Showing papers on "Free electron model published in 2006"

Field emission tip as a nanometer source of free electron femtosecond pulses.

Abstract: We report a source of free electron pulses based on a field emission tip irradiated by a low-power femtosecond laser. The electron pulses are shorter than 70 fs and originate from a tip with an emission area diameter down to 2 nm. Depending on the operating regime we observe either photofield emission or optical field emission with up to 200 electrons per pulse at a repetition rate of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator, could serve as an efficient source for time-resolved electron interferometry, for time-resolved nanometric imaging and for synchrotrons.

Size dependence of refractive index of gold nanoparticles

Abstract: The extinction spectra of spherical gold nanoparticles suspended in a homogeneous media were measured and the results were adjusted with Mie's theory together with an appropriate modification of the optical properties of bulk material considering the limitation that introduces the size of nanoparticles on the dielectric function. Usually, the contribution of free electrons to the dielectric function is modified for particle size, while the contribution of bound electrons is assumed to be independent of size. This work discusses the separated contribution of free and bound electrons on the optical properties of particles and their variation with size for gold nanoparticles. The effects of dielectric function and its changes with size on extinction spectra near plasmon resonance are considered. The damping constant for free electrons was changed with size as usual and a scattering constant of C = 0.8 was used. For the bound electron contribution, two different models were analysed to fit the extinction spectra: on the one hand, the damping constant for interband transitions and the gap energy were used as fitting parameters and on the other, the electronic density of states in the conduction band was made size-dependent. For the first model, extinction spectra corresponding to particles with radius R = 0.7 nm were fitted using two sets of values of the energy gap and damping constant: Eg = 2.3 eV and or Eg = 2.1 eV and . For the second model, a simple assumption for the electronic density of states and its contribution to the dielectric function in terms of size allowed to adjust extinction spectra for all samples explored (from 0.3 to 1.6 nm radius). This last model uses only one parameter, a scale factor R0 = 0.35 nm, that controls the contribution of the bound electrons in nanoparticles. Contrast between the maximum and the minimum in the extinction spectra near the resonance at 520 nm or alternatively the broadening of the plasmon band can be used to determine the size of gold nanoparticles with radius smaller than 2 nm.

Free-electron generation in laser-irradiated dielectrics

Abstract: We study the mechanisms of ultrafast free-electron generation in laser-irradiated dielectrics. The evolution of the free-electron density in the conduction band of laser-irradiated dielectrics is calculated with the recently introduced multiple rate equation. This system of rate equations unifies key points of detailed kinetic approaches and simple rate equations to a widely applicable description, valid on a broad range of time scales. It keeps track of the nonstationary electron energy distribution at the initial stage of ionization and provides the transition to the asymptotic avalanche regime at longer time scales. The analytic solution for the asymptotic regime yields the avalanche parameter entering the standard rate equation and the condition of its applicability. We present results on the establishment of an ionization avalanche, comparing our model with other theoretical approaches. The role of impact ionization as compared to multiphoton ionization is analyzed. A self-similarity of the fraction of impact-ionized electrons, depending only on the product of intensity and pulse duration, is revealed.

Generation of traveling surface plasmon waves by free-electron impact.

Abstract: The injection of a beam of free 50 keV electrons into an unstructured gold surface creates a highly localized source of traveling surface plasmons with spectra centered below the surface plasmon resonance frequency. The plasmons were detected by a controlled decoupling into light with a grating at a distance from the excitation point. The dominant contribution to the plasmon generation appears to come from the recombination of d-band holes created by the electron beam excitation.

Nonlinear terahertz response of n-type GaAs

Abstract: Excitation of an $n$-type GaAs layer by intense ultrashort terahertz pulses causes coherent emission at 2 THz. Phase-resolved nonlinear propagation experiments show a picosecond decay of the emitted field, despite the ultrafast carrier-carrier scattering at a sample temperature of 300 K. While the linear THz response is in agreement with the Drude response of free electrons, the nonlinear response is dominated by the super-radiant decay of optically inverted impurity transitions. A quantum mechanical discrete state model using the potential of the disordered impurities accounts for all experimental observations.

