Distribution Of Low-energy Electrons Research Articles

Low-energy electron distributions from the photoionization of liquid water: a sensitive test of electron mean free paths.

The availability of accurate mean free paths for slow electrons (<50 eV) in water is central to the understanding of many electron-driven processes in aqueous solutions, but their determination poses major challenges to experiment and theory alike. Here, we describe a joint experimental and theoretical study demonstrating a novel approach for testing, and, in the future, refining such mean free paths. We report the development of Monte-Carlo electron-trajectory simulations including elastic and inelastic electron scattering, as well as energy loss and secondary-electron production to predict complete photoelectron spectra of liquid water. These simulations are compared to a new set of photoelectron spectra of a liquid-water microjet recorded over a broad range of photon energies in the extreme ultraviolet (20–57 eV). Several previously published sets of scattering parameters are investigated, providing direct and intuitive insights on how they influence the shape of the low-energy electron spectra. A pronounced sensitivity to the escape barrier is also demonstrated. These simulations considerably advance our understanding of the origin of the prominent low-energy electron distributions in photoelectron spectra of liquid water and clarify the influence of scattering parameters and the escape barrier on their shape. They moreover describe the reshaping and displacement of low-energy photoelectron bands caused by vibrationally inelastic scattering. Our work provides a quantitative basis for the interpretation of the complete photoelectron spectra of liquids and opens the path to fully predictive simulations of low-energy scattering in liquid water.

Intensity-dependent angular distribution of low-energy electrons generated by intense high-frequency laser pulse.

By solving the three-dimensional time-dependent Schrödinger equation, we investigate the angular distributions of the low-energy electrons when an intense high-frequency laser pulse is applied to the hydrogen atom. Our numerical results show that the angular distributions of the low-energy electrons which generated by the nonadiabatic transitions sensitively depend on the laser intensity. The angular distributions evolve from a two-lobe to a four-lobe structure as the laser intensity increases. By analyzing nonadiabatic process in the Kramers-Henneberger frame, we illustrate that this phenomenon is attributed to the intensity-dependent adiabatic evolution of the ground state wavefunction. When the laser intensity further increases, the pathway of nonadiabatic transition from the ground state to the excited state and then to the continuum states is non-negligible, which results in the ring-like structure in the photoelectron momentum distribution. The angular distributions of the low-energy electrons provide a way to monitor the evolution of the electron wavefunction in the intense high frequency laser fields.

An Alternative Interpretation of Impulsive SEP Events Occurring on 1999 January 9–10

Abstract We have examined two impulsive solar energetic particle events that occurred on 1999 January 9–10 (earlier event A and later event B). Both events showed earlier velocity dispersion and later intensity dropout of ions. In particular, in event A, the dropout repeated five times. Through the onset time analysis of solar particles, we find that only at higher energies (&gt;0.2 MeV nucleon−1 for heavy ions and &gt;2.8 keV for electrons) can the analysis provide a consistent path length of ions and electrons. The path length in event A is larger than that in event B. In contrast, at lower energies, the analysis fails to predict the distribution of first arrival of solar particles. The divergence between observation and prediction would increase if the interplanetary scattering of ions were taken into account. We then focus on the lower-energy region, where a negative correlation of ion intensities with plasma β is displayed. We have found that the repeated dropout of ions can be caused by the magnetic reconnection acceleration in the solar wind. In addition, we have discovered an isolated proton dropout event in which a sharply anisotropic pitch-angle distribution of low-energy electrons is also seen. Our observation is consistent with the prediction of Tautz et al. that a minimum power spectral density component parallel to the magnetic field can reduce the magnetic mirroring effect, preventing electron scattering through 90°.

Inverse Compton Scattering Spectra of Gamma-Ray Burst Prompt Emission

Abstract Although the physical origin of gamma-ray burst (GRB) prompt emission is still controversial, synchrotron radiation from accelerated electrons is a promising mechanism. It is believed that electrons are accelerated continuously by ultra-relativistic shocks or magnetic reconnections. At the same time, these electrons will be cooled via several processes (mainly adiabatic expansion, synchrotron radiation, and inverse Compton scattering (ICS)), which regulate the distribution of electrons. An extra high-energy spectrum component is expected to be induced by ICS. However, the gamma–gamma annihilation effect may eliminate the high-energy photons and prevent the observers from distinguishing the extra component. We perform numerical calculations by taking these effects into account and discuss whether the extra ICS component could be observed. By exploring the plausible parameter space for relevant quantities of the GRB ejecta, we present the electron distributions and the corresponding spectra of synchrotron radiation and ICS. It is found that the extra component is observable only for ejecta with a rather large bulk Lorentz factor. A large Lorentz factor means the adiabatic expansion is the leading process in the electron cooling procedure, which makes the low-energy electron distribution spectrum relatively hard. Therefore, the ICS component is more likely to be detected for GRBs that have a hard low-energy photon spectrum.

