Hot Electron Emission Research Articles

Competitive ionization processes of anthracene excited with a femtosecond pulse in the multi-photon ionization regime

To clarify the ionization mechanism of large molecules under multi-photon ionization conditions, photo-electron spectroscopic studies on anthracene have been performed with electron imaging technique. Electron kinetic energy distributions below a few eV reveal that three kinds of ionization channels coexist, viz., vertical ionization, ionization from Rydberg states, and thermionic hot electron emission. Their relative yield is determined by the characteristic of the laser pulse. The duration in particular influences the ratio between the first two processes, while for higher intensities the last process dominates. Our results provide strong evidence that internal conversion plays an important role for the ionization of the molecule.

X-ray variability patterns and radio/X-ray correlations in Cyg X-1

We have studied the X-ray variability patterns and correlations of the radio and X-ray fluxes in all spectral states of Cyg X-1 using X-ray data from RXTE/ASM, CGRO/BATSE, and Swift/BAT. In the hard state, the dominant spectral variability is a changing of normalisation with fixed spectral shape, while in the intermediate state the slope changes, with a pivot point around 10 keV. In the soft state, the low energy X-ray emission dominates the bolometric flux which is only loosely correlated with the high energy emission. In black hole binaries in the hard state, the radio flux is generally found to depend on a power of the X-ray flux, F_R ~ F_X^p. We confirm this for Cyg X-1. Our new finding is that this correlation extends to the intermediate and soft states provided the broad-band X-ray flux in the Comptonization part of the spectrum (excluding the blackbody component) is considered instead of a narrow-band medium-energy X-ray flux. We find an index p=1.7+-0.1 for 15 GHz radio emission, decreasing to p=1.5+-0. at 2.25 GHz. We conclude that the higher value at 15 GHz is due to the effect of free-free absorption in the wind from the companion. The intrinsic correlation index remains uncertain. However, based on a theoretical model of the wind in Cyg X-1, it appears to be close to 1.3, which, in the framework of accretion/jet models, implies that the accretion flow in Cyg X-1 is radiatively efficient. The correlation with the flux due to Comptonization emission indicates that the radio jet is launched by the hot electrons in the accretion flow in all spectral states of Cyg X-1. On the other hand, we rule out the X-ray jet model. Finally, we find that the index of the correlation, when measured using the X-ray flux in a narrow energy band, strongly depends on the band chosen and is, in general, different from that for either the bolometric flux or the flux in the hot-electron emission.

Analysis of two models for metal hot-electron power generation

Hot electron emission means that electrons move over potential barriers to come out of the metal when the metal is being heated. Obviously, voltage will generate between electrons and the metal. Based on this, the model of metal hot-electron power generation is built. Free electron model of Sommerfeld is used to describe the movement of electrons in metal. According to the different width of potential barriers, two models are built. One assumes that electrons move from one metal to another mainly by moving over the potential barrier as the barrier is wide enough. The other assumes that the potential barrier is so narrow that electrons mainly move through the potential barrier by tunnel effect. The first model is analyzed and proved strictly, including the building of model, the calculation of open-circuit voltage and the drawing of volt-ampere characteristic curve. The second model is analysed simply. This paper shows that power generation by metal hot-electron is possible based on the theory and can provide reference for researching in power generation of metal hot-electron.

Field-induced hot-electron emission model for wide-band-gap semiconductor nanostructures

For field emission from wide-band-gap semiconductor nanostructures, nonlinear plots on Fowler–Nordheim (FN) coordinates and unacceptably large field enhancement factors (βFN) are often obtained by fitting based on FN equation. In the present work, the field-induced hot-electron emission model is developed and is found to give theoretical findings consistent with the experimental observation. The hot electrons are produced by heating effect of penetration field into the emitting tip of the nanostructure. This energy is expressed by effective electron temperature Te, which is much higher than the temperature of bulk structure. By combining the effective electron temperature with thermal emission function and Murphy and Good integration function, the relation between emission current density and external field is derived and the field enhancement factor (βTe) can be calculated quantitatively. For evaluation of the theoretical model, ZnO nanostructure is selected as a concrete example. The results are found to agree with experiment findings. Extremely large field enhancement factor is not needed in our model and nonlinear property of saturation region emerges in nature in our calculation.

