Nondipole parameters of TDCS for electron impact ionization and drag current

A comprehensive study is undertaken of angular distributions of electron knock-out from atomic targets by fast electrons with a small transfer of momentum. The general expressions for the parameters of the triple differential cross-section of impact ionization in the optical limit are derived. The calculated parameters are compared with those of the angular distribution of electrons ejected from an atom in the process of photoionization. In these processes, when the multipole transitions are involved, the one-to-one correspondence between the photoionization and impact ionization parameters disappears. The nondipole transitions lead to the backward/forward asymmetry of the angular distribution of ejected electrons that is absent in the dipole approximation for ionization by both fast electrons and photons. Using the He atom as an example, the character of the asymmetry for these two processes is qualitatively different and the backward/forward asymmetry results in macroscopic directed motion of secondary electrons accompanying the passing of a fast electron beam through gas or plasma. The general formulas for this drag current are derived and applied to gaseous He.     

Pion and proton production in forward and backward hemisphere in C-emulsion interaction at 4.5 GeV/c per nucleon

The paper presents an in-depth study on the forward and backward multiplicity correlations in case of pions and protons (from the target) based on the experimental data on ¹²C-emulsion interaction at 4.5 GeV/c per nucleon. The cumulative distribution of their emission angles in the backward hemisphere is presented. The asymmetry parameter (A) and the forward-backward ratio (F/B) as a function of the number of heavy particles (n_h) is also shown. The results reveal many interesting characteristics regarding the backward particle production.     

Multiplicity distribution inK + Al andK + Au collisions at 250 GeV/c and a test of the multiple collision model

Multiplicity distributions, observed inK ⁺ interactions with Al and Au nuclei at 250 GeV/c incident momentum are presented. They are analyzed in the framework of multiple collisions of the incident particle inside a nucleus. The probability distribution of the number of grey tracks is well described by the model of Andersson et al., if a negative binomial distribution is assumed for the distribution of the number of grey protons produced per elementary collision instead of the usual geometrical distribution. The analysis of the average and dispersion of the charge multiplicity distribution supports the validity of the multiple collision model, including results on correlations between forward and backward multiplicities.     

Competition and coupling between forward and backward waves in optically-pumped linear-cavity fir lasers

We study theoretically the gain for a linear-cavity (Fabry-Perot) FIR laser pumped by a travelling-wave infrared beam. Arbitrary field intensities and pressure and Doppler broadenings are considered. The simultaneous presence inside the laser cavity of the forward and backward generated waves produces strong competition and coupling effects between these waves which are interpreted in terms of Lamb-dip and multiphoton processes. These effects determine the main differences in the amplifying behaviour between the bi-directional (linear) and uni-directional (ring) laser configurations.     

Forward–backward multiplicity correlations in ring like and jet like events in <Superscript>16</Superscript>O–AgBr interactions at 60 AGeV

We have studied the forward–backward correlations of charged-particle multiplicities in symmetric bins in pseudorapidity space in order to gain insight into the underlying correlations structure of particle production in case of ring-like and jet-like events of ¹⁶O–AgBr interactions at 60 AGeV. The variance σ_c ² of a suitably defined the forward-backward asymmetry variable C has been determined. The experimental results confirm correlations of the produced particles in the forward and the backward pseudorapidity region for both ring-like and jet-like events.     

Fluctuation Theorems for Entropy Production and Heat Dissipation in Periodically Driven Markov Chains

Asymptotic fluctuation theorems are statements of a Gallavotti-Cohen symmetry in the rate function of either the time-averaged entropy production or heat dissipation of a process. Such theorems have been proved for various general classes of continuous-time deterministic and stochastic processes, but always under the assumption that the forces driving the system are time independent, and often relying on the existence of a limiting ergodic distribution. In this paper we extend the asymptotic fluctuation theorem for the first time to inhomogeneous continuous-time processes without a stationary distribution, considering specifically a finite state Markov chain driven by periodic transition rates. We find that for both entropy production and heat dissipation, the usual Gallavotti-Cohen symmetry of the rate function is generalized to an analogous relation between the rate functions of the original process and its corresponding backward process, in which the trajectory and the driving protocol have been time-reversed. The effect is that spontaneous positive fluctuations in the long time average of each quantity in the forward process are exponentially more likely than spontaneous negative fluctuations in the backward process, and vice-versa, revealing that the distributions of fluctuations in universes in which time moves forward and backward are related. As an additional result, the asymptotic time-averaged entropy production is obtained as the integral of a periodic entropy production rate that generalizes the constant rate pertaining to homogeneous dynamics.     

