Emission Of High-energy Electrons Research Articles

Asymmetric emission of high-energy electrons in the two-dimensional hydrodynamic expansion of large xenon clusters irradiated by intense laser fields

Energy spectra and angular distributions have been measured of electrons that are emitted upon disassembly of ${\mathrm{Xe}}_{n}$ clusters $(n=20000--150000)$ following irradiation by intense ${(10}^{15}--{10}^{16}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2})$ laser pulses whose durations are varied over the 100--2200 fs range. The cluster explosion dynamics occur in the hydrodynamic regime. For the smaller clusters in the range that we have studied, a single-electron temperature adequately describes the measured electron energy distribution; in the case of larger clusters, a two-temperature fit becomes necessary. The total electron emission is found to be unexpectedly asymmetric and exhibits a resonance when the laser-pulse duration is $\ensuremath{\sim}1\mathrm{ps}.$ These results are rationalized by extending the hydrodynamic model to also take into account the force that the light field exerts on the polarization charge that is induced on the surface of the cluster. We show that the magnitude of this electrostrictive force is comparable to those of the Coulombic and hydrodynamic forces, and it exhibits resonance behavior. Contrary to the findings of the only other earlier report, we find that the low-energy component in the electron energy distribution is connected to the resonance in energy absorption by the cluster. The high-energy component seems to be produced by a mechanism that is not so strongly influenced by the resonance.

Spatial distribution of high-energy electron emission from water plasmas produced by femtosecond laser pulses.

High energy electrons emitted by water plasmas produced by a single or a multiple laser pulse are investigated. The multipulse mode greatly enhances the generation and the temperature of hot electrons. Directional emission of high energy electrons over 25 keV is observed in two symmetric directions with respect to the laser axis and at 46 degrees from the directions of the laser electric field. Two-dimensional particle-in-cell simulations reproduce well the experimental results and indicate that the acceleration mechanism of the high energy electrons is due mainly to the resonance absorption at the edge of the spherical droplets formed by the leading pulse.

Emission of high-energy electrons from moderately underdense plasmas produced by TW-fs laser pulses

Self-focusing of the intense laser pulses as well as production of the well-collimated electron beams in the moderately underdense plasma of ne/nc 10% were investigated at the maximum laser intensity of 7 × 10 17 W/cm 2 . Progresses of a relativistic self-focusing in the plasma of a gaussian density profile were observed by an ultra-fast optical probing. Even if the large gas-target of the quasi-uniform density was used, the self-focusing was terminated after traveling approximately 1.2 mm by the abrupt transition of the propagation mode from the smooth channeling to the complex filamentation, which correlated with the emission of MeV-electron beams. High-energy electron beams have the power-law spectrum of E −3.5 reaching the maximum energy of 2 MeV. The maximum number of the electrons was(1.3±0.6)×10 11 /sr/shot. Model calculations suggest that the most plausible mechanism of the production of high-energy electron beam in the moderately underdense plasma is that the phase of the electron oscillation in filaments of the laser beam randomly changes many times.

Secondary-electron emission from thin carbon foils byH0andH+in frozen-charge states

The statistical distributions of the number of simultaneously emitted secondary electrons (SE's) from a thin carbon foil induced by the frozen-charged ${\mathrm{H}}^{0}$ and ${\mathrm{H}}^{+}$ projectiles of 2.5--3.5 MeV have been measured by using the coincidence technique with the foil-transmitted particles. The measurement was carried out at the forward and backward directions of the incident beam separately. For frozen-charged ${\mathrm{H}}^{0},$ the average SE yields per projectile at the forward direction, ${\ensuremath{\gamma}}_{\mathrm{F}},$ and at the backward direction, ${\ensuremath{\gamma}}_{\mathrm{B}},$ are significantly smaller than the corresponding ones for ${\mathrm{H}}^{+}$ due to the screening effect of its bound electron. In addition to the suppression of low-energy electron production for ${\mathrm{H}}^{0},$ the preferential forward emission of high-energy electrons makes the proton-hydrogen difference in the ratio of ${\ensuremath{\gamma}}_{\mathrm{B}}$ to the stopping power more striking. Although the probability of simultaneous n electron emission per unit projectile, ${W}_{n},$ for ${\mathrm{H}}^{0}$ is significantly smaller than that for ${\mathrm{H}}^{+}$ at small n, their difference decreases with increasing n both at the forward and backward directions. This behavior of ${W}_{n}$ also suggests that there is not a large proton-hydrogen difference in the production of high-energy electrons. As a result of a simple model calculation, the difference between the emission statistics by ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^{0}$ can be well reproduced by a Poisson distribution with a mean equal to the difference of their ${\ensuremath{\gamma}}_{\mathrm{F}}$ or ${\ensuremath{\gamma}}_{\mathrm{B}}$ values and the validity of the above-mentioned interpretation on the proton-hydrogen difference of the SE emission is quantitatively confirmed.

