Several Tens Of Electron Volts Research Articles

Experimental method for warm dense matter ionization distribution based on X-ray fluorescence pectroscopy

Warm dense matter (WDM), a state of matter that lies at the frontier between condensed matter and plasma, is one of the main research objects of high energy density physics (HEDP). Comparing with the isolated atom, the electron structure of WDM will change because of the influence of density and temperature effect. Both the accurate theoretical representation and the accurate experimental study of WDM electron structure are challenging, as it is strongly coupled and partially degenerated. In this work, an experimental method of studying the ionization distribution of WDM based on X-ray fluorescence spectroscopy is developed. In the experiment, in a specially designed hohlraum, warm and dense titanium with several tens of electron volts and nearly solid density is produced by simultaneous driving of high-energy X-ray heating and shock compression. Then, using the characteristic line spectrum emitted by the laser irradiation on pump material (Vanadium) as a pump source, the titanium emits fluorescence. The X-ray fluorescence spectra of titanium with different states (cold sample, 1.8–4.5 g/cm<sup>3</sup> and 1–25 eV) are diagnosed by changing the experimental strategy. The experimental results indicate that the line profiles of K<sub><i>α</i></sub> and K<sub><i>β</i></sub> fluorescence spectrum of the heated sample change obviously compared with those of the cold sample. According to the theoretical calculation of the two-step Hartree-Fock-Slater (TSHFS) method, the main reason for the change of the line profile is the change of ionization distribution caused by temperature rising. The future work will focus on optimizing the experimental method of X-ray fluorescence spectrum, such as improving the spectrum resolution, characterizing the temperature and density experimentally, obtaining a set of ionization distribution data, and then studying the influence of dense environment on electronic structure.

Characteristics of the Single-Gap Pseudospark Discharge Under Nanosecond Pulsed Voltages

In this paper, a single-gap pseudospark discharge chamber was built and tested with several electrode gap distances and pressures to explore the characteristics of the pseudospark under nanosecond pulsed voltages. Experimental results show that with the development of the discharge, anode plasma firstly forms near the anode, and then moves towards the cathode. The ionization initial voltage decreases with the increase of the electrode gap distance and pressure in our pseudospark discharge configuration. By analysing the emission spectra, it is concluded that the major emission spectra are composed of the second positive bands, and the electron energy of the plasma is in the level of several tens of electron volts. The mechanism of how the pressure and gap distance influence the characteristics has been discussed with the simulation of the collision ionization process using the Monte Carlo Collision method. Based on the simulation and experimental results, it is inferred that the breakdown voltages is decided by the ionization process of the electron with the neutral gas in the main gap, and a possible reason is put forward to explain the reason that U is a function of p2d.

Scaling of a fast spherical discharge

The influence of the discharge cavity dimensions on the properties of the spherical plasma formed in a fast discharge was studied experimentally. The passage of a current pulse with an amplitude of 30–40 kA and a rise rate of ~1012 A/s (a fast discharge) through a spherical ceramic (Al2O3) cavity with an inner diameter of 11 mm filled with argon at a pressure of 80 Pa results in the formation of a 1- to 2-mm-diameter spherical plasma with an electron temperature of several tens of electronvolts and a density of 1018–1019 cm–3. It is shown that an increase in the inner diameter of the discharge cavity from 11 to 21 mm leads to the fourfold increase in the formation time of the spherical plasma and a decrease in the average ion charge. A decrease in the cavity diameter to 7 mm makes the spherical plasma unstable.

Late-Time Mixing Sensitivity to Initial Broadband Surface Roughness in High-Energy-Density Shear Layers.

Using a large volume high-energy-density fluid shear experiment (8.5 cm^{3}) at the National Ignition Facility, we have demonstrated for the first time the ability to significantly alter the evolution of a supersonic sheared mixing layer by controlling the initial conditions of that layer. By altering the initial surface roughness of the tracer foil, we demonstrate the ability to transition the shear mixing layer from a highly ordered system of coherent structures to a randomly ordered system with a faster growing mix layer, indicative of strong mixing in the layer at a temperature of several tens of electron volts and at near solid density. Simulations using a turbulent-mix model show good agreement with the experimental results and poor agreement without turbulent mix.