On Riccati equations describing impedance relations for forward and backward excitation in the one-dimensional cochlea model

Recent experimental observations of otoacoustic emissions suggest the existence of spontaneous emitters of sound on the basilar membrane. These tend to send off waves not only in the normal direction of propagation. It is therefore significant to study the environmental conditions such an emitter finds inside the cochlea. The impedance relations seen by these emitters are described by the Riccati equation for an inhomogeneous transmission line. The results reported in this paper differ considerably for forward and backward excitation. This reflects the quite different behavior of the cochlea pertaining to waves traveling forward and backward. Because of reflections, backward waves cannot be treated with the Liouville-Green approximation.     

Backward enhanced emission from multiphoton processes in aerosols

We have investigated, both theoretically and experimentally, multiphoton-induced processes in aerosol particles using femtosecond laser pulses. More specifically, we have demonstrated that both multiphoton (1, 2 and 3 photon)-induced fluorescence (MPEF) and laser-induced breakdown (LIB) emissions are strongly enhanced in the backward direction. The backward enhancement increases from 1.8 to 35 (emission ratio between the backward direction and 90°) with increasing non-linear process order n. Application to non-linear lidar of biological aerosols is discussed. Received: 24 April 2002 / Revised version: 3 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-472/431507, E-mail: wolf@lasim.univ-lyon1.fr     

Repetition rate multipliers for efficient implementation of multiphoton and pump-probe fluorescence microscopy

With the advances in pulsed laser systems, microscopic imaging techniques such as multiphoton and pump-probe fluorescence microscopy have developed into effective tools for investigating intensity and time-resolved phenomena inside biological systems. However, pulsed lasers used in these techniques usually are commercial systems with repetition frequencies of around 80 MHz. While these systems have proven to be adequate for multiphoton and pump-probe microscopic imaging applications, the temporal separation of the laser pulse train (around 12.5 ns) is long compared to the fluorescence lifetimes of many common fluorescence species. In this work, we present the designs of repetition rate multipliers based on passive optical components that can be used to increase the efficiency in multiphoton and pump-probe fluorescence microscopy. Depending on the lifetime of fluorescence molecules under investigation, the passive repetition rate multiplier can increase the duty cycle of multiphoton or pump-probe microscopy up to fourfold.     

Transition from a state with a certain momentum into a state with a certain parameter of spinless particle escape in the external field

A distribution function on the parameter of escape of particle B with the amplitude of process A → B in the external stationary field is investigated in this work. This distribution function is directly related to the form factor of the external-field source. By way of illustration consideration has been given to a hydrogen-like atom. It is shown that the distribution into the forward hemisphere comprises information on the spatial structure of an electron cloud, while the distribution into the backward hemisphere reproduces a spatial structure in the vicinity of a nucleus.     

Irreversibility and fluctuation theorem in stationary time series

The relative entropy between the joint probability distribution of backward and forward sequences is used to quantify time asymmetry (or irreversibility) for stationary time series. The parallel with the thermodynamic theory of nonequilibrium steady states allows us to link the degree of asymmetry in the time signal with the distance from equilibrium and the lack of detailed balance among its states. We study the statistics of time asymmetry in terms of the fluctuation theorem, showing that this type of relationship derives from simple general symmetries valid for any stationary time series.     

Improving signal levels in intravital multiphoton microscopy using an objective correction collar

Multiphoton microscopy has enabled biologists to collect high-resolution images hundreds of microns into biological tissues, including tissues of living animals. While the depth of imaging exceeds that possible from any other form of light microscopy, multiphoton microscopy is nonetheless generally limited to depths of less than a millimeter. Many of the advantages of multiphoton microscopy for deep tissue imaging accrue from the unique nature of multiphoton fluorescence excitation. However, the quadratic relationship between illumination level and fluorescence excitation makes multiphoton microscopy especially susceptible to factors that degrade the illumination focus. Here we examine the effect of spherical aberration on multiphoton microscopy in fixed kidney tissues and in the kidneys of living animals. We find that spherical aberration, as evaluated from axial asymmetry in the point-spread function, can be corrected by adjustment of the correction collar of a water immersion objective lens. Introducing a compensatory positive spherical aberration into the imaging system decreases the depth-dependence of signal levels in images collected from living animals, increasing signal by up to 50%.     

Uniform plasmonic near-field nanopatterning by backward irradiation of femtosecond laser

We present localized optical field distribution properties in the vicinity of gold particles on a silicon substrate by backward and forward irradiation. It is technically difficult to fabricate nanostructures on the surface by a conventional forward laser incident to the substrate because gold nanoparticles easily aggregate to form double-layered particle arrays. We calculated enhanced optical field properties in order to pattern the substrate surface only with a template of the bottom-layered particle arrays in the case that the backward irradiation of a femtosecond laser is used in the system of aggregated double-layered gold nanoparticle arrays. With the backward irradiation, the optical field intensity in the substrate for the double-layered hexagonal arrays is found to be only 30% lower than the mono-layered system. Moreover, a near field cannot be generated with the forward irradiation. As a result, only the backward irradiation scheme is found to be effective for uniform surface nanopatterning at enhanced plasmonic near-field zones.     

