Angular Distribution Of Emitted Electrons Research Articles

Manipulating the electron dynamics in the non-sequential double ionization process of Ar atoms by an orthogonal two-color laser field

Electron dynamics during non-sequential double ionization (NSDI) is one of the most attractive areas of research in the field of laser–atom or laser–molecule interaction. Based on the classic two-dimensional model, we study the process of NSDI of argon atoms driven by a few-cycle orthogonal two-color laser field composed of 800 nm and 400 nm laser pulses. By changing the relative phase of the two laser pulses, a localized enhancement of NSDI yield is observed at 0.5π and 1.5π, which could be attributed to a rapid and substantial increase in the number of electrons returning to the parent ion within extremely short time intervals at these specific phases. Through the analysis of the electron–electron momentum correlations within different time windows of NSDI events and the angular distributions of emitted electrons in different channels, we observe a more pronounced electron–electron correlation phenomenon in the recollision-induced ionization (RII) channel. This is attributed to the shorter delay time in the RII channel.

Doubly differential cross sections for ionisation in proton–helium collisions at intermediate energies: energy and angular distribution of emitted electrons

Using the two-centre wave-packet convergent close-coupling approach, we continue our study of the proton–helium collision system. This method uses a correlated two-electron wave function to describe the helium target and discretises the continuum using wave-packet pseudostates. The cross section differential in the electron-emission energy and emission angle is calculated for incident-projectile energies in the intermediate range from 70 to 300 keV, where coupling between various channels and electron–electron correlation effects are important. We also apply an alternative, simpler approach that reduces the target to an effective single-electron system. Overall, the present results from both methods agree well with the available experimental data. This positions both implementations of the two-centre wave-packet convergent close-coupling approach well to further study other doubly differential, as well as fully differential, cross sections of single ionisation in proton–helium collisions.Graphical abstract

Prediction of Betavoltaic Battery Parameters

The approaches for predicting output parameters of betavoltaic batteries are reviewed. The need to develop a strategy for predicting these parameters with sufficient accuracy for the optimization of betavoltaic cell design without using the simple trial and error approach is discussed. The strengths and weaknesses of previously proposed approaches for the prediction are considered. Possible reasons for the difference between the calculated and measured parameters are analyzed. The depth dependencies of beta particles deposited energy for Si, SiC, GaN, and Ga2O3 and 20% purity 63Ni and titanium tritide as radioisotope sources are simulated using the Monte Carlo algorithm taking into account the full beta energy spectrum, the isotropic angular distribution of emitted electrons and the self-absorption inside the radioisotope source for homogeneously distributed emitting points. The maximum short circuit current densities for the same semiconductors and radioisotope sources are calculated. The methodology allowing the prediction of betavoltaic cell output parameters with accuracy no worse than 30% is described. The results of experimental and theoretical investigations of the temperature dependence of betavoltaic cell output parameters are briefly discussed. The radiation damage by electrons with the subthreshold energy and the need to develop models for its prediction is considered.

Single ionization of an asymmetric diatomic system by relativistic charged projectiles

We study single ionization of a heteroatomic system by charged projectiles whose velocity $v$ approaches the speed of light $c$. The system is formed by two loosely bound atomic species, $A$ and $B$, with the ionization potential of $A$ being smaller than excitation energy for a dipole-allowed transition in $B$. In such a case, three single ionization channels occur: (i) single-center ionization of atom $A$, (ii) single-center ionization of atom $B$, and (iii) two-center ionization of $A$. While (i) and (ii) are the well known mechanism of direct impact ionization of a single atom, in channel (iii) ionization of $A$ proceeds via impact excitation of $B$ with consequent radiationless transfer of excitation energy---via (long-range) two-center electron-electron correlations---to $A$, leading to its ionization. We show that, close to the resonance energy, the two-center channel (iii) is so enormously strong that its contribution remains dominant even if the range of emission energies $\ensuremath{\sim}1$ eV, which is orders of magnitude broader than its width, is considered. The influence of relativistic effects, caused by a high collision velocity, on the angular distribution of emitted electrons may be quite strong even at $\ensuremath{\gamma}=1/\sqrt{1\ensuremath{-}{v}^{2}/{c}^{2}}\ensuremath{\approx}2$. However, in the energy distribution and the total cross section, these effects become substantial only at $\ensuremath{\gamma}\ensuremath{\gg}1$. Relativistic effects arising due to a large size of the two-atomic system are shown to be very weak even for a $^{7}\mathrm{Li}\text{\ensuremath{-}}\mathrm{He}$ dimer whose mean size is about 28 \AA{}.