Seasonal variations and north–south asymmetries in polar wind outflow due to solar illumination

Abstract. The cold ions (energy less than several tens of electronvolts) flowing out from the polar ionosphere, called the polar wind, are an important source of plasma for the magnetosphere. The main source of energy driving the polar wind is solar illumination, which therefore has a large influence on the outflow. Observations have shown that solar illumination creates roughly two distinct regimes where the outflow from a sunlit ionosphere is higher than that from a dark one. The transition between both regimes is at a solar zenith angle larger than 90°. The rotation of the Earth and its orbit around the Sun causes the magnetic polar cap to move into and out of the sunlight. In this paper we use a simple set-up to study qualitatively the effects of these variations in solar illumination of the polar cap on the ion flux from the whole polar cap. We find that this flux exhibits diurnal and seasonal variations even when combining the flux from both hemispheres. In addition there are asymmetries between the outflows from the Northern Hemisphere and the Southern Hemisphere.

Very low energy scanning electron microscopy in nanotechnology

The group of low energy electron microscopy at ISI AS CR in Brno has developed a methodology for very low energy scanning electron microscopy at high image resolution by means of an immersion electrostatic lens (the cathode lens) inserted between the illumination column of a conventional scanning electron microscope and the sample. In this way the microscope resolution can be preserved down to a landing energy of the electrons one or even fractions of an electronvolt. In the range of less than several tens of electronvolts the image signal generation processes include contrast mechanisms not met at higher energies, which respond to important features of the 3D inner potential of the target and visualise its local crystallinity as well as the electronic structure. The electron wavelength comparable with interatomic distances allows observation of various wave–optical phenomena in imaging. In addition, the cathode lens assembly secures acquisition of electrons backscattered from the sample at large angles with respect to the surface normal, which are abandoned in standard microscopes although they provide enhanced crystallinity information and surface sensitivity even at medium electron energies. The imaging method is described and illustrated with selected application examples.

Photoionization measurements for the iron isonuclear sequence Fe3+, Fe5+, and Fe7+

Cross sections for single photoionization of Fe${}^{3+}$, Fe${}^{5+}$, and Fe${}^{7+}$ ions were measured at spectral resolutions of 0.04, 0.15, and 0.13 eV, respectively, by merging mass- and charge-selected ion beams with a beam of monochromatized synchrotron undulator radiation. The measurements span photon energy ranges beginning at the ionization thresholds and extending several tens of electron volts to include the most important resonant contributions due to $3p$--$\mathit{nd}$ transitions to autoionizing states. The photoion yield spectra are characterized by narrow resonances and also broad features in Fe${}^{3+}$ and Fe${}^{5+}$ that are believed to result from unresolved fast super-Coster-Kronig transitions following $3p$--$3d$ excitation. Absolute photoionization cross-section measurements were also performed using ion beams containing undetermined fractions of ions in their ground and metastable states. A Rydberg series attributed to $3p$--$\mathit{nd}$ transitions from the ${}^{2}{G}_{1}$ metastable state in Fe${}^{3+}$ was identified. The data are compared with recently published measurements on Fe${}^{3+}$ and Fe${}^{5+}$ using a similar technique at lower spectral resolution.

Genetic-algorithm implementation of atomic potential reconstruction from differential electron scattering cross sections

We demonstrate the successful implementation of genetic algorithm for the retrieval of atomic potentials using elastic differential cross sections (DCSs) between free electrons and atomic ions for electron energies from a few to several tens of electron volts. Since the DCSs over this energy region can be extracted from laser-generated high-energy photoelectron momentum spectra, the results suggest that infrared lasers can be used to image the target structure. Extending to molecular targets, in particular, to transient molecules created by an earlier pump pulse, our results suggest that few-cycle infrared probe lasers can be used for dynamic chemical imaging with temporal resolution of a few femtoseconds.

Binuclear atom as the bound state of a proton and a heavy atom

The existence of a bound state of a proton and a heavy atom is predicted. The atom is described by the Thomas-Fermi method. The electrons screen the field of the proton, which suppresses the repulsive force between the proton and the atomic nucleus. On the other hand, the force of attraction between the proton and the electrons is directed along the electron density gradient (i.e., towards the nucleus). It is concluded that for Z = 80, the two forces are balanced at a distance from the nucleus of about 0.6 of the Bohr radius. It is found that the potential energy minimum of the proton with a depth of several tens of electronvolts lies in the range of negative energies (attraction). It is proposed that such a system be referred to as a binuclear atom. It is emphasized that, in contrast to molecules, in which binding with the hydrogen atom is ensured by a rearrangement of the states of the outer-shell (valence) electrons, a binuclear atom is formed as a result of the collective response of the system of inner electrons to the proton potential.

Radiation factor: Effect on the structure and stress state of condensates produced by ion sputtering of Ti-W-B refractory materials

The paper examines the structurization and stress state of ion-plasma condensates of Ti-W-B refractory materials exposed to radiation in deposition. High imbalance is found, which is characteristic of materials condensed with ion bombardment, the mean atomic energy of film-forming particles ranging from several to several tens of electron volts. Models for optimizing the structure and stress state of condensates exposed to radiation to improve their mechanical properties are discussed.