Analysis of Smith-Purcell free-electron lasers

Abstract: We present an analysis of the beam dynamics in a Smith-Purcell free-electron laser (FEL). In this system, an electron beam interacts resonantly with a copropagating surface electromagnetic mode near the grating surface. The surface mode arises as a singularity in the frequency dependence of the reflection matrix. Since the surface mode is confined very close to the grating surface, the interaction is significant only if the electrons are moving very close to the grating surface. The group velocity of the surface mode resonantly interacting with a low-energy electron beam is in the direction opposite to the electron beam. The Smith-Purcell FEL is therefore a backward wave oscillator in which, if the beam current exceeds a certain threshold known as start current, the optical intensity grows to saturation even if no mirrors are employed for feedback. We derive the coupled Maxwell-Lorentz equations for describing the interaction between the surface mode and the electron beam, starting from the slowly varying approximation and the singularity in the reflection matrix. In the linear regime, we derive an analytic expression for the start current and calculate the growth rate of optical power in time. The analysis is extended to the nonlinear regime by performing a one-dimensional time-dependent numerical simulation. Results of our numerical calculation compare well with the analytic calculation in the linear regime and show saturation behavior in the nonlinear regime. We find that a significant amount of power grows in the surface mode due to this interaction. Several ways to outcouple this power to freely propagating modes are discussed.

Nonlinear propagation of ion-acoustic waves in electron-positron-ion plasma with trapped electrons

Abstract: A theoretical investigation has been made for ion-acoustic waves in an unmagnetized electron-positron-ion plasma. A more realistic situation in which plasma consists of a negatively charged ion fluid, free positrons, and trapped as well as free electrons is considered. The properties of stationary structures are studied by the reductive perturbation method, which is valid for small but finite amplitude limit, and by pseudopotential approach, which is valid for large amplitude. With an appropriate modified form of the electron number density, two new equations for the ion dynamics have been found. When deviations from isothermality are finite, the modified Korteweg-deVries equation has been found, and for the case that deviations from isothermality are small, calculations lead to a generalized Korteweg-deVries equation. It is shown from both weakly and highly nonlinear analysis that the presence of the positrons may allow solitary waves to exist. It is found that the effect of the positron density changes the maximum value of the amplitude and M (Mach number) for which solitary waves can exist. The present theory is applicable to analyze arbitrary amplitude ion-acoustic waves associated with positrons which may occur in space plasma.

Effects of geometry and impurities on quantum rings in magnetic fields

Abstract: We investigate the effects of impurities and changing ring geometry on the energetics of quantum rings under different magnetic field strengths. We show that as the magnetic field and/or the electron number are/is increased, both the quasiperiodic Aharonov-Bohm oscillations and various magnetic phases become insensitive to whether the ring is circular or square in shape. This is in qualitative agreement with experiments. However, we also find that the Aharonov-Bohm oscillation can be greatly phase shifted by only a few impurities and can be completely obliterated by a high level of impurity density. In the many-electron calculations we use a recently developed fourth-order imaginary time projection algorithm that can exactly compute the density matrix of a free electron in a uniform magnetic field.

Free-electron generation in laser-irradiated dielectrics

Abstract: We study the mechanisms of ultrafast free-electron generation in laser-irradiated dielectrics The transient free-electron density in laser-irradiated dielectrics is calculated with a widely applicable new model, the multiple rate equation The system of simple rate equations keeps track of the nonstationary electron energy distribution We clarify the role of different ionization processes in dependence on laser pulse duration and intensity

Universal behavior of nearly free electron states in carbon nanotubes.

Abstract: The nearly free electron state of a carbon nanotube drops rapidly in energy relative to the other conduction bands under alkali doping. A natural (and previously proposed) explanation for this rapid downshift is hybridization with the potassium states. However, we show that the downshift occurs even when the extra electrons are compensated by a uniform positive background, wherein there can be no hybridization, since there are no alkali atoms. Instead, the motion of the nearly free band arises from a universal electrostatic mechanism, which applies for any type of positive countercharge independent of tube diaf/meter and helicity. The nearly free electron state, being weakly bound to the tube wall, is extraordinarily labile and deforms onto the countercharge, whereas the remaining pi* conduction band states are held to the surface of the carbon sheet by the strong carbon potential.

