Electron Time Scales Research Articles

A theoretical and simulation study of the contact discontinuities based on a Vlasov simulation code

[1] Contact discontinuity (CD) is the simplest solution that can be obtained from the magnetohydrodynamics (MHD) Rankine-Hugoniot jump conditions. Due to the limitations of the previous kinetic simulation models, the stability of the CD has become a controversial issue in the past 10 years. The stability of the CD is reexamined analytically and numerically. Our theoretical analysis shows that the electron temperature profile and the ion temperature profile must be out of phase across the CD if the CD structure is to be stable in the electron time scale and with zero electron heat flux on either side of the CD. Both a newly developed fourth-order implicit electrostatic Vlasov simulation code and an electromagnetic finite-size particle code are used to examine the stability and the electrostatic nature of the CD structure. Our theoretical prediction is verified by both simulations. Our results of Vlasov simulation also indicate that a simulation with initial electron temperature profile and ion temperature profile varying in phase across the CD will undergo very transient changes in the electron time scale but will relax into a quasi-steady CD structure within a few ion plasma oscillation periods if a real ion-electron mass ratio is used in the simulation and if the boundary conditions allow nonzero heat flux to be presented at the boundaries of the simulation box. The simulation results of this study indicate that the Vlasov simulation is a powerful tool to study nonlinear phenomena with nonperiodic boundary conditions and with nonzero heat flux at the boundaries of the simulation box.

Spectrally resolved photon echo spectroscopy of Zn(II), Co(II) and Ni(II)–octaethyl porphyrins

Spectrally resolved femtosecond three-pulse photon echo signal from some metal–octaethyl porphyrins (OEPs) like Zn(II)–OEP, Ni(II)–OEP, Co(II)–OEP is reported. Excited state dynamics is studied by time evolving photon echo spectra for different values of coherence and population relaxation times. Dependence on the spectrally resolved photon echo spectra on varying metal center is analyzed. For all these metallo-porphyrins, the electronic relaxation timescale is found to be limited by our laser pulsewidth of 50fs whereas the timescale for intramolecular vibrational relaxation, occurring within the Q00 band was found to be over a picosecond for Co(II)–OEP and Ni(II)–OEP and within a picosecond for Zn(II)–OEP.

Suzaku Observation of the TeV Blazar 1ES 1218+304: Clues on Particle Acceleration in an Extreme TeV Blazar

We observed the TeV blazar 1ES 1218+304 with the X-ray astronomy satellite Suzaku in May 2006. At the beginning of the two-day continuous observation, we detected a large flare in which the 5-10 keV flux changed by a factor of {approx}2 on a timescale of 5 x 10{sup 4} s. During the flare, the increase in the hard X-ray flux clearly lagged behind that observed in the soft X-rays, with the maximum lag of 2.3 x 10{sup 4} s observed between the 0.3?1 keV and 5?10 keV bands. Furthermore we discovered that the temporal profile of the flare clearly changes with energy, being more symmetric at higher energies. From the spectral fitting of multi-wavelength data assuming a one-zone, homogeneous synchrotron self-Compton model, we obtain B {approx} 0.047 G, emission region size R = 3.0 x 10{sup 16} cm for an appropriate beaming with a Doppler factor of {delta} = 20. This value of B is in good agreement with an independent estimate through the model fit to the observed time lag ascribing the energy-dependent variability to differential acceleration timescale of relativistic electrons provided that the gyro-factor {zeta} is 10{sup 5}.

Born–Oppenheimer approximation and beyond for time-dependent electronic processes

Explicit computations of electronic motion in time and space are gradually becoming feasible and available. The knowledge of this motion is of relevance by itself but is also important for understanding available and predicting future experiments on the electronic time scale. In electronic processes of interest, usually several and even many stationary electronic states participate and the obvious question arises on how to describe the accompanying quantum nuclear dynamics at least on the time scale of the process. In this work, we attempt to study the nuclear dynamics in the framework of a fully time-dependent Born-Oppenheimer approximation. Additionally, we attempt to go beyond this approximation by introducing the coupling of several electronic wavepackets by the nuclear wavepackets. In this context, we also discuss a time-dependent transformation to diabatic electronic wavepackets. A simple but critical model of charge transfer is analyzed in some detail on various levels of approximation and also solved exactly.