Comparative study of secondary electron emission from solids under positron and electron impacts

Secondary electron emission from gold caused by the impacts of positrons and electrons at keV or less energy is investigated by means of a Monte Carlo simulation of elastic and inelastic collision processes of the projectiles and cascade electrons inside the solid. The cross-section for the elastic collision of each particle with an atom in the solid is calculated using the partial wave expansion technique, whereas the cross-section for the inelastic collision or for the electron excitation is calculated from the Ashley’s optical-data model. Because of less large-angle elastic scattering of the positron, the calculated backscattering coefficient is smaller for the positron than the electron. At low impact energies (<600 eV), however, the calculated secondary electron yield for the positron is larger than that for the electron, due to the larger inelastic energy loss of the positron and the larger number of excited electrons. The difference between the distributions for the positron and the electron calculated at such low impact energy is due to increasing emission of high-energy electrons excited by the projectile without the cascade process; which is more enhanced for the electrons than the positrons. These calculated results are consistent with the recent findings based on experiments.

Monte Carlo simulation of kinetic electron emission from metal due to impact of heavy ions

A Monte Carlo simulation is performed for study of the dependence of kinetic electron emission on nuclear charge of projectile Z 1, using the nonlinear response theory with the density-functional (DF) formalism to calculate electron excitation cross section. The kinetic yield, energy distribution, excitation depth distribution and emission statistics of emitted electrons showed clear Z 1 oscillations, however, the Z 1 oscillations of them are different from that of the inelastic stopping power, in particular for high Z 1, due to large elastic energy loss of the ions and secondary cascade process of primary excited electrons within the solid. For high Z 1, the linear relationship does not exist between them and the inelastic stopping power, although they are closely related to it. The emission of high-energy primary electrons excited by the ion within shallow depth without experiencing the secondary cascade process, results in the Z 1 dependence in the energy distribution, excitation depth distribution and emission statistics of emitted electrons.

High-energy electron emission from ion-bombarded surfaces of Ga, Ge, In and Sn

Intense, continuous distributions of kinetic emitted electrons, often with energies extending to greater than 100 eV, have been observed for Ga, Ge, In and Sn surfaces bombarded with keV Ar + ions. These broad distributions, which clearly are not associated with Auger excitation, can be understood in terms of a one-electron collisional-excitation model involving orbital interaction, promotion, and direct-to-continuum deexcitation. Possible pathways for electron promotion have been obtained for the above elements from detailed calculations of correlation diagrams which have been used to predict successfully the probability of collisional excitation and deep-level promotion. Element-specific differences observed in the kinetic emission spectra between the two pairs of elements, the Ga and Ge pair and the In and Sn pair (associated with collisional level-coupling and promotion processes), also have been interpreted from these correlation diagrams. We conclude that the high-energy, kinetic electron emission as well as the specific differences observed between the Ga and Ge pair and the In and Sn pair are consistent with this one-electron promotion/deexcitation model.

Trapping of a classical electron between two heavy scattering centers.

This paper shows that a classical electron may get trapped within the field of two atomic nuclei even though it is free, i.e., the total energy of the electron is greater than that at large distances from the attractive centers. The consequences of this phenomenon for the emission of high-energy electrons during slow-atom bombardment of solids are discussed.

On the mechanism of the emission of high-energy electrons from the surfaces of freshly peeled films: Polytetrafluoroethylene

This paper describes the results of an investigation of high-electron emission accompanying the mechanical disruption of adhesive contact in vacuo. Investigations of such emission parameters as the energy of the electrons, their angular distribution, depth of the electron capture center levels on freshly formed polymer surfaces responsible for the emission as well as the establishment of relations governing the time variation of the parameters of the emitted electrons' flux, their relation to the polymer film surface charge, and finally the numerical computations based on these experiments have led the authors to the conclusion that the mechanism of electron emission from freshly formed polymer film surfaces has a field-emission nature proceeding from surface states and is described accurately enough by the analytical expressions derived from the above considerations.

Formation of localized states and electronic excitation in solids during low-energy particle bombardment

The excitation of deep holes (2s, 2p, in Al) in collision cascades is known to be responsible for emission of high-energy secondary electrons from solids bombarded by fast particles. The excitation occurs above certain impact energy threshold ( ≈ 800 eV in the case of Al) which is given by the minimum interatomic distance in the binary atomic collision needed for inner shell-valence band energy crossing ( ≈ 0.5 Å in Al). Experiments indicate however, that electrons may be emitted from solids far below the deep-hole excitation threshold. The classical valence electron-particle binary collision mechanism which is responsible for most of the particle electronic energy losses yields only low energy electrons which cannot overcome the work function of solids. We have found out that in collision cascades the deformation of valence electron clouds in more complex collision can lead to localized levels below the bottom of the conduction band of a metal. When these levels loose electrons during dynamical particle-particle interactions their subsequent filling by Auger processes could lead to electron emission. Using a cluster of 123 Al atoms with 3s and 3p overlapping Gaussian orbitals and using the pseudopotential of solid Al we have calculated the localization and energies of these new time-dependent electronic states for some collision configurations. We have also estimated the probability of the hole occupation of such states induced by the dynamics of particle cascades.

Origin of the high-energy electron emission from metals under laser irradiation.

