Three-photon detachment of electrons from the fluorine negative ion

Absolute three-photon detachment cross sections are calculated for the fluorine negative ion within the lowest-order perturbation theory. The Dyson equation of the atomic many-body theory is used to obtain the ground-state 2p wavefunction with correct asymptotic behaviour, corresponding to the true (experimental) binding energy. We show that in accordance with the adiabatic theory this is crucial for obtaining absolute values of the multiphoton cross sections. Comparisons with other calculations and experimental data are presented.

Infrared emission from tunneling electrons: The end of the rainbow in scanning tunneling microscopy

Electromagnetic radiation originating with localized surface plasmons in the metal-tip/metal-sample nanocavity of a scanning tunneling microscope is demonstrated to extend to a wavelength lambda of at least 1.7 mu m. Progressive spectral extension beyond lambda similar to 1.0 mu m occurs for increasing tip radius above similar to 15 nm, reaching lambda similar to 1.7 mu m for tip radius similar to 100 nm; these observations are corroborated by use of a simple physical model that relates the discrete plasmon mode frequencies to the tip radius. This spectral extension opens up a new regime for scanning tunneling microscope-based optical spectroscopy.

Hot Electrons Transverse Refluxing in Ultraintense Laser-Solid Interactions

We have analyzed the coupling of ultraintense lasers (at similar to 2 X 1019 W/cm(2)) with solid foils of limited transverse extent (similar to 10 s of mu m) by monitoring the electrons and ions emitted from the target. We observe that reducing the target surface area allows electrons at the target surface to be reflected from the target edges during or shortly after the laser pulse. This transverse refluxing can maintain a hotter, denser and more homogeneous electron sheath around the target for a longer time. Consequently, when transverse refluxing takes places within the acceleration time of associated ions, we observe increased maximum proton energies (up to threefold), increased laser-to-ion conversion efficiency (up to a factor 30), and reduced divergence which bodes well for a number of applications.

Modulated dust-acoustic wave packets in a plasma with non-isothermal electrons and ions

The Nonlinear self-modulation of dust acoustic waves is studied in the presence of non-thermal (non-Maxwellian) ion and electron populations. By employing a multiple scale technique, a nonlinear Schrodinger-type equation (NLSE) is derived for the wave amplitude. The influence of non-thermality, in addition to obliqueness (between the propagation and modulation directions), on the conditions for modulational instability to occur is discussed. Different types of localized solutions (envelope excitations) which may possibly occur are discussed, and the dependence of their characteristics oil physical parameters is traced. The ion deviation from a Maxwellian distribution comes out to be more important than the electron analogous deviation alone. Both yield a de-stabilizing effect oil (the amplitude of) DAWs propagating in a dusty plasma with negative dust grains, and thus favour the formation of bright- (rather than dark-) type envelope structures, (solitons) in the plasma. A similar tendency towards amplitude de-stabilization is found for the ease of the presence of positively charged dust in the plasma.

Arbitrary amplitude ion-acoustic solitary excitations in the presence of excess superthermal electrons

Velocity distribution functions with an excess of superthermal particles are commonly observed in space plasmas, and are effectively modeled by a kappa distribution. They are also found in some laboratory experiments. In this paper we obtain existence conditions for and some characteristics of ion-acoustic solitary waves in a plasma composed of cold ions and kappa-distributed electrons, where kappa>3/2 represents the spectral index. As is the case for the usual Maxwell-Boltzmann electrons, only positive potential solitons are found, and, as expected, in the limit of large kappa one recovers the usual range of possible soliton Mach numbers, viz., 1 < M < 1.58. For lower values of kappa, modeling the presence of a greater superthermal component, the range of accessible Mach numbers is reduced. It is found that the amplitude of the largest possible solitons that may be generated in a given plasma (corresponding to the highest allowed Mach number for the given plasma composition) falls off with decreasing kappa, i.e., an increasing superthermal component. On the other hand, at fixed Mach number, both soliton amplitude and profile steepness increase as kappa is decreased. These changes are seen to be important particularly for kappa < 4, i.e., when the electrons have a "hard" spectrum.