Lifetimes of C602− and C702− dianions in a storage ring

Abstract: C602− and C702− dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C702− ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C602− ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200±30meV above the energy of the monoanion plus a free electron an...

Thomson scattering from near-solid density plasmas using soft x-ray free electron lasers

Abstract: We propose a collective Thomson scattering experiment at the VUV free electron laser facility at DESY (FLASH) which aims to diagnose warm dense matter at near-solid density. The plasma region of interest marks the transition from an ideal plasma to a correlated and degenerate many-particle system and is of current interest, e.g. in ICF experiments or laboratory astrophysics. Plasma diagnostic of such plasmas is a longstanding issue. The collective electron plasma mode (plasmon) is revealed in a pump-probe scattering experiment using the high-brilliant radiation to probe the plasma. The distinctive scattering features allow to infer basic plasma properties. For plasmas in thermal equilibrium the electron density and temperature is determined from scattering off the plasmon mode.

Electron-correlation effects in a disordered Fe2TiSn Heusler alloy

Abstract: Recent electrical transport, specific heat and magnetic measurements indicated that the Heusler-type Fe2TiSn alloy could be a candidate for a 3d heavy-fermion system with a quasi-particle effective mass of ~40 times the free electron mass. Ab initio electronic structure calculations yield a nonmagnetic ground state with a pseudogap and a very small number of the density of states at the Fermi level. However, the atomic disorder strongly influences the magnetic and transport properties of Fe2TiSn samples. In contrast to the thermodynamical properties of Fe2TiSn, the infrared studies do not support the notion that the Kondo interaction plays a dominant role in this alloy and rather support the interband transition across a pseudogap responsible for the mass enhancement. The many-body calculations, however, have shown that the narrow d band resulting from Fe/Ti site exchange (i.e. Fe impurity atoms) can be responsible for the unusual temperature dependences of the physical properties of the Fe2TiSn alloy. In this review paper we discuss the implications of our findings for the ground-state properties of Fe2TiSn, in particular with respect to the role of atomic disorder.

Formation of electron bunches for harmonic cascade x-ray free electron lasers

Abstract: The operation of x-ray free electron lasers (FELs) relies on extremely high quality electron beams. Two FEL projects employing the technique of self-amplified spontaneous emission define the state-of-the-art situation: peak current of few kiloamperes, emittance of 1 mm-mrad or less, and an energy spread of 1 MeV or less [1,2]. Creation of electron bunches with these parameters is a difficult and elaborate process consisting of the electron bunch production, acceleration, and compression. A significant understanding was gained in the underlying physics over the past decade [3‐7]. The main phenomena affecting electron bunches includes space charge effects, wakefields, and coherent synchrotron radiation (CSR). Nonlinearity of the waveform of the accelerating field in the linac and nonlinear time-of-flight characteristics of bunch compressors also play an important role. More demanding for the electron beam quality are FELs that are designed to generate temporally coherent x-rays. These FELs, called high-gain harmonic generation FELs or harmonic cascade FELs (HC FELs) [8‐10], employ a laser to seed the radiation at a lower harmonic of the output FEL radiation. Very often several FEL cascades are used to obtain the radiation at the x-ray wavelength. In these cases, the radiation produced in one cascade by one group of electrons proceeds ahead and interacts with other electrons from the same electron bunch in the next cascade. Thus, relatively long electron bunches are needed to accommodate this technique. It is important to have a constant peak current (i.e. a flat electron density distribution) over the entire bunch length, and a discussion of the means to obtain this is one of the objectives of this paper. We also propose a technique aiming for control of the peak current spikes at the edges of the electron bunch. Those spikes often occur after the final bunch compression and are capable of inducing unwanted wakefields and coherent synchrotron radiation. It is largely anticipated that HC FELs will be used for the production of relatively long temporally coherent x-ray pulses with a narrow bandwidth. However, a nonlinear energy modulation of electrons in the electron bunch can cause a frequency chirp in the output signal [11,12] which broadens the bandwidth significantly beyond the Fourier transform limit. 1 The rf harmonic linearizer [13] often helps to remove major nonlinear components in the energy modulation of electrons, but, frequently, even the remaining modulation significantly broadens the bandwidth, in particular, when strong linac structural wakefields are present. In this paper we propose a complementary technique which employs a specially shaped distribution of electron density and the wakefields themselves in order to avoid the remaining energy modulation of electrons. We also give a simple recipe on how to find such a distribution and demonstrate its usefulness with a practical example. The overall goal for a design of the electron beam delivery system responsible for the formation of the electron bunches for HC FEL is to obtain a so-called flat-flat distribution, i.e., flat in the peak current and flat in the energy, which means that there are no peak current and energy modulations of electrons along the electron bunch. The technique discussed in this paper allows achievement of this goal on a macroscale comparable to the bunch length, but does not deal with the peak current and energy fluctuations on a microscale of few tens of microns often caused by the microbunching instability. This problem has already been addressed in the literature (see [4,14] and references therein).