On the mechanism of weak-field coherent control of retinal isomerization in bacteriorhodopsin

Experimental studies of the short time reaction dynamics controlling the chemical branching ratio provide direct evidence for the mechanism of coherent control of the retinal photoisomerization in bacteriorhodopsin in the weak-field limit with respect to the previous report [V. Prokhorenko, A. Nagy, S. Waschuk, L. Brown, R. Birge, R. Miller, Science 313 (2006) 1257]. The phase sensitivity of the reaction dynamics is directly revealed using time- and frequency-resolved pump–probe measurements. The high degree of control of the reaction branching ratio is theoretically explained through a combination of spectral amplitude shaping and phase-dependent coupling to selectively excite vibrations most strongly coupled to the reaction coordinate. Coherent control in this context must involve reaction dynamics that occur on time scales comparable to electronic and vibrational decoherence time scales.

Modeling of dynamic response of SOL-divertor plasmas to an ELM crash

A dynamic five-point model is developed to study responses of scrape-off-layer (SOL) and divertor plasmas to a crash due to edge-localized modes (ELMs). The five-point model is beneficial to wide-ranging studies of SOL-divertor physics and to coupling with the core transport code in the integrated modeling. The five-point model can reproduce static and dynamic features obtained by usual fluid codes. The dynamic behavior obtained by the five-point model agrees fairly well with that by the particle code PARASOL. The influence of the ELM crash on the thermoelectric instability and the resultant asymmetry is investigated. The ELM crash transiently induces the thermoelectric instability and large SOL currents for asymmetric divertor plasmas before the ELM. The resultant currents drive convective heat flows and enhance the asymmetry on the fast time scale of electrons. The ELM crash is found to reverse the asymmetry before and after the ELM. Even for symmetric plasmas before the ELM, the asymmetry is induced on the slow time scale of ions by an ELM crash, because the transient formation of dense and cold divertor plasmas causes the thermoelectric instability.

Time-Resolved Electrochemical Spectroscopy of Charge Migration in Molecular Wires: Computational Evidence for Rich Electronic Dynamics

Electrical conduction through a molecule tethered by thiol bridges between two gold clusters is examined from a time-dependent point of view. The shortest electronic time scale for charge migration is a few femtoseconds transit, which is too swift for coupling to the nuclei, that proceeds by super exchange. An order of magnitude slower transfer occurs sequentially through the lower-in-energy sigma bonds. The electronic structure computations are performed at a high level ab initio density functional theory level where the external electric fields are included as part of the Hamiltonian. The structure computed includes the thiol bridge as well as the gold trimer at either end of the molecule. The results shown in detail are for the saturated dithiohexane −S−(CH2)6−S− bridge between the two gold trimers.

Stability and evolution of one-dimensional relativistic solitons on the ion time scale

The one-dimensional dynamics of trapped relativistic electromagnetic radiation, which appears during laser plasma interaction, is investigated within a relativistic fluid-Maxwell model. The modifications of plasma density due to trapped laser fields are considered for linear as well as circular polarizations. It is shown that standing (V=0) solitons are stable on the electron time scale. However, the stability region does not agree with the prediction from the Vakhitov-Kolokolov criterion. Ions always drive the standing solitons unstable, irrespective of the polarization. The stability of moving (0<V<c) solitons, which have been obtained in the literature as stationary solutions of the fluid-Maxwell equations including ion dynamics, is demonstrated. The problem of soliton generation is addressed. The time evolution of the so called post-solitons, which are generated behind a broad laser pulse propagating in underdense plasma, is analyzed. The effect of finite electron and ion temperatures is briefly discussed.

An electronic time scale in chemistry.

Ultrafast, subfemtosecond charge migration in small peptides is discussed on the basis of computational studies and compared with the selective bond dissociation after ionization as observed by Schlag and Weinkauf. The reported relaxation could be probed in real time if the removal of an electron could be achieved on the attosecond time scale. Then the mean field seen by an electron would be changing rapidly enough to initiate the migration. Tyrosine-terminated tetrapeptides have a particularly fast charge migration where in <1 fs the charge arrives at the other end. A femtosecond pulse can be used to observe the somewhat slower relaxation induced by correlation between electrons of different spins. A slower relaxation also is indicated when removing a deeper-lying valence electron. When a chromophoric amino acid is at one end of the peptide, the charge can migrate all along the peptide backbone up to the N end, but site-selective ionization is probably easier to detect for tryptophan than for tyrosine.

