Modulated electrostatic modes in pair plasmas: Modulational stability profile and envelope excitations

A pair plasma consisting of two types of ions, possessing equal masses and opposite charges, is considered. The nonlinear propagation of modulated electrostatic wave packets is studied by employing a two-fluid plasma model. Considering propagation parallel to the external magnetic field, two distinct electrostatic modes are obtained, namely a quasiacoustic lower moddfe and a Langmuir-like, as optic-type upper one, in agreement with experimental observations and theoretical predictions. Considering small yet weakly nonlinear deviations from equilibrium, and adopting a multiple-scale technique, the basic set of model equations is reduced to a nonlinear Schrodinger equation for the slowly varying electric field perturbation amplitude. The analysis reveals that the lower (acoustic) mode is stable and may propagate in the form of a dark-type envelope soliton (a void) modulating a carrier wave packet, while the upper linear mode is intrinsically unstable, and may favor the formation of bright-type envelope soliton (pulse) modulated wave packets. These results are relevant to recent observations of electrostatic waves in pair-ion (fullerene) plasmas, and also with respect to electron-positron plasma emission in pulsar magnetospheres. (c) 2006 American Institute of Physics.

Modulational instability of dust acoustic waves in dusty plasmas: Modulation obliqueness, background ion nonthermality, and dust charging effects

The oblique modulational instability of dust acoustic (DA) waves in an unmagnetized warm dusty plasma with nonthermal ions, taking into account dust grain charge variation (charging), is investigated. A nonlinear Schrodinger-type equation governing the slow modulation of the wave amplitude is derived. The effects of dust temperature, dust charge variation, ion deviation from Maxwellian equilibrium (nonthermality) and constituent species' concentration on the modulational instability of DA waves are examined. It is found that these parameters modify significantly the oblique modulational instability domain in the k-theta plane. Explicit expressions for the instability rate and threshold have been obtained in terms of the dispersion laws of the system. The possibility and conditions for the existence of different types of localized excitations are also discussed. The findings of this investigation may be useful in understanding the stable electrostatic wave packet acceleration mechanisms close to the Moon, and also enhances our knowledge on the occurrence of instability associated to pickup ions around unmagnetized bodies, such as comets, Mars, and Venus.

Nonlinear modulation of ion-acoustic waves in two-electron-temperature plasmas

The amplitude modulation of ion-acoustic waves IS investigated in a plasma consisting of adiabatic warm ions, and two different populations of thermal electrons at different temperatures. The fluid equations are reduced to nonlinear Schrodinger equation by employing a multi-scale perturbation technique. A linear stability analysis for the wave packet amplitude reveals that long wavelengths are always stable, while modulational instability sets in for shorter wavelengths. It is shown that increasing the value of the hot-to-cold electron temperature ratio (mu), for a given value of the hot-to-cold electron density ratio (nu): favors instability. The role of the ion temperature is also discussed. In the limiting case nu = 0 (or nu -> infinity). which correspond(s) to an ordinary (single) electron-ion plasma, the results of previous works are recovered.

Redistribution of vibrational population in a molecular ion with nonresonant strong-field laser pulses

We present an experimental demonstration of nonresonant manipulation of vibrational states in a molecule by an intense ultrashort laser pulse. A vibrational wave packet is generated in D-2(+) through tunnel ionization of D-2 by a few-cycle pump pulse. A similar control pulse is applied as the wave packet begins to dephase so that the dynamic Stark effect distorts the electronic environment of the nuclei, transferring vibrational population. The time evolution of the modified wave packet is probed via the D-2(+) photodissociation yield that results from the application of an intense probe pulse. Comparing the measured yield with a quasiclassical trajectory model allows us to determine the redistribution of vibrational population caused by the control pulse. ©

Superluminal electromagnetic solitary waves in electron-positron plasmas

The propagation of an electromagnetic wave packet in an electron-positron plasma, in the form of coupled localized electromagnetic excitations, is investigated, from first principles. By means of the Poincare section method, a special class of superluminal localized nonlinear stationary solutions, existing along a separatrix curve, are proposed as intrinsic electromagnetic modes in a relativistic electron-positron plasma. The ratio of the envelope time scale to the carrier wave time scale of these envelope solitary waves critically depends on the carrier's phase velocity. In the strongly superluminal regime, v(ph)/c >> 1, the large difference between the envelope and carrier time scales enables us to carry out a multiscale perturbative analysis resulting in an analytical form of the solution envelope. The analytical prediction thus obtained is shown to be in agreement with the solution obtained via a direct numerical integration. Copyright (c) EPLA, 2012