Resonant single-photon double ionization driven by combined intra- and interatomic electron correlations

The double ionization of an atom by single-photon absorption in the presence of a neighbouring atom is studied. The latter is, first, resonantly photoexcited and, afterwards, transfers the excitation energy radiationlessly to the other atom, leading to its double ionization. The process relies on the combined effect of interatomic and intra-atomic electron correlations. It can dominate over the direct double photoionization by several orders of magnitude at interatomic distances up to a few nanometers. The relative position of the neighbouring atom is shown to exert a characteristic influence on the angular distribution of emitted electrons.

A quantum relaxation-time approximation for finite fermion systems

We propose a relaxation time approximation for the description of the dynamics of strongly excited fermion systems. Our approach is based on time-dependent density functional theory at the level of the local density approximation. This mean-field picture is augmented by collisional correlations handled in relaxation time approximation which is inspired from the corresponding semi-classical picture. The method involves the estimate of microscopic relaxation rates/times which is presently taken from the well established semi-classical experience. The relaxation time approximation implies evaluation of the instantaneous equilibrium state towards which the dynamical state is progressively driven at the pace of the microscopic relaxation time. As test case, we consider Na clusters of various sizes excited either by a swift ion projectile or by a short and intense laser pulse, driven in various dynamical regimes ranging from linear to strongly non-linear reactions. We observe a strong effect of dissipation on sensitive observables such as net ionization and angular distributions of emitted electrons. The effect is especially large for moderate excitations where typical relaxation/dissipation time scales efficiently compete with ionization for dissipating the available excitation energy. Technical details on the actual procedure to implement a working recipe of such a quantum relaxation approximation are given in appendices for completeness.

Quantitative modeling of electron spectroscopy intensities for supported nanoparticles: The hemispherical cap model for non-normal detection

Nanoparticles of one element or compound dispersed across the surface of another substrate element or compound form the basis for many materials of great technological importance, such as heterogeneous catalysts, fuel cells and other electrocatalysts, photocatalysts, chemical sensors and biomaterials. They also form during film growth by deposition in many fabrication processes. The average size and number density of such nanoparticles are often very important, and these can be estimated with electron microscopy or scanning tunneling microscopy. However, this is very time consuming and often unavailable with sufficient resolution when the particle size is ~1nm. Because the probe depth of electron spectroscopies like X-Ray Photoelectron Spectroscopy (XPS) or Auger Electron Spectroscopy (AES) is ~1nm, these provide quantitative information on both the total amount of adsorbed material when it is in the form of such small nanoparticles, and the particle thickness. For electron spectroscopy conducted with electron detection normal to the surface, Diebold et al. (1993) derived analytical relationships between the signal intensities for the adsorbate and substrate and the particles' average size and number density, under the assumption that all the particles have hemispherical shape and the same radius. In this paper, we report a simple angle- and particle-size-dependent correction factor that can be applied to these analytical expressions so that they can also be extended to measurements made at other detection angles away from the surface normal. This correction factor is computed using numerical integration and presented for use in future modeling. This correction factor is large (>2) for angles beyond 60°, so comparing model predictions to measurements at both 0° and ≥60° will also provide a new means for testing the model's assumptions (hemispherical shape and fixed size particles). The ability to compare the hemispherical cap model at several angles simultaneously also should enable more accurate estimates of surface structural parameters when elastic diffraction effects cause strong peaks in the angular distributions of emitted electrons.

Atomic ionization of hydrogen-like ions by twisted photons: angular distribution of emitted electrons

We investigate the angular distribution of electrons that are emitted in the ionization of hydrogen-like ions by twisted photons. Analysis is performed based on the first-order perturbation theory and the non-relativistic Schrödinger equation. Special attention is paid to the dependence of the electron emission pattern on the impact parameter b of the ion with respect to the centre of the twisted wave front. In order to explore such a dependence, detailed calculations were carried out for the photoionization of the 1s ground and 2 py excited states of neutral hydrogen atoms. Based on these calculations, we argue that for relatively small impact parameters, the electron angular distributions may be strongly affected by altering the position of the atom within the wave front. In contrast, if the atom is placed far from the front centre, the emission pattern of the electrons is independent of the impact parameter b and resembles that observed in the photoionization by plane wave photons.

The dependence of the forward-backward asymmetry parameter on the molecular orientation in angular distributions of emitted electrons in the H2 ionisation induced by bare ion impact

We have studied the angular distributions of electron emission in collisions of bare ions with H2 oriented target using molecular wave functions calculated with the Schwinger variational iteractive method within the Continuum Distorted Wave-Eikonal Initial State approximation (CDW-EIS-SVIM). By calculating the forward-backward asymmetry parameter for different molecular orientations we show that this parameter depend not only on the perturbation strength, Sp = Zp/vp, where Zp and vp are respectively the projectile charge and velocity (two-centre effect) and on the outgoing electron velocity, but may also depend on how the molecule is aligned with respect to the projectile incident axis. Doubly differential cross sections (DDCS) for fixed in space molecular target were calculated. The asymmetry parameter was compared with both experimental and theoretical orientation averaged results.