Relativistic electron events in 2002: Studies of pitch angle isotropization

Energization of electrons to relativistic energies in the Earth's outer radiation belt is not fully understood. Recent renewed interest in the problem has resulted in a number of models which invoke a variety of physical processes ranging from diffusive particle transport to in situ acceleration. Evolution of local pitch angle distributions and global flux isotropization timescales of relativistic electrons may be important discriminators of the underlying physical processes of energization, transport, and loss. Data from SAMPEX, Polar, and LANL suite of satellites were used to study electron isotropization and pitch angle evolution for relativistic electron enhancement events during the year 2002. It was found that during electron energization events there is generally increased precipitation of electrons and rapid electron flux isotropization on a global scale. Case studies using pitch angle resolved data collected at geosynchronous distances suggest an in situ broadening of the local pitch angle distribution in agreement with the corresponding global isotropization measurements. These observations suggest that electron energization in the magnetosphere is intimately associated with rapid pitch angle scattering leading to a flattening of the pitch angle distributions of energized electrons.

Parallel electric fields in a diverging flux tube with upward current: Nature of quasi‐static fields

Using one‐dimensional (1‐D) particle‐in‐cell (PIC) simulations of a diverging magnetic flux tube, the spatial distribution of parallel electric field in response to an applied potential drop is studied. Hot and cold plasma populations as appropriate for the upward current region are included. Simulations start with an initially empty flux tube representing an auroral density cavity. The cavity refills with plasmas flowing from the ionosphere at the bottom and the magnetosphere at the top. The ionospheric cold ions are significantly accelerated by means of plasma expansion processes in the cavity. When an appropriate warm secondary electron population at the ionospheric boundary is included in the simulations, a significant feature of the expanding ionospheric plasma is a rarefaction shock (RFS) near the bottom of the density cavity. Dynamics of the shock and its role in setting up a low‐altitude double layer (DL) are examined. Even after the shock's decay due to the heating of the ionospheric cold electrons by wave‐particle interactions, electron heating at low altitudes sustains the enhanced role of plasma expansion in upward acceleration of ions. The applied potential drop does localize like that in a strong DL, but only rarely a strong monotonic DL forms; often the DL is highly dynamic and evolves continually via plasma turbulence consisting of both electron and ion timescales. Averaging the fast fluctuations at the electron timescales, the potential distribution across the quasi‐static DL yields oscillatory and spiky parallel fields, which evolve with ion‐acoustic and ion hole turbulence. We compare our results with observations from satellites.

Self-similarity in Hall plasma discharges: Applications to particle models

Electron transport is a key process in the physics of Hall thruster discharges. Therefore, a kinetic description of the heavy particles (Xe) as well as electrons is required. The ideal numerical model would be a particle model for all the species. Nowadays, such a model is unpractical because it would need too large an amount of computation time due to the very different time scales of electrons and heavy particles dynamics. For this purpose two scalings to speed-up the execution time of a two-dimensional fully kinetic Particle-in-Cell/Monte Carlo Collision simulation of the Hall thruster SPT-100 are proposed. These two different scaling schemes generate self-similar systems of the acceleration channel including the process of secondary electron emission from the dielectric walls. Instead of using the common approach of a smaller neutral and ion mass or a larger vacuum permittivity the channel dimensions are reduced keeping the main dimensionless physics parameters constant. This leads to scaling laws for the input (magnetic field, mass flow rate, current and voltage discharge, etc.) and output parameters obeying self-similarity. This scaling methodology makes the simulation faster and allows improved modeling of electron interactions and fundamental processes. This model has demonstrated its outstanding capability in improving the physics insight into the processes in SPT-100 under the scaling constraints for the geometrical reduction. The application to particle models of different plasma based devices is suggested for such systems where a linearization of the Boltzmann equation is possible.