On transition rates in surface hopping

Trajectory surface hopping (TSH) is one of the most widely used quantum-classical algorithms for nonadiabatic molecular dynamics. Despite its empirical effectiveness and popularity, a rigorous derivation of TSH as the classical limit of a combined quantum electron-nuclear dynamics is still missing. In this work, we aim to elucidate the theoretical basis for the widely used hopping rules. Naturally, we concentrate thereby on the formal aspects of the TSH. Using a Gaussian wave packet limit, we derive the transition rates governing the hopping process at a simple avoided level crossing. In this derivation, which gives insight into the physics underlying the hopping process, some essential features of the standard TSH algorithm are retrieved, namely (i) non-zero electronic transition rate ("hopping probability") at avoided crossings; (ii) rescaling of the nuclear velocities to conserve total energy; (iii) electronic transition rates linear in the nonadiabatic coupling vectors. The well-known Landau-Zener model is then used for illustration. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770280]

Femtosecond real-time probing of reactions. 12. Vectorial dynamics of transition states

Femtosecond time-resolved techniques with KETOF (kinetic energy time-of-flight) detection in a molecular beam are developed for studies of the vectorial dynamics of transition states. Application to the dissociation reaction of IHgI is presented. For this system, the complex [I---Hg---I](++)* is unstable and, through the symmetric and asymmetric stretch motions, yields different product fragments: [I---Hg---I](++)* -> HgI(X^2/sigma^+) + I(^2P_3/2) [or I*(^2P_l/2)] (1a); [I---Hg---I](++)* -> Hg(^1S_0) + I(^2P_3/2) + I(^2P_3/2) [or I* (^2P_1/2)] (1 b). These two channels, (1a) and (1b), lead to different kinetic energy distributions in the products. It is shown that the motion of the wave packet in the transition-state region can be observed by MPI mass detection; the transient time ranges from 120 to 300 fs depending on the available energy. With polarized pulses, the vectorial properties (transition moments alignment relative to recoil direction) are studied for fragment separations on the femtosecond time scale. The results indicate the nature of the structure (symmetry properties) and the correlation to final products. For 311-nm excitation, no evidence of crossing between the I and I* potentials is found at the internuclear separations studied. (Results for 287-nm excitation are also presented.) Molecular dynamics simulations and studies by laser-induced fluorescence support these findings.

Time-resolved studies of neutral and ionized Na_n clusters with femtosecond light pulses

The real-time dynamics of Na_n (n=3-21) cluster multiphoton ionization and fragmentation has been studied in beam experiments applying femtosecond pump-probe techniques in combination with ion and electron spectroscopy. Three dimensional wave packet motions in the trimer Na_3 ground state X and excited state B have been observed. We report the first study of cluster properties (energy, bandwidth and lifetime of intermediate resonances Na_n^*) with femtosecond laser pulses. The observation of four absorption resonances for the cluster Na_8 with different energy widths and different decay patterns is more difficult to interpret by surface plasmon like resonances than by molecular structure and dynamics. Timeresolved fragmentation of cluster ions Na_n^+ indicates that direct photo-induced fragmentation processes are more important at short times than the statistical unimolecular decay.

Femtosecond time-resolved molecular multiphoton ionization: the Na_2 system

We report here the first experimental study of femtosecond time-resolved molecular multiphoton ionization. Femtosecond pump-probe techniques are combined with time-of-flight spectroscopy to measure transient ionization spectra of Na_2 in a molecular-beam experiment. The wave-packet motions in different molecular potentials show that incoherent contributions from direct photoionization of a singly excited state and from excitation and autoionization of a bound doubly excited molecular state determine the observed transient ionization signal.

Femtosecond dynamics of molecular and cluster ionization and fragmentation

The real-time dynamics of molecular (Na_2 . Na_3) and cluster Na_n (n=4-2l) multiphoton ionization and -fragmentation has been studied in beam experiments applying femtosecond pump-probe techniques in combination with ion and electron spectroscopy. Wave packet motion in the dimer Na_2 reveals two independent multiphoton ionization processes while the higher dimensional motion in the trimer Na_3 reflects the chaotic vibrational motion in this floppy system. The first studies of cluster properties (energy, bandwidth and lifetime of intermediate resonances Na^*_n) ) with femtosecond laser pulses give a striking illustration of the transition from "molecule-like" excitations to "surfaceplasma"-like resonances for increasing cluster sizes. Time-resolved fragmentation of cluster ions Na_n^* indicate that direct photo-induced fragmentation processes are more important at short times than the statistical unimolecular decay.

Fundamental interactions of molecules (Na_2, Na_3) with intense femtosecond laser pulses

The real-time dynamics of multiphoton ionization and fragmentation of molecules Na_2 and Na_3 has been studied in molecular beam experiments employing ion and electron spectroscopy together with femtosecond pump-probe techniques. Experiments with Na_2 and Na_3 reveal unexpected features of the dynamics of the absorption of several photons as seen in the one- and three-dimensional vibrational wave packet motion in different potential surfaces and in high laser fields: In Na_2 a second major resonance-enhanced multiphoton ionization (REMPI) process is observed, involving the excitation of two electrons and subsequent electronic autoionization. The possibility of controlling a reaction by controlling the duration of propagation of a wave packet on an electronically-excited surface is demonstrated. In high laser fields, the contributions from direct photoionization and from the second REMPI process to the total ion yield change, due to different populations in the electronic states participating in the multiphoton ionization (MPI) processes. In addition, a vibrational wave packet motion in the electronic ground state is induced through stimulated emission pumping by the pump laser. The 4^1 \summe^+_g shelf state of Na_2 is given as an example for performing frequency spectroscopy of highlying electronic states in the time domain. Pure wave packet effects, such as the spreading and the revival of a vibrational wave packet, are investigated. The three-dimensional wave packet motion in the Na_3 reflects the normal modes in the X and B states, and shows in addition the pseudorotational motion in the B state in real time.