Change of the 7Be electron capture half-life in metallic environments

Abstract: For the electron capture of 7Be in the metallic environments Pd and In the 7Be half-life was observed to increase by 0.9±0.2 and 0.7±0.2%, respectively, while in the insulator Li2O it was unchanged within experimental error (all samples cooled to T = 12K). The observations are consistent with the predictions of the Debye plasma model applied to the quasi-free electrons in the metals.

Electron release in the afterglow of a pulsed inductively-coupled radiofrequency oxygen plasma

Abstract: A pulsed inductively-coupled radiofrequency plasma in oxygen is investigated by means of time-resolved microwave interferometry in a wide pressure range from 0.5 to 200 Pa. In the afterglow a peak of the electron density is observed. The effect is maximum for pressures around 50 Pa. The time-resolved measurements of the electron density are interpreted in the framework of a fluid model. This model points out the significance of negative ions. The overall electron density is comparatively small. Attachment and detachment processes nearly balance during the power-on phase. But when the power is switched off, the electron temperature drops very quickly. This means that the production of new negative ions is inhibited so that the negative ions are destroyed by collisions. These reactions quickly set free electrons in the afterglow and are the reason for the observed peak in the electron density after switching off the power.

Concentration dependence of carrier localization in InN epilayers

Abstract: The authors studied the concentration dependence of carrier localization in InN epilayers using time-resolved photoluminescence (PL). Based on the emission-energy dependence of the PL decays and the PL quenching in thermalization, the localization energy of carriers in InN is found to increase with carrier concentration. The dependence of carrier concentration on the localization energy of carriers in InN can be explained by a model based on the transition between free electrons in the conduction band and localized holes in the deeper tail states. They suggest that carrier localization originates from the potential fluctuations of randomly located impurities.

EOSTA—an improved EOS quantum mechanical model in the STA opacity code

Abstract: The STA model is extended to include calculations of thermodynamical quantities required for equation of state (EOS). For that purpose the plasma free electrons are now treated quantum mechanically accounting for shape resonances. The resulting gradual orbital ionization assures a regular behavior of all the thermodynamical quantities vs. density and temperature. The relativistic quantum mechanical framework that we have applied in a new code named EOSTA follows Liberman's Inferno model with several improvements that accomplish higher accuracy. These improvements include: a numerical technique to trace all the resonances and follow their detailed structure and application of the phase amplitude method that allows the inclusion of higher angular momenta partial waves and higher energies of the free orbitals. In addition we employ two complementary methods to treat the exchange potential in the calculation of orbital wave functions: (1) for EOS calculations Local Density Approximation is used and (2) for ionization lowering and orbital energies required in the opacity calculations we have found two satisfactory alternatives: (a) the optimized effective potential (OEP) and (b) first order corrected Local Density Approximation. In both alternatives the resulting orbitals are used to calculate the detailed exchange term that includes a proper reduction of the self energy. A new approach for calculating the electronic pressure is presented. The relativistic virial theorem expresses the pressure as a sum of the total energy and a local density term. This form allows consistent calibration of the correlation energy to comply with the periodic table zero pressure density points and higher density pressures. Results are presented describing the various thermodynamical quantities vs. density and temperature in comparison with other calculations and experiments.

