy surface (APES) is mainly populated by a radiationless transition from the tetragonal T minimum of the same APES.Furthermore, a comparison of parameters for the impurity centre luminescence corresponding to the tetragonal emission band in KI : In and KI : Tl has been drawn.     

Fluorescence lifetimes and radiative rate constants of alkanes at 25°C

Fluorescence lifetimes of 33 alkanes were measured at 25°C using a 0.7 ns pulse radiolysis system and the relationship between molecular structure, fluorescence lifetime (τ), quantum yield (Φ_f) and radiative rate constant (R = $\text{Φ}{}_{\mathrm{f}}\text{τ}$) is discussed. The τ's showed a large dependence on the molecular structure, however, the R-values were approximately constant for groups of alkanes: R = 1.3×10⁶s^?1 for n-alkanes, R ≈ 4×10⁶ s^?1 for aliphatic vicinally substituted dimethyl alkanes, R = 7×10⁶s^-1 for cyclohexane and alkylcyclohexanes, and R = 10×10⁶s^?1 for di- and trimethylcyclohexanes. The excited state lifetimes of some non-fluorescing alkanes were also determined from steady-state photolytic quenching data and found to be very low (? 0.3 ns).     

Multiple quadrupolar spin echoes in the La-filled skutterudite LaOs4As12

We report experimental results of ¹³⁹La pulse NMR studies in LaOs₄As₁₂. Measurements have been performed on a powder sample obtained from high quality single crystals. For the first time the pattern of quadrupole echoes for ¹³⁹La nuclei (I=7/2) was obtained. All the allowed quadrupolar echoes expected for spin I=7/2 were observed at times t=(4/3)τ, (3/2)τ, (5/3)τ, 2τ, (5/2)τ, 3τ, 4τ. The presence of quadrupolar echoes is the fingerprint of the deviation from perfect cubic symmetry of the structure and can be used as a simple and fast test of the sample quality.     

Surface ablation оf aluminum and silicon by ultrashort laser pulses of variable width

Single-shot thresholds of surface ablation of aluminum and silicon via spallative ablation by infrared (IR) and visible ultrashort laser pulses of variable width τ_las (0.2–12 ps) have been measured by optical microscopy. For increasing laser pulse width τ_las < 3 ps, a drastic (threefold) drop of the ablation threshold of aluminum has been observed for visible pulses compared to an almost negligible threshold variation for IR pulses. In contrast, the ablation threshold in silicon increases threefold with increasing τ_las for IR pulses, while the corresponding thresholds for visible pulses remained almost constant. In aluminum, such a width-dependent decrease in ablation thresholds has been related to strongly diminished temperature gradients for pulse widths exceeding the characteristic electron-phonon thermalization time. In silicon, the observed increase in ablation thresholds has been ascribed to two-photon IR excitation, while in the visible range linear absorption of the material results in almost constant thresholds.