Quasi-free Electrons Research Articles

Generalized Thermionic Emission for Arbitrary Dimension

Thermionic emission for quasi-free electron metals is investigated in arbitrary dimension. The thermionic emission for a constant temperature and work function is calculated for one, two and three dimensions. The generalized current density of the thermionic emission is calculated for arbitrary dimension. The current density is shown as a function of temperature for different work functions. The current density is also shown as a function of dimension for different temperatures and work functions.

Observation of dissociative quasi-free electron attachment to nucleoside via excited anion radical in solution

Damage to DNA via dissociative electron attachment has been well-studied in both the gas and condensed phases; however, understanding this process in bulk solution at a fundamental level is still a challenge. Here, we use a picosecond pulse of a high energy electron beam to generate electrons in liquid diethylene glycol and observe the electron attachment dynamics to ribothymidine at different stages of electron relaxation. Our transient spectroscopic results reveal that the quasi-free electron with energy near the conduction band effectively attaches to ribothymidine leading to a new absorbing species that is characterized in the UV-visible region. This species exhibits a nearly concentration-independent decay with a time constant of ~350 ps. From time-resolved studies under different conditions, combined with data analysis and theoretical calculations, we assign this intermediate to an excited anion radical that undergoes N1-C1′ glycosidic bond dissociation rather than relaxation to its ground state.

Probing the coupling of a dipole-bound electron with a molecular core.

A dipolar molecule can weakly bind an electron in a diffuse orbital. However, the spin-orbit coupling between this weakly bound electron and the electrons in the molecular core is not known. Here we probe this coupling using the linear C2P- anion with the 3Σ+ ground state, which possesses dipole-bound excited states because neutral C2P (2Π) has a sufficiently large dipole moment. Photodetachment spectroscopy and resonant photoelectron spectroscopy are used to probe the nature of the dipole-bound states. Two dipole-bound excited states are observed with a binding energy of 37 cm-1, corresponding to the two spin-orbit states of neutral C2P (2Π1/2 and 2Π3/2). The current study demonstrates that the weakly bound electron in the dipole-bound excited states of C2P- is not spin-coupled to the electrons in the C2P core and can be considered as a quasi-free electron.

From strong-field physics in and at nanoscale matter to photonics-based laser accelerators

New ways of controlling quasi-free and free electrons by means of phase-controlled ultrashort laser pulses are demonstrated: from strong-field physics in the conducting 2-d material graphene and at the surface of nanostructures, to laser acceleration of free electrons with a nanophotonic structure, and the demonstration of the longitudinal Kapitza-Dirac effect.

Development of a 20MeV proton particle-induced X-ray emission analysis method in a helium atmosphere

We developed a 20[Formula: see text]MeV particle-induced X-ray emission (PIXE) analysis method using a medical cyclotron, which is conventionally used for positron emission tomography analysis performed in vacuo, during which the target sample is damaged. For non-destructive analysis and ease of switching between target samples, we developed a technique allowing 20[Formula: see text]MeV proton PIXE analysis to be performed at the atmospheric pressure. We filled the PIXE analysis chamber with helium and checked that the continuous background of the Compton tails of nuclear reaction [Formula: see text]-rays increased only minimally, and that the quasi-free electron bremsstrahlung (QFEB) did not increase at all, in the X-ray energy spectrum.

Influence of electron quantum confinement on the strength of carbon nanotube bundles

Radial confinement and specific boundary conditions lead to inhomogeneous spatial distributions for conduction electrons in metallic carbon nanotubes.Such uneven spread of negative charge on the surface of a carbon nanotube can induce Coulomb-type interactions between parallel tubes, contributing to the relative friction. Thus, the axial conduction electron density on a single idealized carbon nanotube closed at both ends was obtained in the framework of the two-dimensional quasi-free electron approximation. A Coulomb potential energy between two parallel nanotubes was calculated as a function of the longitudinal offset between them. The study provides a simple method for estimating the friction force related to Coulomb inter-tube interactions appearing during a parallel sliding motion, with implications in the stretching resistance of various carbon nanotube bundles and in designing various nano-electromechanical devices.

Transverse Momentum Transfer Distributions Following Single Ionization in 3.6 MeV/amu Au53+ + He Collisions: A 4-Body Classical Treatment

A four-body classical model based on the resolution of Hamilton equations of motion was used here to determine and analyze ionization doubly-differential cross sections following 3.6 MeV/amu Au53+ + He collisions. Our calculation was not able to reproduce the binary peaks experimentally observed in the transverse momentum distributions for electron emission energies larger than 10 eV. Surprisingly, by introducing a large number of free or quasi-free electrons that followed the projectile at the same velocity, the agreement between the experiment and our calculation was improved, since our model reproduced, at least qualitatively, the experimental binary peaks. The origin of the presence of such electrons is discussed.