Femtosecond dynamics of molecules and clusters

The real-time dynamics of multiphoton ionization and fragmentation of molecules - Na_2 , Na_3 - and clusters - Na_n, Hg_n - has been studied in molecular beam experiments employing ion and electron spectroscopy together with femtosecond pump-probe techniques. Experiments with Na_2 and Na_3 reveal unexpected features of the dynamics of the absorption of several photons as seen in the one- and three dimensional vibrational wave packet motion in different potential surfaces and in high laser fields. Cluster size dependent studies of physical properties such as absorption resonances, lifetimes and decay channels have been performed using tunable femtosecond light pulses in resonance enhanced multiphoton ionization (REMPI) of the cluster size under investigation. This method failed in ns-laser experiments due to the ultrafast decay of the studied cluster. For Na_n, cluster we find that for cluster sizes n \le 21 molecular excitations and properties prevail over collective excitations of plasmon-like resonances. In the case of Hg_n cluster prompt formation of singly and doubly charged cluster are observed up to n \approx 60. The transient multiphoton ionization spectra show a 'short' time wave packet dynamics, which is identical for singly and doubly charged mercury clusters while the 'long' time fragmentation dynamics is different.

The formation mechanism of the Bonin high in August

The Bonin high is a subtropical anticyclone that is predominant near Japan in the summer. This anticyclone is associated with an equivalent-barotropic structure, often extending throughout the entire troposphere. Although the equivalent-barotropic structure of the Bonin high has been known for years among synopticians because of its importance to the summer climate in east Asia, there are few dynamical explanations for such a structure. The present paper attempts to provide a formation mechanism for the deep ridge near Japan. We propose a new hypothesis that this equivalent-barotropic ridge near Japan is formed as a result of the propagation of stationary Rossby waves along the Asian jet in the upper troposphere (‘the Silk Road pattern’). First, the monthly mean climatology is examined in order to demonstrate this hypothesis. It is shown that the enhanced Asian jet in August is favourable for the propagation of stationary Rossby waves and that the regions of descent over the eastern Mediterranean Sea and the Aral Sea act as two major wave sources. Second, a primitive-equation model is used to simulate the climatology of August. The model successfully simulates the Bonin high with an equivalent-barotropic structure. The upper-tropospheric ridge is found to be enhanced by a height anomaly of more than 80 m at 200 hPa, when a wave packet arrives. Sensitivity experiments are conducted to show that the removal of the diabatic cooling over the Asian jet suppresses the Silk Road pattern and formation of an equivalent-barotropic ridge near Japan, while the removal of the diabatic heating in the western Pacific does not. Copyright © 2003 Royal Meteorological Society

Baroclinic stationary waves in aquaplanet models

An aquaplanet model is used to study the nature of the highly persistent low-frequency waves that have been observed in models forced by zonally symmetric boundary conditions. Using the Hayashi spectral analysis of the extratropical waves, the authors find that a quasi-stationary wave 5 belongs to a wave packet obeying a well-defined dispersion relation with eastward group velocity. The components of the dispersion relation with k ≥ 5 baroclinically convert eddy available potential energy into eddy kinetic energy, whereas those with k < 5 are baroclinically neutral. In agreement with Green’s model of baroclinic instability, wave 5 is weakly unstable, and the inverse energy cascade, which had been previously proposed as a main forcing for this type of wave, only acts as a positive feedback on its predominantly baroclinic energetics. The quasi-stationary wave is reinforced by a phase lock to an analogous pattern in the tropical convection, which provides further amplification to the wave. It is also found that the Pedlosky bounds on the phase speed of unstable waves provide guidance in explaining the latitudinal structure of the energy conversion, which is shown to be more enhanced where the zonal westerly surface wind is weaker. The wave’s energy is then trapped in the waveguide created by the upper tropospheric jet stream. In agreement with Green’s theory, as the equator-to-pole SST difference is reduced, the stationary marginally stable component shifts toward higher wavenumbers, while wave 5 becomes neutral and westward propagating. Some properties of the aquaplanet quasi-stationary waves are found to be in interesting agreement with a low frequency wave observed by Salby during December–February in the Southern Hemisphere so that this perspective on low frequency variability, apart from its value in terms of basic geophysical fluid dynamics, might be of specific interest for studying the earth’s atmosphere.