Dynamics of electron-beam holes induced by self-trapping

A phase-space distribution of a weak electron-beam injected into a cold plasma is experimentally investigated in electron time scales. It is shown that electron-beam holes in phase-space evolve dynamically from appearance to collapse, as synchronized with a wave packet dominated by a beam mode. The dependence of the hole velocity radii on the packet crest amplitudes indicates that the holes are induced by self-trapping of the beam. It is found that the beam is well trapped while the packet grows linearly, but easy to be detrapped after that.

Quantum fluctuation of electronic wave-packet dynamics coupled with classical nuclear motions

An ab initio electronic wave-packet dynamics coupled with the simultaneous classical dynamics of nuclear motions in a molecule is studied. We first survey the dynamical equations of motion for the individual components. Reflecting the nonadiabatic dynamics that electrons can respond to nuclear motions only with a finite speed, the equations of motion for nuclei include a force arising from the kinematic (nuclear momentum) coupling from electron cloud. To materialize these quantum effects in the actual ab initio calculations, we study practical implementation of relevant electronic matrix elements that are related to the derivatives with respect to the nuclear coordinates. Applications of the present scheme are performed in terms of the configuration state functions (CSF) using the canonical molecular orbitals as basis functions without transformation to particular diabatic basis. In the CSF representation, the nonadiabatic interaction due to the kinematic coupling is anticipated to be rather small, and instead it should be well taken into account through the off-diagonal elements of the electronic Hamiltonian matrix. Therefore it is expected that the nonadiabatic dynamics based on this CSF basis neglecting the kinematic coupling may work. To verify this anticipation and to quantify the actual effects of the kinematic coupling, we compare the dynamics with and without the kinematic-coupling terms using the same CSF set. Applications up to the fifth electronically excited states in a nonadiabatic collision between H(2) and B(+) shows that the overall behaviors of these two calculations are surprisingly similar to each other in an average sense except for a fast fluctuation reflecting the electronic time scale. However, at the same time, qualitative differences in the collision events are sometimes observed. Therefore it turns out after all that the kinematic-coupling terms cannot be neglected in the CSF-basis representation. The present applications also demonstrate that the nonadiabatic electronic wave-packet dynamics within ab initio quantum chemical calculation is feasible.

Beam-Self-Trapping Phenomena in an Electron-Beam-Plasma

It is observed that electron-beam holes in phase-space evolve dynamically from arising to collapsing in electron time scales in an electron-beam-plasma system. The holes are synchronized with a wave packet dominated by a beam mode of the two-stream instability, and their velocity radii depend on the amplitudes of the packet crests. This shows that the holes are induced by the self-trapping of the beam. The observed images indicate that the holes keep alive due to the hard detrapping of the beam during the packet linear growth, but begin to collapse due to the easy detrapping in the packet saturation.

Electronic excitations and tunneling spectra of metallic nanograins

Tunneling-induced electronic excitations in a metallic nanograin are classified in terms of {\em generations}: subspaces of excitations containing a specific number of electron-hole pairs. This yields a hierarchy of populated excited states of the nanograin that strongly depends on (a) the available electronic energy levels; and (b) the ratio between the electronic relaxation rate within the nano-grain and the bottleneck rate for tunneling transitions. To study the response of the electronic energy level structure of the nanograin to the excitations, and its signature in the tunneling spectrum, we propose a microscopic mean-field theory. Two main features emerge when considering an Al nanograin coated with Al oxide: (i) The electronic energy response fluctuates strongly in the presence of disorder, from level to level and excitation to excitation. Such fluctuations produce a dramatic sample dependence of the tunneling spectra. On the other hand, for excitations that are energetically accessible at low applied bias voltages, the magnitude of the response, reflected in the renormalization of the single-electron energy levels, is smaller than the average spacing between energy levels. (ii) If the tunneling and electronic relaxation time scales are such as to admit a significant non-equilibrium population of the excited nanoparticle states, it should be possible to realize much higher spectral densities of resonances than have been observed to date in such devices. These resonances arise from tunneling into ground-state and excited electronic energy levels, as well as from charge fluctuations present during tunneling.