Deposited dielectrics on metal thin films using silicon and glass substrates for hot electron-induced electrochemiluminescence

Electrochemiluminescence by tunnel emission of hot electrons into aqueous solution is a sensitive method for detection of luminophores e.g. rare-earth chelates, which may be used as labels in bioassays. Electrons are injected into solution from an insulating film-coated working electrode, working against a platinum counter electrode. Conductive silicon electrodes with various tunnel dielectric materials e.g. thermal oxide have been used in previous work. In this paper we explore the use of metal thin film electrodes on silicon and glass substrates, using tunneling dielectrics of aluminum oxide and silicon dioxide made by the low-temperature processes of atomic layer deposition or plasma-enhanced chemical vapor deposition.

Collimated hot electron jets generated from subwavelength grating targets irradiated by intense short-pulse laser

Hot electron emission from subwavelength grating targets irradiated by a 60 fs short-pulse laser was investigated at moderate intensities in excess of 1015 W/cm2. Collimated hot electron jets with a divergence angle as small as 5° were observed at the specular reflection direction when the laser beam was incident at the resonance angle for surface plasmon excitation. When the incident angle, which departs from the resonant angle, is increased, the collimated hot electron jet remains near the specular reflection direction, but its intensity attenuates gradually. At the same time, the number of the hot electrons with very large spreading angle emitting along the target normal direction increases with the incident angle.

Ultrafast imaging of photoelectron packets generated from graphite surface

We present an electron projection imaging method to study the ultrafast evolution of photoelectron density distribution and transient fields near the surface. The dynamical profile of the photoelectrons from graphite reveals an origin of a thermionic emission, followed by an adiabatic process leading to electron acceleration and cooling before a freely expanding cloud is established. The hot electron emission is found to couple with a surface charge dipole layer formation, with a sheet density several orders of magnitude higher than that of the vacuum emitted cloud.

Electrogenerated chemiluminescence of chosen terbium complexes deposited into Al 2O 3 layer at the aluminium cathode

Cathodic electrogenerated luminescence of terbium(III)-(acetylacetonate) 3-1,10-phenanthroline, terbium(III)-(salicylate) 3 and terbium(III)-(salicylate) 3-1,10-phenanthroline complexes were studied. In this experiment terbium complexes were coated upon aluminium porous oxide layer by physical adsorption from methanolic or acetonate solution. The hexagonally ordered porous aluminium oxide has been prepared in a two-step anodization process in sulphuric acid solution. The primary step of cathodic ECL in this method is tunnel emission of hot electrons through barrier oxide layer. Electrochemiluminescence was achieved by recombination of the radical species, such as reduced ligand's aromatic moiety and strong oxidizing agent, i.e. sulphate radicals. This study was conducted to investigate ECL spectra, kinetic and integral ECL intensity relative to the porous aluminium oxide layer thickness.

Ultrathin tunnel insulator films on silicon for electrochemiluminescence studies

Thin insulating films on conductive substrates have been used for tunnel emission of hot electrons into aqueous solution. The hydrated electrons thus formed induce electrochemiluminescence (ECL) in various luminophores, e.g. rare-earth metal chelates, which can be detected in sub-nanomolar concentrations. The luminophores can be used as labels for antibodies, enabling simple and highly sensitive immunoassays. This paper compares thermal silicon dioxide, low pressure chemical vapor deposited silicon nitride, plasma enhanced chemical vapor deposited silicon dioxide and nitride, atomic layer deposited alumina, and liquid phase deposited silicon dioxide for electrodes in ECL applications.

Quasielectrostatic whistler wave radiation from the hot electron emission of a laser-produced plasma

Measurements are presented of radiated wave electric fields which result from the creation of a dense, laser-produced plasma within a large, uniform background magnetoplasma. The radiated field patterns are consistent for waves propagating along the quasielectrostatic branch of the whistler wave dispersion curve calculated from the background plasma parameters. The energy source of these waves coincides with an observed energetic tail electron population escaping the laser-produced plasma. A prominent feature of the radiated electric fields is a bipolar spike in both time and space, with a cross-field size near that of the initial escaping electron burst and a duration equivalent to one oscillation at the lower hybrid frequency within the background plasma. Additionally, time-windowed snapshots of the whistler wave radiation patterns are shown to provide a remote diagnostic of the cross-field speed of the laser-produced plasma.