Спектр и угловое распределение низкоэнергетических электронов из распада 152Eu

Спектр и угловое распределение низкоэнергетических электронов из распада 152Eu

How to Really Measure Low Energy Electrons in Space

There is little argument in the space plasma physics community that in-situ, low energy electron measurements are technically challenging. The primary obstacle has been the effects of spacecraft charging on the measured three-dimensional electron velocity space distribution. A successful spacecraft charging correction algorithm used with the three-dimensional electron instrument aboard the Ulysses spacecraft has clarified the role of spacecraft and instrument parameters in the eventual reconstruction of low energy electron distributions. Suggestions for instrument and spacecraft modifications that can minimize spacecraft charging effects are presented in this paper. The emphasis is on designs that lend themselves to robust correction algorithms.

Low energy electron emission from Uracil and oxygen in collisions with 42 MeV bare C ions

We report doubly differential (energy and angle) cross-sections (DDCS) of low energy electrons emitted from uracil (C4H4N2O2) in collisions with 42 MeV energy C ions. For a comparative study we have also used O2 as a target. The DDCS are obtained for electron energy ranging from 3-640 eV and at angles from 30° to 135°. The electron spectrum shows that the cross-sections are largest for low energy electrons and falls sharply with energy. Auger peaks corresponding to C, N and O were also observed. Angular distributions of low energy electrons were plotted. Large forward-backward asymmetry was observed compared to that for oxygen.

Suprathermal plasma analyzer for the measurement of low-energy electron distribution in the ionosphere

It is commonly believed that an energy transfer from thermal to suprathermal electrons (<several eV) greatly affects the energy budget of the ionosphere. However, observation of electron energy spectrum in this energy range is quite limited because of technical difficulties of measurement. We have developed an instrument to measure electron energy distribution from thermal to suprathermal energy continuously with high-energy resolution of about 0.15 eV. The measurement principle is based on the combination of a retarding potential analyzer with a channel electron multiplier (CEM) and the Druyvesteyn method, which derives energy distribution from the current-voltage characteristics. The capability of detecting plasma space potential enables absolute calibration of electron energy. The instrument with a small vacuum pump, which is required for the CEM to work in low-vacuum region, was first successfully tested by a sounding rocket S-310-37 in the ionospheric E region. The instrument is expected to provide new opportunities to measure energy distribution of thermal and non-thermal electrons in low-density plasma, where a Langmuir probe cannot measure electron temperature because of low plasma density.

Secondary electron imaging at gas pressures in excess of 1kPa

Environmental scanning electron microscopy (ESEM) enables electron imaging of gas-mediated, direct-write nanolithography processes, liquids, and hydrated biomaterials. However, ESEM is limited by poor image quality at gas pressures in excess of ∼600Pa. Here the authors achieve high quality secondary electron imaging at 2kPa of H2O by optimizing boundary conditions that govern beam scatter and the energy distribution of low energy electrons in the gas, dielectric breakdown of the gas, and detector collection efficiency. The presented high pressure imaging method will enable imaging of hydrated materials at close to room temperature, and gas-mediated surface modification processes occurring at high pressures.

The Effect of Model Approximations on Single-Collision Distributions of Low-Energy Electrons in Liquid Water

The development of cross sections for the inelastic interaction of low-energy electrons with condensed tissue-like media is best accomplished within the framework of the dielectric theory. In this work we investigate the degree to which various model approximations, used in the above methodology, influence electron single-collision distributions. These distributions are of major importance to Monte Carlo track structure codes, namely, the energy-loss spectrum, the inelastic inverse mean free path, and the ionization efficiency. In particular, we make quantitative assessment of the influence of (1) the optical data set, (2) the dispersion algorithm, and (3) the perturbation and exchange Born corrections. It is shown that, although the shape and position of the energy-loss spectrum remains almost fixed, its peak height may vary by up to a factor of 1.5. Discrepancies in the calculated inelastic inverse mean free path are largely within 20-30% above 100 eV; they increase drastically, though, at lower energies. Exchange and perturbation Born corrections increase gradually below 1 keV leading to a approximately 30 to 40% reduction of the inverse mean free path at 100 eV. The perturbation effect contributes more than the exchange effect to this reduction. Similar to the dispersion situation, the effect of Born corrections at lower energies is also unclear since the models examined disagree strongly below 100 eV. In comparison, the vapor data are higher than the liquid calculations by 20 to 50% as the energy decreases from 1 to 0.1 keV, respectively. The excitation contribution is the main cause of this difference, since the ionization efficiency in the liquid levels off at approximately 90%, whereas the plateau value for the vapor is approximately 70%. It is concluded that electron inelastic distributions for liquid water, although in some respects distinctively different from the vapor phase, have associated uncertainties that are comparable in magnitude to the phase differences. The situation below 100 eV is uncertain.

