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Showing papers on "Free electron model published in 1994"


Journal ArticleDOI
TL;DR: In this paper, a tunable laser-atomic beam photoelectron source with sub-milli-electron-volt resolution, producing energy-variable free electrons (current 10-12 A) by resonant two-step laser photo-ionization of metastable Ar*(4s 3P2) atoms in a collimated beam, is described.
Abstract: A novel tunable laser-atomic beam photoelectron source with sub-milli-electron-volt resolution, producing energy-variable free electrons (current 10-12 A) by resonant two-step laser photo-ionization of metastable Ar*(4s 3P2) atoms in a collimated beam, is described. Operation and application of the source are demonstrated by a series of laser photoelectron attachment experiments, in which the process e-(E)+SF6 to SF6- is investigated in the electron energy range 0

55 citations


Journal ArticleDOI
TL;DR: In this paper, electron spin resonance (ESR) was applied to microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition in both undoped and residual-gas-doped material.
Abstract: Microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition has been investigated by electron spin resonance (ESR) in the temperature range 10–300 K. In both undoped and residual-gas-doped material, two resonances are observed: a resonance at g = 2.0052 attributed to dangling bonds and a resonance at g= 1.9983 attributed to free conduction electrons. This result has important implications on the band structure of this composite material. It indicates that in some regions of the crystalline phase of the material the Fermi level is close to the conduction band, whereas at the grain boundaries or in the remaining amorphous phase the Fermi level is not high enough to fill dangling-bond states to give the negatively charged D− configuration. At temperatures below T=60 K, we observe a light-induced ESR signal with a considerable enhancement of the free-electron resonance. The decay of the light-induced ESR signal has a fast and a very slow component with recombination times of more ...

51 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate the application of optoelectronically generated terahertz electromagnetic transients to realtime detection of coherent oscillations by free electrons in modulation doped semiconductor heterostructures, due to the cyclotron resonance.
Abstract: We demonstrate the application of optoelectronically generated terahertz electromagnetic transients to real‐time detection of coherent oscillations by free electrons in modulation doped semiconductor heterostructures, due to the cyclotron resonance. The damping of these oscillations gives a direct measure of the electron dephasing which can be compared with scattering rates obtained from dc transport measurements.

42 citations


Journal ArticleDOI
TL;DR: These results link the geometry of adlayer molecules with localization of excess electrons at interfaces with evidence for electron localization in 2D at metal-dielectric interfaces is reported.
Abstract: Evidence for electron localization in 2D at metal-dielectric interfaces is reported. A nondispersive peak, derived from the image-potential state and observed in angle-resolved two-photon photoemission, is assigned to localized excess electrons. The image electron on alkane monolayers is a free electron. Bilayers of chain or ring alkanes show both localized and delocalized states; electrons are localized on trilayer n-pentane. Bilayers and trilayers of neopentane show no localization. Our results link the geometry of adlayer molecules with localization of excess electrons at interfaces.

40 citations


Journal ArticleDOI
TL;DR: Results indicate that a Raman free-electron maser can operate at high power and efficiency with stable single-mode output in the long pulse (equilibrium) regime.
Abstract: Detailed studies of the spectral characteristics and spatial mode structure of a single-mode, high-power Raman free-electron maser (FEM) oscillator operating with a Bragg resonator are reported. The FEM oscillator generated approximately 1 MW of microwave power in a single axial mode at 27.47 GHz, with an electron beam energy of 320 keV and a transmitted current of 30 A, yielding an efficiency of 10.3%, in good agreement with nonlinear simulations. These results indicate that a Raman free-electron maser can operate at high power and efficiency with stable single-mode output in the long pulse (equilibrium) regime.