Hot electron emission limited by self-excited fields from targets irradiated by ultra-intense laser pulses

A large number of hot electrons exceeding the Alfvén current can be produced when an ultra-intense laser pulse irradiates a solid target. Self-excited electrostatic and magnetic fields at the target rear could influence the hot electron transport. To investigate the influence, we measure the hot electrons when a pre-plasma is created on the target rear surface and verify an increase of the electron number by a factor of 2 comparing to the no rear plasma case. The increase may be caused because of changes in the electrostatic potential formation process. The retardation of the potential formation is shown using a particle-in-cell simulation. The electron number escaping within the retardation time duration is consistent with our estimation taking the Alfvén current into account.

Analysis of nanometer vacuum gap formation in thermo-tunneling devices

Combined thermionic emission and tunneling of hot electrons (thermo-tunneling) hasemerged as a potential new solid-state cooling technology. Practical implementation ofthermo-tunneling, however, requires the formation of a nanometer-sized gap spanningmacroscopically significant surfaces. This paper describes a numerical and experimentalinvestigation into the formation of a nanometer-sized tunneling gap based on the combinedaction of electrostatic, elastic and Lorentz forces. Experimental data reported herewere used to tune the model and extract estimates for the size of the tunnelingarea and the gap size, respectively. The effect of changing the strength of themagnetic field was also investigated. The presented one-dimensional (1D) analysisof the relative magnitudes of these forces indicates possible stable operation.

Metal-to-semiconductor emission of hot electrons excited on catalytic reaction

The properties necessary for the chemoemission of hot electrons through the interface between a metal film and a semiconductor are established for a metal-semiconductor structure in which hot electrons are excited during a catalytic reaction at the interface between the metal film and the gas mixture. The possibility of generating electric energy using a method based on this effect is substantiated.

Influence of Electrostatic and Magnetic Fields on Hot Electron Emission in Ultra-Intense Laser Matter Interactions

We studied the influence of electrostatic and magnetic fields at the rear surface of solid targets on the hot electron emission generated by ultra-intense laser pulses. The number of emitted electrons increases in experiments when a pre-plasma is created on the target rear surface. The formation of the electrostatic potential is clearly retarded in particle-in-cell simulations resulting in the elongation of time window for hot electron emission. The increase in the number of electrons is consistent with the Alfven current within the elongated time window.

Electron radiography using hot electron jets from sub-millijoule femtosecond laser pulses

Electron jets produced in the intermediate intensity range of 1015 to 1017 W/cm2 from submillijoule 120 fs Ti:Sapphire laser pulses focused to spots of a few microns in diameter have been characterized. The experimental results show strong emission of hot electrons with energies from 80 keV to above 250 keV from microplasmas created with both p- and s-polarized 250 μJ laser pulses. The electron jets with energies above 250 keV are observed to be highly directional. The electron jets are observed in the plane of polarization of the laser electric field for both p- and s-polarized laser pulses. The hot electrons emitted from these femtosecond laser plasmas have also been used for radiographic imaging. It is expected that the short initial duration of these electron pulses would make them useful for time resolved applications.

Generation of free radicals and electrochemiluminescence from simple aromatic molecules in aqueous solutions

Oxide-covered aluminium electrodes were used to demonstrate that aromatic compounds, such as the simple derivatives of benzene, can be electrochemically excited at cathodically pulse-polarized conductor/insulator/electrolyte (C/I/E) tunnel junction electrodes (e.g. oxide-covered aluminium electrodes). The primary cathodic process at these electrodes was a tunnel emission of hot electrons into an aqueous electrolyte solution. Fluorescence (FL) and electrochemiluminescence (ECL) spectra were compared and the dependence of the electrochemiluminescence on the concentrations of benzene, toluene, phenol, p-cresol and aniline were measured and detailed mechanisms for the present electrochemiluminescence are proposed.