Rates and channels of water ionization by a high-current high-voltage pulsed discharge

The experimentally observed growth of the plasma density in a high-current high-voltage pulsed discharge in a liquid medium is compared with the results of calculations based on the effective cross sections for electron-impact ionization and other elementary processes. It is found that, in the initial stage of the discharge, the plasma density grows linearly with time, whereas at densities above 3 × 1010 cm−3, the growth becomes exponential due to the collective acceleration of plasma electrons. The gas-vapor fraction of the water medium is ionized by two groups of electrons: low-energy electrons, with energies about several tens of electronvolts, and high-energy ones, with energies in the kiloelectronvolt range. The energy spent on water ionization is estimated and is found to be several times higher than the energy required to ionize a rarefied gas.

Inelastic electron interaction (attachment/ionization) with deoxyribose.

We have investigated experimentally the formation of anions and cations of deoxyribose sugar (C(5)H(10)O(4)) via inelastic electron interaction (attachment/ionization) using a monochromatic electron beam in combination with a quadrupole mass spectrometer. The ion yields were measured as a function of the incident electron energy between about 0 and 20 eV. As in the case of other biomolecules (nucleobases and amino acids), low energy electron attachment leads to destruction of the molecule via dissociative electron attachment reactions. In contrast to the previously investigated biomolecules dehydrogenation is not the predominant reaction channel for deoxyribose; the anion with the highest dissociative electron attachment (DEA) cross section of deoxyribose is formed by the release of neutral particles equal to two water molecules. Moreover, several of the DEA reactions proceed already with "zero energy" incident electrons. In addition, the fragmentation pattern of positively charged ions of deoxyribose also indicates strong decomposition of the molecule by incident electrons. For sugar the relative amount of fragment ions compared to that of the parent cation is about an order of magnitude larger than in the case of nucleobases. We determined an ionization energy value for C(5)H(10)O(4) (+) of 10.51+/-0.11 eV, which is in good agreement with ab initio calculations. For the fragment ion C(5)H(6)O(2) (+) we obtained a threshold energy lower than the ionization energy of the parent molecular ion. All of these results have important bearing for the question of what happens in exposure of living tissue to ionizing radiation. Energy deposition into irradiated cells produces electrons as the dominant secondary species. At an early time after irradiation these electrons exist as ballistic electrons with an initial energy distribution up to several tens of electron volts. It is just this energy regime for which we find in the present study rather characteristic differences in the outcome of electron interaction with the deoxyribose molecule compared to other nucleobases (studied earlier). Therefore, damage induced by these electrons to the DNA or RNA strands may start preferentially at the ribose backbone. In turn, damaged deoxyribose is known as a key intermediate in producing strand breaks, which are the most severe form of lesion in radiation damage to DNA and lead subsequently to cell death.

Photoelectron energy shift induced by microfocused x rays in micrometer-thick insulating layers

The temporal changes of the kinetic energy spectra of photoelectrons emitted from micrometer-thick insulating layers, SiO2, and photoresist layers, were investigated with microfocused soft x rays in soft x-ray spectromicroscopy. The energy spectra of the insulators shifted up to several tens of electronvolts toward lower energies within seconds of the initial exposure. The amount of the energy shift depended on the thickness of the insulators. For the photoresist insulator, which was susceptible to radiation damage, the energy shift then decreased as the exposure time increased. The main cause of this decrease is attributed to the increase of conductivity by the x-ray-induced chemical state change of the insulator along the x-ray path. It was also demonstrated that by choosing appropriate time and energy in detecting photoelectrons the spectromicroscopy could be used as a depth probe of the conducting microstructures covered by insulating layers.

Fluorination effects in electron-scattering processes—comparative study

Systematic studies of electron interaction with perfluorinated targets carried out recently in our laboratory revealed a distinct increase of the total cross section, Qtot, over the impact energy range from a dozen or so to several tens of electronvolts. A likely reason for this striking feature is a significant contribution of elastic processes in this energy range. The ratio of the total ionization electron-scattering cross section to the total cross section can be expressed with a simple regression formula. That formula was used to estimate the GeH4 ionization cross section.