Oblique electrostatic excitations in a magnetized plasma in the presence of excess superthermal electrons

The nonlinear propagation of ion-acoustic waves is considered in a magnetized plasma, composed of kappa distributed electrons and an inertial ion fluid. The fluid-dynamical system of equations governing the dynamics of ion-acoustic waves is reduced to a pseudoenergy-balance equation. The properties of arbitrary amplitude, obliquely propagating ion-acoustic solitary waves are thus investigated via a mechanical-motion analog (Sagdeev potential) approach. The presence of excess superthermal electrons is shown to influence the nature of magnetized ion-acoustic solitons. The influence on the soliton characteristics of relevant physical parameters such as obliqueness (the angle between soliton propagation direction and magnetic field), the electron deviation from a Maxwellian ("superthermality") and the soliton speed is investigated.

Dust ion acoustic solitons in a plasma with kappa-distributed electrons

Dust ion acoustic solitons in an unmagnetized dusty plasma comprising cold dust particles, adiabatic fluid ions, and electrons satisfying a kappa distribution are investigated using both small amplitude and arbitrary amplitude techniques. Their existence domain is discussed in the parameter space of Mach number M and electron density fraction f over a wide range of values of kappa. For all kappa > 3/2, including the Maxwellian distribution, negative dust supports solitons of both polarities over a range in f. In that region of parameter space solitary structures of finite amplitude can be obtained even at the lowest Mach number, the acoustic speed, for all kappa. These cannot be found from small amplitude theories. This surprising behavior is investigated, and it is shown that f(c), the value of f at which the KdV coefficient A vanishes, plays a critical role. In the presence of positive dust, only positive potential solitons are found. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3400229]

Electrostatic solitary waves in the presence of excess superthermal electrons: modulational instability and envelope soliton modes

The nonlinear dynamics of electrostatic solitary waves in the form of localized modulated wavepackets is investigated from first principles. Electron-acoustic (EA) excitations are considered in a two-electron plasma, via a fluid formulation. The plasma, assumed to be collisionless and uniform (unmagnetized), is composed of two types of electrons (inertial cold electrons and inertialess kappa-distributed superthermal electrons) and stationary ions. By making use of a multiscale perturbation technique, a nonlinear Schrodinger equation is derived for the modulated envelope, relying on which the occurrence of modulational instability (MI) is investigated in detail. Stationary profile localized EA excitations may exist, in the form of bright solitons (envelope pulses) or dark envelopes (voids). The presence of superthermal electrons modifies the conditions for MI to occur, as well as the associated threshold and growth rate. The concentration of superthermal electrons (i.e., the deviation from a Maxwellian electron distribution) may control or even suppress MI. Furthermore, superthermality affects the characteristics of solitary envelope structures, both qualitatively (supporting one or the other type, for different.) and quantitatively, changing their characteristics (width, amplitude). The stability of bright and dark-type nonlinear structures is confirmed by numerical simulations.

Transmission and Trapping of Cold Electrons in Water Ice

Experiments are reported which show that currents of low energy ("cold") electrons pass unattenuated through crystalline ice at 135 K for energies between zero and 650 meV, up to the maximum studied film thickness of 430 bilayers, indicating negligible apparent trapping. By contrast, both porous amorphous ice and compact crystalline ice at 40 K show efficient electron trapping. Ice at intermediate temperatures reveals metastable trapping that decays within a few hundred seconds at 110 K. Our results are the first to demonstrate full transmission of cold electrons in high temperature water ice and the phenomenon of temperature-dependent trapping.