Erratum: “Angular distribution of emitted electrons due to intense p-polarized laser foil interaction” [Phys. Plasmas 17, 033101 (2010)

Erratum: “Angular distribution of emitted electrons due to intense <i>p</i>-polarized laser foil interaction” [Phys. Plasmas 17, 033101 (2010)

Discrete dipole approximation for low-energy photoelectron emission from NaCl nanoparticles

This work presents a model for the photoemission of electrons from sodium chloride nanoparticles 50–500 nm in size, illuminated by vacuum ultraviolet light with energy ranging from 9.4 to 10.9 eV. The discrete dipole approximation is used to calculate the electromagnetic field inside the particles, from which the two-dimensional angular distribution of emitted electrons is simulated. The emission is found to favor the particle's geometrically illuminated side, and this asymmetry is compared to previous measurements performed at the Lawrence Berkeley National Laboratory. By modeling the nanoparticles as spheres, the Berkeley group is able to semi-quantitatively account for the observed asymmetry. Here however, the particles are modeled as cubes, which are closer to their actual shape, and the interaction of an emitted electron with the particle surface is also considered. The end result shows that the modeled emission asymmetry for these low-energy electrons is more sensitive to the interaction with the particle-surface than to the specific particle shape, i.e., a sphere or cube.

Angular distribution of emitted electrons due to intense p-polarized laser foil interaction

The angular distribution of electrons emitting from a foil surface illuminated by p-polarized laser pulses is studied using particle-in-cell simulation for incident angles of θ1=22.5°, 45°, 67.5° and laser amplitudes of a=0.5, 1, 2. Theoretical prediction of the emission direction, based on canonical momentum conservation along the target surface, is verified. Surface ablation, the Alfvén current limit, as well as self-generated electromagnetic fields on the surface are numerically investigated and found to play important roles in the modulation of the angular distribution of the emitted electrons. The emitted electrons of higher energy are found to be directly accelerated to near the polarization direction of the incident laser light. The simulation results agree very well with the recent experimental results from Al targets irradiated by a 60 fs, 180 mJ laser pulse.

Scintillator avalanche photoconductor with high resolution emitter readout for low dose x-ray imaging: Lag

A new concept of indirect conversion flat-panel imager with avalanche gain and field emitter array (FEA) readout is being investigated. It is referred to as scintillator avalanche photoconductor with high resolution emitter readout (SAPHIRE). The present work investigates the temporal performance, i.e., lag, of SAPHIRE. Since the temporal performance of the x-ray detection materials, i.e., the structured scintillator and avalanche amorphous selenium (a-Se) photoconductor, has been studied previously, the investigation is focused on lag due to the FEA readout method. The principle of FEA readout is similar to that of scanning electron beam readout used in camera tubes, where the dominant source of lag is the energy spread of electrons. Since the principles of emission and beam focusing methods for FEA are different from thermionic emission used in camera tubes, its electron beam energy spread and hence lag is expected to be different. In the present work, the energy spread of the electrons emitted from a FEA was investigated theoretically by analyzing different contributing factors due to the FEA design and operations: The inherent energy spread of field emission, the FEA driving pulse delay, and the angular distribution of emitted electrons. The electron energy spread determined the beam acceptance characteristic curve of the photoconductive target, i.e., the accepted beam current (I(a)) as a function of target potential (V(t)), from which lag could be calculated numerically. Lag calculation was performed using FEA parameters of two prototype HARP-FEA image sensors, and the results were compared with experimental measurements. Strategies for reducing lag in SAPHIRE were proposed and analyzed. The theoretical analysis shows that the dominant factor for lag is the angular distribution of electrons emitted from the FEA. The first frame lags for two prototype sensors with 4 and 25 microm HARP layer thicknesses were 62.1% and 9.1%, respectively. A lag clearance procedure can be implemented by turning on all the FEA pixels simultaneously between subsequent frames without negative impact of readout speed. For large-area SAPHIRE, the bias electrode for the HARP needs to be divided into strips to allow parallel readout. With typical cardiac detector parameters, SAPHIRE with 128 parallel strips can provide real-time readout (30 frames/s) with first frame lag of -4%. The investigation of lag in SAPHIRE shows that the angular distribution of emitted electrons from FEA can result in substantial lag if the readout was performed pixel by pixel. Effective strategies for reducing lag include dividing the bias electrode into multiple strips to allow parallel readout and the incorporation of rapid charge clearance procedure between subsequent frames or rows.