Time-resolved studies of scintillation materials with VUV harmonic ultrashort pulses laser source

High energy excitation, after a very fast electron-electron diffusion cascade, leads to the creation of excited region with many electrons and holes of typical energies from few to several tens of electron-volts. At this stage, the scintillation process involves the same electronic relaxation and energy transfer mechanisms as those occurring under VUV excitation. Time-resolved luminescence spectroscopy with VUV and X-synchrotron radiation became today a principal investigation method of energy relaxation mechanisms in scintillation materials. High-order harmonic (HH) generation is a very promising new XUV source for these studies. The emission of HH occurring when an intense linearly polarized femtosecond laser pulse interacts with an atomic gas. We present the results of characterization of HH source mounted at C.E.L.I.A. laser (20 mJ, 20 fs, 1 kHz). The experimental setup giving the possibilities of many applications in physics and chemical-physics as well as a first time-resolved luminescence measurements of scintillation materials are described. The main advantages of harmonic radiation for spectroscopic study are: quasi continuous spectrum in the VUV region (10-100 eV), ultra-short pulse (ps and sub-ps), high pulse intensity (up to 106 photons/pulse/eV), relatively high frequency of pulses (1 kHz) and table-top size.

Electron energy distribution function in a Hall-type thruster (abstract)

The electron energy distribution function measured at the channel exit of a small Hall-type thruster is interpreted as a beam-plasma electron distribution function with electron beam energy of several tens of electron volts. Although unstable, the distribution function does not correspond to the completely quasilinear relaxed beam function. Comparison with the measured function in the vicinity of the channel exit shows that the electron beam energy is about 30 eV for the total voltage applied ≅200 V. The contribution of the electron beam is shown to be essential in the ionization of the working gas, even when single ionization processes only are considered.

On the electron energy distribution function in a Hall-type thruster

The electron energy distribution function measured at the channel exit of a small Hall-type thruster is interpreted as a beam–plasma electron distribution function with electron beam energy of several tens of electronvolts. Though unstable, the distribution function does not correspond to the completely quasilinear relaxed beam function. Comparison with the measured function in the vicinity of the channel exit shows that the electron beam energy is about 30 eV for the total voltage applied ∼200 V. The contribution of the electron beam is shown to be essential in the ionization of the working gas, even when only the single ionization processes are considered.

Highly repetitive, extreme-ultraviolet radiation source based on a gas-discharge plasma

An extreme-ultraviolet (EUV) radiation source near the 13-nm wavelength generated in a small (1.1 J) pinch plasma is presented. The ignition of the plasma occurs in a pseudosparklike electrode geometry, which allows for omitting a switch between the storage capacity and the electrode system and for low inductive coupling of the electrically stored energy to the plasma. Thus energies of only a few joules are sufficient to create current pulses in the range of several kiloamperes, which lead to a compression and a heating of the plasmas to electron densities of more than 10(17) cm(-3) and temperatures of several tens of electron volts, which is necessary for emission in the EUV range. As an example, the emission spectrum of an oxygen plasma in the 11-18-nm range is presented. Transitions of beryllium- and lithium-like oxygen ions can be identified. Current waveform and time-resolved measurements of the EUV emission are discussed. In initial experiments a repetitive operation at nearly 0.2 kHz could be demonstrated. Additionally, the broadband emission of a xenon plasma generated in a 2.2-J discharge is presented.

Exact energy values of ‘‘low-energy ion beams’’

Ion beams with energies of several tens of electron volts are used for forming special films with excellent physical and chemical properties by ion-beam deposition. Although the ion energy values are considered very important for controlling physical and chemical characteristics of the deposited films, most reported values for such film formation have been considerably inaccurate since they have been given simply by multiplying the power supply voltages to the discharge chambers with electron charge e. In practice, the plasma potential at which ions are generated is several to several tens of volts higher than the discharge chamber, so that the real beam energy is that much higher. Although the excess energy originating from the plasma floating potential has been negligible for usual high-energy ion implantation, it cannot be neglected for ion-beam deposition where low-energy ion beams below 100 eV are used. In this article it is indicated that knowing electron temperatures of plasmas is necessary to get the exact energy values of low-energy ion beams. Thus, it is also clarified that ion energy spreads of usual ion beams may be caused by possible fluctuation of the electron temperatures as well as by thermal motion of ions in the plasma.

Optical characterization of the cathode plasma sheath during the biasing step for diamond nucleation on silicon

Abstract In-situ nucleation by negatively biasing the substrate in the microwave plasma is of crucial importance for diamond deposition on silicon, especially for the production of oriented diamond nuclei. We have characterized the cathode plasma sheath above the substrate by measuring the optical emission of the hydrogen Balmer lines during the biasing step. From the Stark splitting of the lines we deduce a field strength of about 1.5kV cm −1 directly at the silicon surface, increasing to about 3.5kV cm −1 when the surface is covered with a diamond film. At the same time the voltage drop moves closer to the cathode and the current increases by an order of magnitude. The superimposed broadening due to the Doppler effect is decomposed into several components. The broadest component results from hydrogen atoms with several tens of electronvolts of kinetic energy.