40 citations


Journal ArticleDOI
TL;DR: In this article, a comparison of measured rate coefficients for Rydberg electron attachment with those calculated from the measured free electron cross sections on the basis of the quasi-free electron model was made.
Abstract: New experimental results on attachment reactions involving free electrons at sub-meV resolution allow for the first time a conclusive comparison of measured rate coefficients for Rydberg electron attachment with those calculated from the measured free electron cross sections on the basis of the quasi-free electron model for Rydberg electron collisions. Using classical velocity distributions for the highn Rydberg electrons and our measured free electron attachment cross sections, we calculate Rydberg electron attachment rate coefficientsknl for the two cases SF6 and HI for Rydberg binding energies |En| of 0.1–40 meV. We find a significant increase inknl towards lower binding energies, especially for HI, which is due to the deviation of the free electron cross section from the limitings-wave behaviour σ0∼E−1/2. The increase at |En|≲2 meV is in qualitative agreement with our highn Rydberg data (n≳80) if l-mixing due to residual electric fields is taken into account. For low l, Rydberg rate coefficientsknl(|En|) are significantly larger than free electron rate coefficientske(E=|En|), while for circular orbits (l=n−1) they agree. On average, attachment reactions of Rydberg electrons in low l orbits proceed with an effective collision energy substantially smaller than the binding energy |En|.

39 citations


Journal ArticleDOI
TL;DR: In this article, the strong removal of free electrons in the interpulse period of copper HyBrID lasers when HBr is added is presented. But the influence of the low number of remanent electrons on various discharge characteristics is ultimately responsible for the higher efficiencies, larger specific average output powers and superior beam characteristics of that type of laser compared with conventional copper lasers.

33 citations


Journal ArticleDOI
TL;DR: An experiment in which the attenuation of a spin-polarized free electron beam is measured by direct transmission through an ultrathin ferromagnetic layer and it is found that the transmission coefficient of minority spin electrons is about 0.7 times that of majority spin electrons.
Abstract: We present an experiment in which the attenuation of a spin-polarized free electron beam is measured by direct transmission through an ultrathin ferromagnetic layer. The self-supported metal target consists of a 1-nm-thick cobalt film sandwiched between 21- and 2-nm-thick gold layers. Measurements are performed throughout a wide energy range (incident electron energies from 4 eV to 50 eV above the Fermi level). At low energy, close to the clean surface vacuum level, we find that the transmission coefficient of minority spin electrons is about 0.7 times that of majority spin electrons.

31 citations


Journal ArticleDOI
TL;DR: In this article, an extended Wannier model is formulated in which the electrons recede from the reaction zone being at equal distances from the nucleus but in arbitrary directions, which accounts properly for the dynamics of the electron angular correlations near the double ionization threshold.
Abstract: In order to account properly for the dynamics of the electron angular correlations near the double ionization threshold the extended Wannier model is formulated in which the electrons recede from the reaction zone being at equal distances from the nucleus but in arbitrary directions. 1Se and 1Po states of the two-electron continuum are considered. The boundary conditions on the border of the reaction zone are imposed corresponding to the double photoionization process. The wavepacket propagation to the free electron regime is calculated numerically. The asymmetry parameter beta is in good agreement with recent experiments.

25 citations


Journal ArticleDOI
TL;DR: These observations give direct evidence of free-electron multiple photon absorption in the conduction band, in keeping with recent Monte Carlo simulation results.
Abstract: We measure the kinetic energy of photoelectrons emitted from an $\ensuremath{\alpha}$-quartz surface irradiated by short (100 fs) and intense laser pulses. In the high intensity regime (above 100 GW/${\mathrm{cm}}^{2}$), we observe a high-energy tail in the photoemission spectra at 620 and 560 nm. This effect disappears for the doubled frequencies. These observations give direct evidence of free-electron multiple photon absorption in the conduction band, in keeping with recent Monte Carlo simulation results.