Time-Resolved Detection of Hot Electron-Induced Electrochemiluminescence of Fluorescein in Aqueous Solution

Strong electrogenerated chemiluminescence (ECL) of fluorescein is generated during cathodic pulse polarization of oxide-covered aluminum electrodes and the resulting decay of emission is so sluggish that time-resolved detection of fluorescein is feasible. The present ECL in aqueous solution is based on the tunnel emission of hot electrons into the aqueous electrolyte solution, which probably results in the generation of hydrated electrons and hydroxyl radicals acting as redox mediators. The successive one-electron redox steps with the primary radicals result in fluorescein in its lowest excited singlet state. The method allows the detection of fluorescein (or its derivatives containing usable linking groups to biomolecules) over several orders of magnitude of concentration with detection limits well below nanomolar concentration level. The detection limits can still be lowered, e.g., by addition of azide or bromide ions as coreactants. The results suggest that the derivatives of fluorescein, such as fluorescein isothiocyanate (FITC), can be detected by time-resolved measurements and thus be efficiently used as electrochemiluminescent labels in bioaffinity assays.

Energetic electrons and protons generated from the interaction of ultrashort laser pulses with microdroplet plasmas

Generation of fast electrons and protons from ultrashort laser interactions with microdroplet plasmas has been simulated by two-dimensional particle-in-cell simulations. At weakly relativistic laser intensities, two jets of hot electrons are found to emit symmetrically in the backward direction when there is preplasma formed around the droplet surface. These electron jets are associated with resonance absorption around the target surface with a spherical geometry. At relativistic laser intensities, electron bunches generated by the laser ponderomotive force separated by a laser period are superimposed to the resonant hot-electron jets. Relevant with the hot-electron emission, energetic protons can be separated into two groups: one with higher energies mainly emits in certain angles within the laser polarization plane and another with lower energies emits nearly isotropically in all directions. The protons in the first group are accelerated by the electrostatic fields induced by the hot-electron jets due to the resonance absorption, while those in the second group are generated by the ambipolar expansion of the microdroplet plasma.

Thermionic electron emission enhancement of PZT thin films sputter deposition

In this study, PZT thin films are deposited using a novel dual-target (PZT/PbO) RF magnetron sputtering system coupled with plasma enhancement by hot filament thermionic electron emission. The plasma enhancement is expected to allow the deposition temperature to be decreased while retaining the desired properties of the PZT thin films. The influence of the intensified plasma to the structure of the deposited PZT films is investigated and correlated to its ferroelectric properties. The results show that with thermionic enhancement, high remnant and saturation polarization values of 31 μC/cm 2 and 76 μC/cm 2, respectively, are obtained at a deposition temperature of 550 °C if the higher PZT to PbO target power ratio is used; polarization behavior is not observed for the same deposition temperature without thermionic support. For the lower PZT to PbO target power ratio, improved polarization behavior is observed for the coating deposited at 580 °C with thermionic support. Moreover, film growth rate at this power ratio is increased on average by a factor of 1.7 with thermionic enhancement. This study presents the possibility of utilizing a thermionically enhanced plasma to aid in the deposition of ferroelectric PZT thin films by allowing either the deposition time or temperature to be reduced.

Temperature measurements in plasmas generated by using lasers at different intensities

The temperature of laser-generated pulsed plasmas is an important property that depends on many parameters, such as the particle species and the time elapsed from the laser interaction with the matter and the surface characteristics. Laser-generated plasmas with low intensity (<1010 W/cm2) at INFN-LNS of Catania and with high intensity (>1014 W/cm2) in PALS laboratory in Prague have been investigated in terms of temperatures relative to ions, electrons, and neutral species. Time-of-flight (ToF) measurements have been performed with an electrostatic ion energy analyzer (IEA) and with different Faraday cups, in order to measure the ion and electron average velocities. The IEA was also used to measure the ion energy, the ion charge state, and the ion energy distribution. The Maxwell–Boltzmann function permitted to fit the experimental data and to extrapolate the ion temperature of the plasma core. The velocity of the neutrals was measured with a special mass quadrupole spectrometer. The Nd:Yag laser operating at low intensity produced an ion temperature core of the order of 400 eV and a neutral temperature of the order of 100 eV for many ablated materials. The ToF of electrons indicates the presence of hot electron emission with an energy of ∼1 keV.