Dry Excess Electrons in Room-Temperature Ionic Liquids

In this article, we describe general trends to be expected at short times when an excess electron is generated or injected in different room-temperature ionic liquids (RTILs). Perhaps surprisingly, the excess electron does not localize systematically on the positively charged cations. Rather, the excess charge localization pattern is determined by the cation and anion HOMO/LUMO gaps and, more importantly, by their relative LUMO alignments. As revealed by experiments, the short-time (ps/ns) transient UV spectrum of excess electrons in RTILs is often characterized by two bands, a broad band at low energies (above 1000 nm) and another weaker band at higher energies (around 400 nm). Our calculations show that the dry or presolvated electron spectrum (fs) also has two similar features. The broad band at low energies is due to transitions between electronic states with similar character on ions of the same class but in different locations of the liquid. The lower-intensity band at higher energies is due to transitions in which the electron is promoted to electronic states of different character, in some cases on counterions. Depending on the chemical nature of the RTIL, and especially on the anions, excess electrons can localize on cations or anions. Our findings hint at possible design strategies for controlling electron localization, where electron transfer or transport across species can be facilitated or blocked depending on the alignment of the electronic levels of the individual species.

Production of ultracollimated bunches of multi-MeV electrons by 35 fs laser pulses propagating in exploding-foil plasmas

Very collimated bunches of high energy electrons have been produced by focusing super-intense femtosecond laser pulses in submillimeter under-dense plasmas. The density of the plasma, preformed with the laser exploding-foil technique, was mapped using Nomarski interferometry. The electron beam was fully characterized: up to 10(9) electrons per shot were accelerated, most of which in a beam of aperture below 10(-3) sterad, with energies up to 40 MeV. These measurements, which are well modeled by three-dimensional numerical simulations, validate a reliable method to generate ultrashort and ultracollimated electron bunches. (C) 2002 American Institute of Physics.

Measurement of DNA damage by electrons with energies between 25 and 4000 eV.

All ionizing radiations deposit energy stochastically along their tracks. The resulting distribution of energies deposited in a small target such as the DNA helix leads to a corresponding spectrum in the severity of damage produced. So far, most information about the probable spectra of DNA lesion complexity has come from Monte Carlo studies which endeavour to model the relationship between the energy deposited in DNA and the damage induced. The aim of this paper is to establish methods of determining this relationship by irradiating pBR322 plasmid DNA using low energy electrons with energies comparable with the minimum energy thought to produce critical damage. The technique of agarose gel electrophoresis has been used to ascertain the fraction of DNA single- and double-strand breaks induced by monoenergetic electrons with energies as low as 25 eV. Our data show that the threshold electron energy for induction of single-strand breaks is

Role of low-energy electrons in Ar emission from low-pressure radio frequency discharge plasma

Optical emission spectra from a low-pressure Ar plasma were studied with high spatial resolution. It has been shown that the intensity ratios of Ar lines excited through metastable levels to those excited directly from the ground state are sensitive to the shape of electron energy distribution function. From these measurements, important information on the spatial variation of plasma parameters can be obtained. (C) 1999 American Institute of Physics. [S0003-6951(99)01629-0].

Electron-scale electrostatic solitary waves and shocks: the role of superthermal electrons

The propagation of electron-acoustic solitary waves and shock structures is investigated in a plasma characterized by a superthermal electron population. A three-component plasma model configuration is employed, consisting of inertial (“cold”) electrons, inertialess ? (kappa) distributed superthermal (“hot”) electrons and stationary ions. A multiscale method is employed, leading to a Korteweg-de Vries (KdV) equation for the electrostatic potential (in the absence of dissipation). Taking into account dissipation, a hybrid Korteweg-de Vries-Burgers (KdVB) equation is derived. Exact negative-potential pulse- and kink-shaped solutions (shocks) are obtained. The relative strength among dispersion, nonlinearity and damping coefficients is discussed. Excitations formed in superthermal plasma (finite ?) are narrower and steeper, compared to the Maxwellian case (infinite ?). A series of numerical simulations confirms that energy initially stored in a solitary pulse which propagates in a stable manner for large ? (Maxwellian plasma) may break down to smaller structures or/and to random oscillations, when it encounters a small-? (nonthermal) region. On the other hand, shock structures used as initial conditions for numerical simulations were shown to be robust, essentially responding to changed in the environment by a simple profile change (in width).