Positronium formation in collisions of fast positrons impacting on vapour water molecules

Abstract: Positronium formation through electron capture by fast positrons impinging on vapour water molecules is studied theoretically at intermediate and high impact energies. This multi-electron system is treated within the framework of the independent electron model. The charge transfer process is described by employing the continuum distorted-wave final-state approximation. In this model, the final state of the collision is distorted by two Coulomb wavefunctions associated with the interactions of both the positron and the active electron (the captured one) with the residual ionic target. The water molecule is described by an expansion of monocentric Slater-like functions centred on the oxygen atom. Total cross sections are computed by using a partial-wave technique and compared with results obtained using the simpler Coulomb– Born approximation. Theoretical results for the production of H2O + ions in the final channel of the reaction through charge transfer processes (positronium formation and ionization) are also presented.

Observation of quantum interference effect in solids

Abstract: In order to achieve quantum interference of free electrons inside a solid, we have modified the geometry of the solid so that de Broglie waves interfere destructively inside the solid. Quantum interference of de Broglie waves leads to a reduction in the density of possible quantum states of electrons inside the solid and increases the Fermi energy level. This effect was studied theoretically within the limit of the quantum theory of free electrons inside the metal. It has been shown that if a metal surface is modified with patterned indents, the Fermi energy level will increase and consequently the electron work function will decrease. This effect was studied experimentally in both Au and SiO2 thin films of special geometry and structure. Work function reductions of 0.5eV in Au films and 0.2eV in SiO2 films were observed. Comparative measurements of work function were made using the Kelvin probe method based on compensation of internal contact potential difference. Electron emission from the same thin fil...

The atomic and electron structure of ZrO 2

Abstract: The atomic structure of amorphous and crystalline zirconium dioxide (ZrO2) films is studied using X-ray diffraction and extended X-ray absorption fine structure techniques. The electron structure of ZrO2 is experimentally determined using X-ray and UV photoelectron spectroscopy, and the electron energy band structure is theoretically calculated using electron density functional method. According to these data, the valence band of ZrO2 consists of three subbands separated by an ionic gap. The upper subband is formed by the O2p states and Zr4d states; the medium subband is formed by the O2s states; and the narrow lower subband is formed predominantly by the Zr4p states. The bandgap width in amorphous ZrO2, as determined using the electron energy loss spectroscopy data, amounts to 4.7 eV. The electron band structure calculations performed for a cubic ZrO2 phase point to the existence of both light (0.3m 0) and heavy (3.5m 0) holes, where m 0 is the free electron mass. The effective masses of band electrons in ZrO2 fall within (0.6–2.0)m 0.

Electron transfer from sodium to oriented nitromethane, CH3NO2: probing the spatial extent of unoccupied orbitals.

Abstract: Beams of sodium atoms with energies of a few eV are crossed with a beam of oriented CH3NO2 molecules to study the effect of collision energy and orientation on electron transfer. The electron transfer produces Na+ ions and free electrons, parent negative ions (CH3NO2-), and fragmentation ions NO2- and O- in proportions that depend on the collision energy. The steric asymmetry is very small or zero and suggests that production of all of the ions is favored by sideways attack with respect to the permanent dipole along the C−N axis. In these experiments, the electron appears to be transferred into the 2B1 state of the anion comprising mainly the π*NO LUMO, producing a valence-bound state rather than a dipole-bound state.