19 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the quantum size effects of amorphous silicon/amorphous silicon carbide (a-Si(:H)/a -SiC (:H)) multilayer films prepared by a dual rf magnetron sputtering method by measurements of vacuum ultraviolet photoelectron Spectroscopy (UPS) and optical absorption.
Abstract: Quantum size effects of amorphous silicon/amorphous silicon carbide (a-Si(:H)/a-SiC(:H)) multilayer films prepared by a dual rf magnetron sputtering method are investigated by measurements of Vacuum Ultraviolet Photoelectron Spectroscopy (UPS) and optical absorption. Valence band offsets of a-Si/a-SiC and a-Si:H/a-SiC:H heterojunctions are 0.3 and 0.05 eV, respectively. Optical gaps of a-Si, a-SiC, a-Si:H and a-SiC:H are 1.22, 1.52, 1.87 and 2.20 eV, respectively. In a-Si:H/aSiC:H multilayer film the quantum well layer thickness dependence of the optical gap is found to be in good agreement with a 1-dimensional quantum well model. It is obtained that the effective mass of electron in a-Si:H film is 0.15 mo (mo; the mass of free electron). In a-Si/a-SiC multilayer film quantum size effect does not appear. It can be considered that the effective mass of hole in a-Si film is more than hundred times as large as that of free electron.

Journal ArticleDOI
TL;DR: In this paper, the free electron distribution in δ-doped high electron mobility transistors with pseudomorphic InGaAs wells is calculated self-consistently, and the electron distribution is calculated from the single-particle wave functions obtained from a four-band k⋅p theory.
Abstract: The free electron distribution in δ‐doped high electron mobility transistors with pseudomorphic InGaAs wells is calculated self‐consistently. The electron distribution is calculated from the single‐particle wave functions obtained from a four‐band k⋅p theory. The Hartree part of the Coulomb interaction is obtained from the Poisson equation and the exchange‐correlation part from density functional theory within the local‐density approximation. The calculated energy separations between the electron and hole subbands agree well with observed peak positions in photoluminescence data. In addition, it is found that for spacer layers thicker than about 40 A and a δ‐doping density of about 5×1012 cm−2, the δ layer can form a channel as deep as the p well and can draw electrons from the latter.

Journal ArticleDOI
TL;DR: The commonly assumed quasiequilibrium particle distribution with the same quasi-Fermi-level for all quantum-dot carriers in the same energy (conduction or valence) band is found not to be valid for a wide range of temperatures at the inversion populations and bound energy separations used in the literature.
Abstract: The commonly assumed quasiequilibrium particle distribution with the same quasi-Fermi-level for all quantum-dot carriers in the same energy (conduction or valence) band is found not to be valid for a wide range of temperatures at the inversion populations and bound energy separations (greater than a LO phonon energy) used in the literature. Bound state occupation factors obtained from the steady state solution of rate equations describing the ionization balance in room-temperature 100-\AA{}-radius GaAs quantum dots whose centers are separated by 400 \AA{} are found to be very different from the quasiequilibrium distribution used in an example from the literature. In such quantum dots, bound state transitions result from collisions between charged particles via the Coulomb interaction, and from interband and intraband radiative processes. The critical free electron concentration above which collisional processes can establish a quasiequilibrium in the conduction band is found to exceed ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. Our numerical solution is in good agreement with Pitaevskii's model from atomic physics of an electron random walk in energy as modeled by a Fokker-Planck equation. In our simple model, electrons are captured into a bound conduction band state via three-body recombination and phonon emission, and drop into lower energy bound states via a series of collisional deexcitations before combining with a valence band hole. Solution of the rate equations is standard in numerical studies of stimulated emission in atomic plasmas, but our present discussion is, to our knowledge, the first in the literature on semiconductor quantum-dot lasers.

Journal ArticleDOI
TL;DR: In this paper, the linear polarizability and hyperpolarizability of a free electron gas confined to a spherical shell are calculated, and the magnitude of the calculated polarizabilities of C60 and C70 molecules were found to be in reasonable agreement with some of the experiments.

Journal ArticleDOI
TL;DR: In this article, the contribution of surface free electrons to the selective Al CVD using dimethylaluminum hydride (DMAH) and hydrogen has been investigated, and it has been shown that free electrons on the Si surface contribute to the surface reaction.