Electronic ricochet explosion of metal wire

An unknown type of phase transition of a metal from the solid state to a liquefaction one at a temperature T<740 K is established, which is much lower than the melting point of the investigated Ni. This effect we called an electronic ricochet. The transition arises in the ordinary lamp inside its nickel wire due to an electric explosion of the tungsten filament connected to it. The medium of its propagation was quasi free electrons, which removed the ricochet far from the site of the explosion. The ratio of the volume Vd of the formed Ni drops to the volume Vc of the solid conductor from which these droplets increased was Vd/Vc≈0.2. This means that at least 20% of the mass of the solid metallic conductor was subjected to such an unusual phase transition. The result of the action of the electronic ricochet was found in six lamps out of hundreds of tested samples.

Energy of the quasi-free electron in CO and HD: Probing intermolecular potentials within the local Wigner-Seitz model.

We present the quasi-free electron energy V0(ρ) in the weakly polar fluids CO and HD from gas to liquid densities ρ, on noncritical isotherms, and at a temperature near the critical isotherm. These results represent the first systematic investigation of V0(ρ) in polar fluids across a broad density range and illustrate that field enhanced photoemission can be used to obtain data in such systems. We show that the local Wigner-Seitz model for V0(ρ), when coupled with thermodynamic data for the fluids, can yield optimized intermolecular potential parameters, as well as the magnitude of the zero kinetic energy electron scattering length.

Low-Energy Electron Emission in the Strong-Field Ionization of Rare Gas Clusters.

Clusters and nanoparticles have been widely investigated to determine how plasmonic near fields influence the strong-field induced energetic electron emission from finite systems. We focus on the contrary, i.e., the slow electrons, and discuss a hitherto unidentified low-energy structure (LES) in the photoemission spectra of rare gas clusters in intense near-infrared laser pulses. For Ar and Kr clusters we find, besides field-driven fast electrons, a robust and nearly isotropic emission of electrons with <4 eV kinetic energies that dominates the total yield. Molecular dynamics simulations reveal a correlated few-body decay process involving quasifree electrons and multiply excited ions in the nonequilibrium nanoplasma that results in a dominant LES feature. Our results indicate that the LES emission occurs after significant nanoplasma expansion, and that it is a generic phenomenon in intense laser nanoparticle interactions, which is likely to influence the formation of highly charged ions.

Self-Assembly of Metal Oxide Nanoparticles in Liquid Metal toward Nucleation Control for Graphene Single-Crystal Arrays

Summary We have achieved the spontaneous assembly of alumina nanoparticles on a liquid metal surface and further synthesized a very-large-scale graphene single-crystal array. The effective electrical field established as a result of the electrical charge of the alumina nanoparticles in the liquid metal ensures the ordered self-assembly of the alumina islands and lays the foundation for precise spatial control of graphene nucleation, leading to the formation of graphene single-crystal arrays. In addition, controlling the density of the alumina in the liquid metal can precisely tune the periodicity of the arrays. This simple strategy, which is unlike the usual solid/aqueous solution system, opens up new territory for nanoparticle assembly. Focusing on the origin of the nanoparticle interaction that drives the self-assembly of ordered structure in the liquid metal system is a promising avenue for triggering more interesting applications.

Extraction efficiency of drifting electrons in a two-phase xenon time projection chamber

We present a measurement of the extraction efficiency of quasi-free electrons from the liquid into the gas phase in a two-phase xenon time-projection chamber. The measurements span a range of electric fields from 2.4 to 7.1 kV/cm in the liquid xenon, corresponding to 4.5 to 13.1 kV/cm in the gaseous xenon. Extraction efficiency continues to increase at the highest extraction fields, implying that additional charge signal may be attained in two-phase xenon detectors through careful high-voltage engineering of the gate-anode region.

Positronium collisions with atoms and molecules

We review recent theoretical efforts to explain observed similarities between electron-atom and positronium(Ps)-atom scattering which also extends to molecular targets. In the range of the projectile velocities above the threshold for Ps ionization (break-up) this similarity can be explained in terms of quasi-free electron scattering and impulse approximation. However, for lower Ps velocities more sophisticated methods should be developed. Our calculations of Ps scattering by heavy noble-gas atoms agree well with experiments at Ps velocities above the Ps ionization threshold. However, in contrast to electron scattering cross sections, at lower velocities they exhibit maxima whereas the experimental cross sections tend to decrease toward lower velocities indicating the same similarity with electron scattering cross section observed above the threshold. Our preliminary results for Ps-N2 scattering confirm experimental observation of a resonance similar to the ∏g resonance in electron-N2 scattering.

Quantum field theory treatment of magnetic effects on a system of free electrons

The effects of a magnetic field on the energy and on the spin of free electrons are computed in the theoretical framework of quantum field theory. In the case of a static moderate field and with relatively slow electrons, the derived formulae are particularly simple. A comparison with the approaches of classical physics and of quantum mechanics shows essential differences and important analogies. The relevance to the magnetic effects of the initial polarization components of the electron states and the possible existence of special values of these quantities are discussed in the final conclusions, which might be useful to explain recent experiments on quasi-free electrons in chiral systems in biology.