Quantum mechanical model for the study of pressure ionization in the superconfiguration approach

Abstract: The knowledge of plasma equation of state and photoabsorption requires suitable and realistic models for the description of ions. The number of relevant electronic configurations of ions in hot dense plasmas can be immense (increasing with atomic number Z). In such cases, calculations relying on the superconfiguration approximation appear to be among the best statistical approaches to photoabsorption in plasmas. The superconfiguration approximation enables one to perform rapid calculation of averages over all possible configurations representing excited states of bound electrons. We present a thermodynamically consistent model involving detailed screened ions (described by superconfigurations) in plasmas. The density effects are introduced via the ion-sphere model. In the usual approaches, bound electrons are treated quantum mechanically while free electrons are described within the framework of semi-classical Thomas–Fermi theory. Such a hybrid treatment can lead to discontinuities in the thermodynamic quantities when pressure ionization occurs. We propose a model in which all electrons (bound and free) are treated quantum mechanically. Furthermore, resonances are carefully taken into account in the self-consistent calculation of the electronic structure of each superconfiguration. The model provides the contribution of electrons to the main thermodynamic quantities, together with a treatment of pressure ionization, and gives a better insight into the electronic properties of hot dense plasmas.

Zitterbewegung of nearly-free and tightly bound electrons in solids

Abstract: We show theoretically that nonrelativistic nearly-free electrons in solids should experience a trembling motion (Zitterbewegung, ZB) in absence of external fields, similarly to relativistic electrons in vacuum. The Zitterbewegung is directly related to the influence of periodic potential on the free electron motion. The frequency of ZB is $\omega\approx E_g/\hbar$, where $E_g$ is the energy gap. The amplitude of ZB is determined by the strength of periodic potential and the lattice period and it can be of the order of nanometers. We show that the amplitude of ZB does not depend much on the width of the wave packet representing an electron in real space. An analogue of the Foldy-Wouthuysen transformation, known from relativistic quantum mechanics, is introduced in order to decouple electron states in various bands. We demonstrate that, after the bands are decoupled, electrons should be treated as particles of a finite size. In contrast to nearly-free electrons we consider a two-band model of tightly bound electrons. We show that also in this case the electrons should experience the trembling motion. It is concluded that the phenomenon of Zitterbewegung of electrons in crystalline solids is a rule rather than an exception.

Plasmons in isolated single-walled carbon nanotubes

Abstract: The collective excitation modes of individual single-walled carbon nanotubes are studied theoretically in the nearest-neighbour tight-binding approximation. Umklapp terms are taken into account and local-field effects are studied. Plasmon mode dispersion relations for carbon nanotubes with radius R~7 A are obtained. We report a strong dependence of the plasmon dispersion on the chirality of the carbon nanotubes. In particular, armchair and zigzag tubes have dispersive plasmons, but some of the chiral tubes have essentially dispersionless plasmons. Our results offer an alternative explanation to the origin of the experimentally observed low-energy dispersionless modes found in momentum-dependent EELS experiments. Important differences between a tight-binding model and free electron gas model are discussed and it is shown that some important qualitative features cannot be captured by a free electron gas model. An acoustic plasmon mode is found in all metallic carbon nanotubes, giving support to the experimental findings in Raman scattering.

Thermal Processes using Attosecond Laser Pulses : When time matters

Abstract: Introduction.- Wave phenomena: an overview.- Hyperbolic partial differential equations and wave phenomena.- The discrete Boltzmann equation.- The existence of a solution for a hyperbolic equation.- Causal thermal phenomena, classical description.- Fundamentals of the rapid thermal processes.- The thermal wave as the solution of HHT.- Causal thermal phenomena, quantal description.- Discretization of the thermal excitation in high excited matter.- Klein-Gordon thermal equation.- Application of the quantum heat transport equation.- The Pauli-Heisenberg model.- Ballistic and diffusion heat transport.- Causal thermal phenomena in a Planck Era.- The time arrow in a Planck gas. -Klein-Gordon thermal equation for a Planck gas. -Attophysics and technology with ultra-short laser pulses. -Quantum heat transport: from basics to applications.- Looking into the nanoworld. -Faster, brighter, shorter. - Hyperbolic heat transport induced by zeptosecond laser pulse.- Possible thermal waves generation by femtosecond TESLA free electron lasers (FEL).- New laser facility LUX - Linac based ultra-fast X-ray laser facility.- Fundamental physics with attosecond laser pulses. -Precision measurements of the fundamental constant of nature.- The life of the universe.- Epilogue. The emergence of quantum dynamics in a classical world.- Key questions.- Schroedinger-Newton wave mechanics. The model.- Appendices.