Nonlinear Evolution of a Wave Packet in Electron-Beam-Plasma

The temporal evolution of a wave packet in electron-beam-plasma is observed experimentally. It is shown that the packet develops on electron time scales not into self-contraction but into asymmetrical widening. Particle trapping, indicated the distribution function of electron-beam, plays an important role in causing this phenomenon. When the waves in an electron-beam-plasma system are strong, nonlinear phenomena characteristi- cally appear. In regard to this phenomena, it is important to discuss solitary waves or modulation instabilities (1). A. Y. Wong et al. (2) showed experimentally a 3-D Langmuir-wave collapse of ion time scales driven by a pulsed electron-beam. T. Intrator et al. (3) reported the longitudinal self-contraction of modulationally unstable electron wave packets developing on electron time scales. We (4) showed that a wave packet can be stabilized by emitting a series of several burst waves. In order to investigate nonlinear phenomena at long distances, we introduce a new beam-plasma device, the length of which is 2 times longer than that of the previous one. In this paper, we report the nonlinear behavior of the electron time scales of the packet in plasma, induced by a low density electron-beam, and the distribution function of the electron-beam.

Ultrafast Dynamics of Porphyrins in the Condensed Phase: II. Zinc Tetraphenylporphyrin

Femtosecond spectroscopic studies of zinc tetraphenylporphyrin (ZnTPP) in benzene and dichloromethane are reported, combining both fluorescence up-conversion and transient absorption measurements. The purpose is to investigate the initial electronic and vibrational relaxation of the S_1 and S_2 excited states, in a system in which interference from solvent rearrangement is insignificant as evidenced by the small Stokes shift in the fluorescence. Excitation of the low-lying singlet excited state (S_1) results in nanosecond relaxation, while excitation to S_2, the Soret band, leads to multiple electronic and vibrational relaxation time scales of S_2 and S_1 populations, from hundreds of femtoseconds to tens of picoseconds. The systematic and detailed studies reported here reveal that the Soret fluorescence band decays with a lifetime in benzene of 1.45 ps for excitation at 397 nm, while emission monitored at the same wavelength, but for two-photon 550 nm excitation, decays biexponentially with 200 fs and 1.0 ps time constants. In addition, the Soret fluorescence decay lifetime for 397 nm excitation is distinctly longer than the rise time of S_1 fluorescence for the same excitation, which varies with wavelength. These observations are consistent with the model presented here in which the Soret band structure consists of absorption from S_0 to two manifolds of states with distinct electronic and vibrational couplings to S_1 and higher electronic states. To compare with literature, we also measured the S_2 lifetime in dichloromethane and found it to be 1.9 ps, a lengthening from its value in benzene. However, the transient fluorescence intensity is greatly reduced. These observations in dichloromethane provide evidence of an ultrafast (<100 fs) channel for electron transfer from ZnTPP to dichloromethane for a subset of excited molecules in favorably oriented contact with the solvent, that is, a bifurcation of population. Finally, solvent-induced vibrational relaxation of the S_1 population following internal conversion from S_2 occurs over a range of time scales (picoseconds to tens of picoseconds) depending on the wavelength (fluorescence or transient absorption), and the observed rate indeed changes with solvent.

Effect of particle acceleration process on the flare characteristics of blazars

Following the kinetic equation approach, we study the flare processes in blazars in the optical-to-X-ray region, considering energy dependent acceleration time-scale of electrons and synchrotron and adiabatic cooling as their dominant energy loss processes.

Thermally Self-Consistent Monte Carlo Device Simulations

We present details of a Monte Carlo simulation code which is coupled to a Heat Diffusion Equation (HDE) solver. Through an iterative procedure, which bypasses the differences in electronic and thermal timescales, this coupled code is capable of producing steady-state thermally self-consistent device characteristics. Electronically-generated thermal flux is calculated by monitoring the net rate of phonon emission, which may be resolved both spatially and by phonon type. The thermal solution is extracted through use of a novel analytical thermal resistance matrix technique which avoids calculation of temperatures beyond the electronically important device region while including the large-scale boundary conditions. On application to a GaAs MESFET the expected ‘thermal droop’ behaviour is obtained in the I-V characteristics and we find a linear relationship between peak lattice temperature and applied source-drain bias. At moderate biases the contribution of intervalley phonons to the thermal power output surpasses that of optical phonons.