Enegretics of the Ps Formation: Role of the Solvated Electron

Taking liquid water as an example, we have considered energetics of the positronium formation. It is shown that quasi-free Ps as well as the Ps localized in a bubble cannot decay into hydrated electron and positron. The most probable value of the positron work function in water, V0+, is 1.5…2 eV. Ps formation from the hydrated electron and positron is energetically possible. By the end of thermalization major fraction of positrons escapes the blob and hydrates outside. The low-mobile е+aq has no time to diffuse back and form Ps with intrablob e-. These е+aq mostly annihilate as “free positrons”. Positrons, which are thermalized within the blob, recombine with the quasi-free intrablob electrons and form Ps.

Resonant scattering of positronium as a quasifree electron

In order to clarify the physics underlying the observations of the electronlike behavior of positronium (Ps) and its resonant scattering from CO2, we have measured the Ps+N2 total cross section and found it also to exhibit significant structure. Analysis of the resonances reveals that Ps is distorted in the collisions and classical trajectory Monte Carlo calculations indicate that the electron is on average closer to the target than the positron, which may in turn bind resonantly to the ensuing temporary negative ion. This description of the nature of Ps resonances agrees with long-standing theoretical predictions.

Plasmon effects in photoemission

Recently, we observe an increasing interest in the laser-assisted processes that take place at the solids surfaces or at the interfaces of semiconductor heterostructures. These processes are affected by the plasmon effects induced by the interaction of quasi-free electrons in solids with laser fields. The plasmon effects lead to a significant enhancement of the oscillating in time electric field in a vicinity of the surface. The latter strongly modifies processes such as the high-order harmonic generation from atoms passing close to the surface, the electron photoemission from metal tips or the tunneling processes in semiconductor heterostructures. In our paper, we focus on the photoemission phenomenon accounting for the space-dependence of an oscillating in time, effective electric field created at the surface.

Visualizing the interface state of PTCDA on Au(111) by scanning tunneling microscopy

We have investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS) the electronic structure of PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) molecular monolayers grown on Au(111). Thanks to our STM/STS measurements, performed under ultra-high vacuum conditions and low temperature, an interface state directly derived from the Shockley-type surface state of pristine Au(111) has been detected. Low bias voltage STM images show the formation of standing wave patterns both on Au(111) and on Au(111) covered by a PTCDA monolayer. These patterns result from the scattering of quasi-free 2D electron surface states with surface defects. By Fourier transforming STM images, the corresponding wavevectors have been extracted. In particular, the simultaneous imaging of both pristine and PTCDA covered Au(111) areas has allowed to measure the Fermi contours and the Fermi wavevectors of both systems. These measurements show that one monolayer PTCDA on Au(111) presents an interface state with an isotropic circular Fermi contour and smaller Fermi wavector () than the corresponding Fermi wavector of pristine Au(111) (). This picture is consistent with an upward shift of the Shockley-type surface state due to the presence of the molecular monolayer.

Alternative treatment for the energy-transfer and transport cross section in dressed electron-ion binary collisions

A formula for determining the electronic stopping power and the transport cross section in electron-ion binary collisions is derived from the induced density for spherically symmetric potentials using the partial-wave expansion. In contrast to the previous one found in many textbooks, the present formula converges to the Bethe and Bloch stopping-power formulas at high ion velocities and agrees rather well with experimental stopping-power data, as shown here for Al, C, and ${\mathrm{H}}_{2}\mathrm{O}$ targets. It can be employed in plasma physics and particularly in any application that requires electronic stopping-power values of quasifree electrons with high accuracy.

Red and blue shift of liquid water’s excited states: A many body perturbation study

In the present paper, accurate optical absorption spectrum of liquid H2O is calculated in the energy range of 5–20 eV to probe the nature of water’s excited states by means of many body perturbation approach. Main features of recent inelastic X-ray measurements are well reproduced, such as a bound excitonic peak at 7.9 eV with a shoulder at 9.4 eV as well as the absorption maximum at 13.9 eV, followed by a broad shoulder at 18.4 eV. The spectrum is dominated by excitonic effects impacting the structures of the spectrum at low and higher energy regimes mixed by single particle effects at high energies. The exciton distribution of the low-energy states, in particular of S1, is highly anisotropic and localized mostly on one water molecule. The S1 state is essentially a HOCO-LUCO (highest occupied crystal orbital - lowest unoccupied crystal orbital) transition and of intra-molecular type, showing a localized valence character. Once the excitation energy is increased, a significant change in the character of the electronically excited states occurs, characterized through emergence of multiple quasi-particle peaks at 7.9 eV in the quasi-particle (QP) transition profile and in the occurring delocalized exciton density distribution, spread over many more water molecules. The exciton delocalization following a change of the character of excited states at around 7.9 eV causes the blue shift of the first absorption band with respect to water monomer S1. However, due to reduction of the electronic band gap from gas to liquid phase, following enhanced screening upon condensation, the localized S1 state of liquid water is red-shifted with respect to S1 state of water monomer. For higher excitations, near vertical ionization energy (11 eV), quasi-free electrons emerge, in agreement with the conduction band electron picture. Furthermore, the occurring red and blue shift of the excited states are independent of the coupling of resonant and anti-resonant contributions to the spectrum.