Y+SO2反应动力学成像

本文利用时间切片离子速度成像技术、交叉分子束和激光溅射技术研究了高碰撞能下(36 kcal/mol)钇原子与二氧化硫分子的反应动力学. 利用多光子电离在482∽615 nm的波长范围内得到了产物YO的时间切片离子速度成像. YO的切片图像显示其较宽的速度分布和前-后向散射为主的角分布,其中前向散射信号明显强于后向散射. 这种空间分布暗示了该反应通过一个中间体进行,且中间体的寿命不超过一个转动周期. 中间体的形成意味着该氧化反应电子转移机理的发生.     

Pion-Muon Asymmetry Revisited

Long ago an unexpected and unexplainable phenomena was observed. The distribution of muons from positive pion decay at rest was anisotropic with an excess in the backward direction relative to the direction of the proton beam from which it was produced. Although this effect was observed by several different groups with pions produced by different means and detected by different methods, the result was not accepted by the physics community, because it is in direct conflict with a large set of other experiments indicating that the pion is a pseudoscalar particle. It is possible to satisfy both sets of experiments if helicity-zero vector particles exist and the pion is such a particle. Helicity-zero vector particles have direction but no net spin. For the neutral pion to be a vector particle requires an additional modification to conventional theory as discussed herein. An experiment is proposed which can prove that the asymmetry in the distribution of muons from pion decay is a genuine physical effect because the asymmetry can be modified in a controllable manner. A positive result will also prove that the pion is not a pseudoscalar particle.     

Charge asymmetry in inelastic π−p interactions at 205 GeV/c for particles with transverse momentum > 1.0 GeVc

In 205 GeV/cπ^?p inelastic interactions, negative particles with transverse momentum greater than 1.0 GeV/c moving forward in the center of mass outnumber similar positive particles by a factor of 3.7 to 1, greatly in excess of the corresponding ratio for small transverse momentum. The asymmetry is reversed in the backward direction. The forward asymmetry is most prominent in 2-, 4-, and 6-prong interactions, but both forward and backward asymmetries are also substantial for higher multiplicity interactions.     

Raman spectroscopy of infrared multiple-photon excited molecules

The method of spectroscopy of spontaneous Raman scattering (RS) with time resolution has been applied for the first time to diagnose the process of multiphoton ir molecular excitation (MPE). Some aspects of RS diagnostics of MPE processes are being analyzed. It has been shown experimentally on SF₆ and CF₃I molecules that it is possible to study such important characteristics of excitation process as the fraction of molecules involved in the process of excitation, vibrational energy distribution of molecules, stochastization of inner molecular energy.     

Observation of large quadrupolar effects in a slow photoelectron imaging experiment

We have studied nondipolar effects in resonance-enhanced multiphoton ionization of Xe and have observed an azimuthal dependence of the photoelectron angular distribution on a quadrupole resonance, as well as a very large asymmetry with respect to the direction of the laser propagation close to the resonance, which is understood in terms of interference between dipole- and quadrupole-allowed ionization channels. The observed asymmetry in the photoelectron angular distribution provides insight into the ejection of slow photoelectrons near an ionization threshold.     

Enhanced fluorescence sensing using microstructured optical fibers: a comparison of forward and backward collection modes

A general model of excitation and fluorescence recapturing by the forward and backward modes of filled microstructured optical fibers (MOFs) is presented. We also present experimental results for both backward and forward fluorescence recapturing within a MOF as a function of fiber length and demonstrate a good qualitative agreement between the numerical model and experimental results. We demonstrate higher efficiency of fluorescence recapturing into backward modes in comparison with that of forward modes.     

Enhanced backward-directed multiphoton-excited fluorescence from dielectric microcavities

We demonstrate theoretically and experimentally that one-, two-, and three-photon excited fluorescence from dye molecules in spherical microcavities has an asymmetrical angular distribution and is enhanced in the backward direction. The enhancement ratios (of intensities at 180 degrees and 90 degrees ) are 9, 5, and 1.8 for three-, two-, and one-photon excitation, respectively. Even larger ratios are expected for microspheres with an index of refraction larger than that used in the experiments. Because of the reciprocity principle and concentration of the incident wave inside particles, the backward enhancement is expected to occur even with nonspherical particles.