Journal ArticleDOI
TL;DR: In this article, the tunability of a tapered wiggler free electron laser amplifier operating in the neighborhood of 94 GHz was investigated and it was shown that it is tunable over a reasonably wide range of frequencies by small adjustments in the energy and current of the electron beam.
Abstract: Free electron laser amplifiers are tunable sources under development as high power, high frequency radiation sources for magnetic fusion applications. High efficiencies can be achieved by varying the wiggler field strength and/or the wiggler period. In addition to the requirement of high efficiency, the free electron laser must be tunable for electron cyclotron heating and current drive applications in magnetic fusion devices. Although the tunability of free electron lasers is well established, the tunability of a tapered free electron laser amplifier has not been studied. In this paper we present an investigation of the tunability of a tapered wiggler free electron laser amplifier operating in the neighborhood of 94 GHz. The configuration of the free electron laser is one in which a sheet electron beam propagates through a rectangular waveguide in the presence of a planar wiggler field with tapered period. We found that the tapered free electron laser amplifier is tunable over a reasonably wide range of frequencies by small adjustments in the energy and current of the electron beam.

Journal ArticleDOI
TL;DR: In this article, the doubly differential cross sections for the electron emis- sion occurring in fast strongly asymmetric collisions are presented and a relativistic quantum mechanical model based on the electron impact approximation is given.
Abstract: A systematic study of the doubly differential cross sections for the electron emis- sion occurring in fast strongly asymmetric collisions is presented. Protons and atomic hyd- rogen (0.5 MeV) were collided with the heavy targets krypton and xenon under single collision conditions. The ejected electrons were observed at laboratory angles in the range 0'<8,5180"and with energies E,from 25 to 1400eV. In addition to the familiar strong ejection in the forward direction (0,%0') arising from capture and loss to continuum states there is pronounced emission in the backward hemisphere for the case of atomic hydrogen projectiles. This.is accompanied by large varia- tions in the angular dependent singly differential cross section and in the energy and width of the electron loss peak as measured at particular angles These Rudtuations in the ability of a heavy target to ionize the structured projectile are a manifestation of a Ramsauer- Townsend efect and can be understood within the framework of quasi-free electron scatter- ing by the target potential. We give a relativistic quantum mechanical model based on the electron impact approximation which shows good agreement with experiment for these strongly asymmetric collision systems.



Journal ArticleDOI
TL;DR: In this article, the nearly free electron approach used in periodic structures is exploited to provide accurate energy level expressions for the electronic ground state in a number of important quantum wells with different shapes.
Abstract: Shallow quantum wells are widely used in electronic and optoelectronic heterostructure devices. However, to determine the effective band edges, one needs numerical techniques unless the barrier height is infinite. In this letter the nearly free electron approach used in periodic structures is exploited to provide accurate energy level expressions for the electronic ground state in a number of important quantum wells with different shapes. These include the square quantum well without and with an applied transverse electric field.



Journal ArticleDOI
TL;DR: In this article, the elastic and ionization scattering cross sections have been calculated as a function of the width of the well using the Born approximation for scattering of the excitons by the free carriers.

Journal ArticleDOI
TL;DR: In this paper, a two-step radiation heating model is utilized to characterize transient temperatures of the electron system and the lattice system, which can result in lattice heating, thermal stress, melting, and evaporation.
Abstract: Free electrons in metal films absorb laser light and then transfer the absorbed photon energy to the metal lattice through electron-lattice collisions, which can result in lattice heating, thermal stress, melting, and evaporation. This work studies nanosecond laser heating of gold films both theoretically and experimentally. A two-step radiation heating model is utilized to characterize transient temperatures of the electron system and the lattice system. Results show that in the nanosecond regime electrons and the lattice are in thermal equilibrium and the classical Fourier heat conduction model is applicable. Microstructures and morphology of films before and after laser pulse heating are characterized with optical and electron microscopes. Two different types of thermal and mechanical responses of gold films are observed. For thin films, thermal stress plays a significant role in laser-film interactions, which can lead to structure changes of films at a temperature much lower than the melting point. Fo...