Whistler waves, Langmuir waves and single loss cone electron distributions inside a magnetic cloud: Observations

Whistler waves propagating along the ambient magnetic field are observed within a coronal mass ejection (on January 10, 1997) associated in time with Langmuir waves and electron distributions of a single loss cone type. In addition, background observations are made on the plasma wave activity in the sheath and foreshock regions that precede the magnetic cloud, on the observed radio emissions (including a type II radio burst) and on the geometry of the cloud. All the data comes from the WIND spacecraft. The whistler waves are identified using full magnetic waveforms while possible evidence of coexisting parallel, and antiparallel propagating Langmuir modes are found in the waveform and spectral wave data from the WAVES experiment. A few hundred low energy electron distributions from the Three‐Dimensional Plasma (3DP) experiment are investigated. Finally, we tentatively suggest that this type of plasma wave particle activity is linked to the type II emission observed, i.e., that the emission mechanisms are proceeding and taking place within the magnetic cloud instead of at the shock region as usually thought. The extra suprathermal electrons could source from electrons accelerated at reconnection sites between the magnetic cloud and the ambient interplanetary magnetic field. A linear instability study using observed properties of the electron distributions is to be presented in a following paper.

Ionization dynamics in fast ion-atom collisions. I. Energy and angular distributions of low-energy electrons emitted in ionization of He by bare carbon ions

We have studied the two-center effect (TCE) in ion-atom ionization by measuring the energy and angular distributions of the double-differential cross sections (DDCSs) ${(d}^{2}\ensuremath{\sigma}/d\ensuremath{\varepsilon}d\ensuremath{\Omega})$ of the low-energy electrons emitted in a collision of He atoms with 2.5 MeV/nucleon ${\mathrm{C}}^{6+}$ ions. The electrons with energies between 0.1 and 300 eV were detected for 13 different emission angles between $15\ifmmode^\circ\else\textdegree\fi{}$ and $160\ifmmode^\circ\else\textdegree\fi{}.$ From the measured DDCSs we have deduced the single differential cross sections such as $d\ensuremath{\sigma}/d\ensuremath{\Omega}$ and $d\ensuremath{\sigma}/d\ensuremath{\varepsilon}$ and the total cross section. The data have been compared with the continuum distorted wave eikonal initial state calculations with H-like and Hartree-Fock-Slater wave functions for the initial and final state of the electron and first Born calculations. The forward-backward asymmetry parameter also has been deduced to study the TCE in detail. In the following paper [Lokesh C. Tribedi et al., Phys. Rev. A 58, 3626 (1998)] we have explored the collision dynamics by deducing the momentum distributions of the electron, the recoil ion, and the projectile.

Angular distributions of low-energy electrons from proton-atom collisions

The forward-backward angular asymmetry in the low-energy electron emission by ion impact is studied in this work. A three-body classical trajectory Monte Carlo (CTMC) method is used to calculate the doubly differential cross sections of ionization in proton-H, Ne atom collisions as a function of the electron emission angle. Our CTMC results are compared with the existing theoretical and experimental data.

Measurement of the Energy Distribution of Low Energy Electrons as a Function of Gas Density: Determination of Absolute Differential Electron-Atom Scattering Cross Sections

The development of sophisticated radiation detector and the study of its energy and directional response require accurate excitation and ionisation cross sections for electrons as a function of energy and angle. However, there is a serious lack of accurate scattering cross sections for electrons of energies of less than a few keV, which usually make up the main body of secondary electrons. A procedure is described with which absolute differential cross sections for electron collision processes can be measured at low scattering angles. With this procedure, absolute differential cross sections for the 1'S®2'P transition in He are measured for electron energies from 0.6 to 2.0 keV at scattering angles of less than 1.0o. Furthermore, the differential cross sections for the 2P®3S transition in Ne are determined for energies from 1.0 to 2.0 keV in the small angle range.