We interpret recent experimental results concerning high-energy-electron production when an aluminum surface is irradiated by laser light in the visible range of 35-ps pulse duration in a moderate (${10}^{6}$ to 2\ifmmode\times\else\texttimes\fi{}${10}^{8}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) intensity range. We show that two-photon absorption governs the process of photoemission. High electron energies measured in such experiments are shown to be an effect due to the transport of a cloud of electrons. Its physical origin is the work of the Coulomb field on the center-of-mass trajectory of the electron cloud, depending on the external fields needed to cancel the space-charge effects.

Cooling by plasmon emission of high energy electrons in semiconductors

The authors present a Monte Carlo study of the Coulomb interaction in polar semiconductors involving both single-particle (screened carrier-carrier interaction) and collective excitations (plasmon scattering). Relaxation of high energy carriers, created by laser excitation, occurs through the interaction with the lattice (mainly via optical phonon emission) and with other carriers. The lattice (mainly via optical phonon emission) and with other carriers. The importance of the latter mechanism has been demonstrated recently by time-resolved and CW hot (e, AA) luminescence experiments which allow the authors to distinguish the separate contributions due to phonon and intercarrier scattering. They study the energy exchange mechanisms in a multi-component plasma using an ensemble Monte Carlo simulation where the intercarrier scattering is treated with two different approaches, the first based on a k-space model that allows the separation of short range (single-particle collisions) and long range (plasmon scattering) Coulomb potential, the second adopting a molecular dynamics scheme. The results of the simulation and the comparison between the two models are discussed for both GaAs and InP. A very good agreement with the experimental results is found.

High energy electron emission phenomenon from ferroelectric crystals

Abstract The energy of electrons We emitted by ferroelectric crystals is calculated and measured. Its magnitude for LiTaO3 under pyroelectric effect condition reaches 120 keV without any extraction field and may be changed greatly by crystal thickness variation or by changing the distance between the emitting crystal surface and the collector. It is shown that the escaping electrons are accelerated in the field of unscreened pyroelectric charge and the electron energy is determined by the crystal surface potential. An artificial β-source based on this phenomenon is discussed.

Determination of overlayer growth by angle-resolved Auger electron spectroscopy

Single-scattering cluster theory, augmented by the inclusion of spherical wave effects, is used to model the substrate and overlayer angle-resolved electron emission intensity to determine the mode of growth of the overlayer. For large inelastic mean-free-path (IMFP) electrons, a new method for identifying first- and second-layer completion is described which will have wide application to intermediate and high-energy electron emission. Calculations for azimuthally integrated data (cylindrical mirror analyzer based measurements) finds that the standard determination of the IMFP is too large by ∼20%.

Electromagnetic probes of nuclear collisions at intermediate energy

We show that emission of high-energy electrons and photons in nuclear collisions at intermediate energies is sensitive to the space-time evolution of the reaction. The electron and photon spectra measure related but complementary quantities connected with the nuclear charge distribution. We show that at 60 MeV/u beam energy production of 50 MeV electrons is predicted to measurable probability.

Importance of multiple forward scattering in medium- and high-energy electron emission and/or diffraction spectroscopies.

We examine the role of multiple scattering in emission and/or diffraction spectroscopies that involve medium- and high-energy electrons. We use quantum-mechanical and classical methods to demonstrate the importance of multiple forward scattering in these techniques. Single-scattering theory produces errors because it neglects multiple forward scattering from atoms which shadow other atoms.

Emission of High-Energy Electrons During Friction and Wear of Dielectrics

Emission of High-Energy Electrons During Friction and Wear of Dielectrics

On the mechanism of the emission of high-energy electrons from the surfaces of freshly peeled films: Polytetrafluoroethylene

This paper describes the results of an investigation of high-electron emission accompanying the mechanical disruption of adhesive contact in vacuo. Investigations of such emission parameters as the energy of the electrons, their angular distribution, depth of the electron capture center levels on freshly formed polymer surfaces responsible for the emission as well as the establishment of relations governing the time variation of the parameters of the emitted electrons' flux, their relation to the polymer film surface charge, and finally the numerical computations based on these experiments have led the authors to the conclusion that the mechanism of electron emission from freshly formed polymer film surfaces has a field-emission nature proceeding from surface states and is described accurately enough by the analytical expressions derived from the above considerations.

Emission Phenomena Accompanying the Triboelectrification Process in Vacuum

It was experimentally found that the triboelectrification of a dielectric is accompanied by the intense emission of high-energy electrons, both in the rolling friction and in the sliding friction. A detailed study was done in the electron emission during the course of triboelectrification in the process of mutual rolling of two dielectric rolls. The experimental results have shown that in triboelectrification the parameters of electron emission depend on the physico-chemical properties of the surfaces of contacting bodies.

2188. Emission of high-energy electrons during mechanical treatment of alkali halides: J Wollbrandt et al,Phys Stat Sol (a), 27 (2), 1975, K35–K55

2188. Emission of high-energy electrons during mechanical treatment of alkali halides: J Wollbrandt et al,Phys Stat Sol (a), 27 (2), 1975, K35–K55