Journal ArticleDOI
TL;DR: In this article, the lifetime and drift velocity of free electrons in an ionization chamber were determined by analysing the decay of the induced current following a short radiation pulse, and the collected electron-charge and therefore the deficit of negative ions can be found.
Abstract: The collection of free electrons in an ionization chamber results in a deficit of negative ions. The recombination of the ions in the chamber gas is therefore smaller than maintained by the classical recombination theory, which assumes an equal number of positive and negative ions. Under certain conditions the current induced by the electrons in the outer circuit of the chamber can be measured. If the lifetime and the drift velocity of the free electrons in the chamber gas are known, the collected electron-charge and therefore the deficit of negative ions can be found. The determination of the lifetime and the drift velocity is achieved by analysing the decay of the induced current following a short radiation pulse.

Journal ArticleDOI
TL;DR: In this article, an observation of millimeter-wave (94 GHz) amplification in a sheet beam, short period, planar wiggler, free electron laser amplifier was reported.
Abstract: We report an observation of millimeter‐wave (94 GHz) amplification in a sheet beam, short period, planar wiggler, free electron laser amplifier. The observed gain is about 5 dB for a 530 keV, 4 A beam through a 54 cm wiggler. Wave energy absorption was also observed when the beam energy is off‐resonance. Experimental results are in good agreement with numerical simulation. This amplifier configuration has potential for producing equally high output power but at relatively low voltage compared with longer period free electron lasers.

Journal ArticleDOI
TL;DR: In this article, the concept of compact free-electron lasers that are based on beam-plasma interactions and that operate in the vacuum ultraviolet and X-ray wavelength ranges is discussed.
Abstract: Concepts of compact free-electron lasers that are based on beam-plasma interactions and that operate in the vacuum ultraviolet and X-ray wavelength ranges are discussed. Coherent radiation can not only be produced by periodic transverse motions of an electron beam, but also by its longitudinal motions. In this latter case, coherent transition radiation is generated when an electron beam passes through a structure with a deep periodic modulation of the plasma electron density. A number of structures are considered as short-wavelength radiators: standing Langmuir solitons or collapsing caverns, fast nonlinear longitudinal plasma waves, artificial periodical structures that can be converted into the plasma state by a powerful current generator or by a laser pulse, and periodic z-pinches produced by a thin wire.


Journal ArticleDOI
TL;DR: In this paper, a nonlinear frequency shift in dissipative trapped electron mode turbulence is shown to give rise to a relaxation oscillation in the saturated power density spectrum, and a simple non-Markovian closure for the coupled evolution of ion momentum and electron density response is developed to describe the oscillations.
Abstract: A nonlinear frequency shift in dissipative trapped electron mode turbulence is shown to give rise to a relaxation oscillation in the saturated power density spectrum. A simple non‐Markovian closure for the coupled evolution of ion momentum and electron density response is developed to describe the oscillations. From solutions of a nonlinear oscillator model based on the closure, it is found that the oscillation is driven by the growth rate, as modified by the amplitude‐dependent frequency shift, with inertia provided by the memory of the growth rate of prior amplitudes. This memory arises from time‐history integrals common to statistical closures. The memory associated with a finite time of energy transfer between coupled spectrum components does not sustain the oscillation in the simple model. Solutions of the model agree qualitatively with the time‐dependent numerical solutions of the original dissipative trapped electron model, yielding oscillations with the proper phase relationship between the fluctu...

Journal ArticleDOI
TL;DR: In this paper, the authors extend the correlated electron model to the case of coupled layers and show that the nature of the non-Fermi liquid ground state leads to the absence of correlated electron states.
Abstract: We extend the correlated electron model of Baskaran (Mod. Phys. Lett.B5, 643 (1991)) to the case of coupled layers. We show that the nature of the non-Fermi liquid ground state leads to the absence...