Measurement of the Energy Distribution of Low Energy Electrons as a Function of Gas Density: Determination of Absolute Differential Electron-Atom Scattering Cross Sections

The development of sophisticated radiation detector and the study of its energy and directional response require accurate excitation and ionisation cross sections for electrons as a function of energy and angle. However, there is a serious lack of accurate scattering cross sections for electrons of energies of less than a few keV, which usually make up the main body of secondary electrons. A procedure is described with which absolute differential cross sections for electron collision processes can be measured at low scattering angles. With this procedure, absolute differential cross sections for the 1'S®2'P transition in He are measured for electron energies from 0.6 to 2.0 keV at scattering angles of less than 1.0o. Furthermore, the differential cross sections for the 2P®3S transition in Ne are determined for energies from 1.0 to 2.0 keV in the small angle range.

Resonant and nonresonant multiphoton ionization of helium.

We have investigated the multiphoton ionization of helium at wavelengths between 310 and 330 nm at intensities between 8\ifmmode\times\else\texttimes\fi{}${10}^{13}$ and 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W/${\mathrm{cm}}^{2}$ and at 630 nm at intensities of 1\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$. We characterize the ionization processes from photoelectron energy and angular distributions observed concurrently with photoion spectra. At the shorter wavelengths we find that resonant enhancement via the ac Stark shifted six-photon resonant states (1s3d and 1s3s) is a dominant ionization path as described previously by Perry, Sz\oke, and Kulander [Phys. Rev. Lett. 63, 1058 (1989)] and by Rudolph et al. [Phys. Rev. Lett. 66, 3241 (1991)]. At intensities above those required for resonant enhancement, and at wavelengths longer than those required for six-photon resonance, we observe that nonresonant seven-photon ionization dominates. This process gives rise to continuous distributions of low-energy electrons with characteristic angular distributions that peak near 0\ifmmode^\circ\else\textdegree\fi{} and 60\ifmmode^\circ\else\textdegree\fi{} relative to the laser polarization. At yet higher intensities, above the threshold where the nonresonant seven-photon channel closes, the dominant ionization path occurs via seven-photon resonant states with odd parity. This path gives rise to angular distributions characteristic of intermediate states with f character.

Projectile Coulomb center effects on low-energy electron emission from H + → Ne collisions

We present doubly differential energy distributions of low-energy electrons emitted in collisions of 106 keV H + on Ne atoms. We find a relevant dependence of the measured distribution of low-energy electrons on the physical extension of the gas target and discuss a correction procedure. Our measurements enable a quantitative analysis of the shape of the soft electron peak, which is clearly evidenced by measured contour lines. Present results indicate that “two center effects” must be considered in order to account for the strong asymmetry of the soft electron peak observed experimentally.

Ion-induced electron emission in grazing ion-surface collisions

The distribution of low-energy electrons emitted from a solid when a swift ion interacts with its surface under grazing incidence conditions is obtained using first-order time-dependent perturbation theory. The response of the surface is calculated from the specular-reflection model and the random-phase approximation for the dielectric function of the bulk material. The electrons are described in terms of independent states, that are solutions of the Schrödinger equation with a finite square barrier potential. A broad peak in energy, corresponding to the excitation of surface plasmons, is predicted to appear in the energy spectrum. The angular dependence of the emission is also analyzed. The electrons are found to be preferentially emitted in the normal and forward directions.

Experimental evidence of the asymmetry of the soft electron peak in ion-atom ionization.

We have measured the doubly differential energy and angular distribution of low-energy electrons emitted in collisions of ${\mathrm{H}}^{+}$ and $^{3}\mathrm{He}^{2+}$ on Ne at 106 keV/u. In this way, we are able to obtain information about the shape of the soft electron ionization peak. Against current belief, but in accordance with a two-Coulomb-center interaction of the emitted electron, we find it to be strongly asymmetric in the forward-backward direction. We discuss the shape of the cross section and introduce a parametric expression for fitting the data.

Diffusion of Low-Energy Electrons in Tissue-Like Liquids

The spatial-energetic distribution of low-energy electrons was studied for a source located in a liquid medium simulating biological tissue. A time-independent Boltzmann equation was used to model this distribution microscopically. Ionization was treated as a perturbation to a quasi-elastic collision process between the electron and the medium. A diffusion limit was obtained by using a scale parameter, leading to a sequence of recursive partial differential equations whose solutions, associated with a macroscopic scale, were obtained by numerical approximations. As an application, electron ranges were estimated based on these solutions and then compared with values reported in the open literature based on experimental results and on Monte Carlo calculation. Local dosimetry, i.e., the energy imparted to a volume of a sphere with radius equal to the range of low-energy electrons, of low-energy electrons from internal emitters can benefit by the knowledge of the ranges estimated for biological tissue. Auger electron emitters, for example, have been the object of a number of investigations because of their radiobiological significance.