Sequential two-photon double ionization of Kr atoms

Sequential two-photon double ionization of Kr atoms is theoretically considered. The angular distribution of emitted electrons is calculated with the dipole amplitudes evaluated within the multiconfigurational Hartree–Fock and Dirac–Fock approaches. The difference in angular distributions for two-photon and single-photon ionization is discussed.

Electron emission induced by grazing impact ofH+andHe+ions on a Cu(001) surface: Low-energy electron diffraction study

Electron emission induced by ${\text{H}}^{+}$ and ${\text{He}}^{+}$ ions with energies up to 30 keV is studied for grazing impact on a clean and flat Cu(001) surface by making use of electron diffraction effects. Depending on the azimuthal orientation of low index directions in the crystal surface with respect to the projectile beam, we observe intensity spots in the angular distributions of emitted electrons. The data are analyzed in the framework of electron diffraction at the ordered surface and reveal interesting details on the emission and coherence mechanisms for the excitation of conduction electrons by fast atomic projectiles in the surface region of a metal.

Atomic and molecular signatures for charged-particle ionization

Analysis of the ejection of electrons in a plane perpendicular to an incident electron beam reveals unexpected differences between the ionization behaviour of atoms and molecules. For molecules that have nuclei at their centres of mass, the angular distribution of emitted electrons is similar to that of atoms. But for those that don’t, the shape of this distribution is qualitatively different.

Measurement of double beta decay of 100Mo to excited states in the NEMO 3 experiment

The double beta decay of 100Mo to the 0 1 + and 2 1 + excited states of 100Ru is studied using the NEMO 3 data. After the analysis of 8024 h of data the half-life for the two-neutrino double beta decay of 100Mo to the excited 0 1 + state is measured to be T 1 / 2 ( 2 ν ) = [ 5.7 −0.9 +1.3 ( stat. ) ± 0.8 ( syst. ) ] × 10 20 y . The signal-to-background ratio is equal to 3. Information about energy and angular distributions of emitted electrons is also obtained. No evidence for neutrinoless double beta decay to the excited 0 1 + state has been found. The corresponding half-life limit is T 1 / 2 ( 0 ν ) ( 0 + → 0 1 + ) > 8.9 × 10 22 y (at 90% C.L.). The search for the double beta decay to the 2 1 + excited state has allowed the determination of limits on the half-life for the two neutrino mode T 1 / 2 ( 2 ν ) ( 0 + → 2 1 + ) > 1.1 × 10 21 y (at 90% C.L.) and for the neutrinoless mode T 1 / 2 ( 0 ν ) ( 0 + → 2 1 + ) > 1.6 × 10 23 y (at 90% C.L.).

Dissociative ionization of H2 by fast protons: three-body break-up and molecular-frame electron emission

Highly differential cross sections have been obtained for dissociative single ionization of H2 by 6 MeV proton impact by measuring the momentum vectors of the electron and the H+ fragment in coincidence. The investigation of the momentum balance of the fragments along the projectile beam provided detailed insight into the four-particle dynamics, even though the H atom is not detected. Within the axial recoil approximation, molecular-frame angular distributions of emitted electrons have been determined for molecules oriented perpendicular to the projectile beam. They are compared with the predictions of a CDW-EIS calculation.

Angular distribution of emitted electrons in sodium clusters: A semiclassical approach

We present a theoretical study of the angular distribution of emitted electrons of a sodium cluster, irradiated by short and intense laser pulses. While the polarization of the excitation field tends to focus a directional emission, the dynamical correlations tend to thermalize the electrons, giving rise to a more isotropic ionization. The competition between these processes is investigated using a semiclassical model Vlasov-\"Uhling-Uhlenbeck, where the dynamical correlations are taken in account by the electron-electron correlations in the Markovian approximation, the widely known \"Uhling-Uhlenbeck collision term. The results are compared to a semiclassical pure mean-field propagation (Vlasov equation) to work out the influence of dynamical correlations on the angular distribution of the electron emission. The trends with laser intensity and frequency are explored. The time evolution of the angular distributions shows that direct emission processes are stronger in the early phase of the processs, while isotropic thermal emission dominates later.

‘Characterization of the intermediate and final states of the2p3/2−14presonant Auger spectra of HCl

The intensity and angular distribution of emitted electrons have been measured at several photon energies at the Cl ${2p}_{3/2}\ensuremath{\rightarrow}4p(\ensuremath{\sigma},\ensuremath{\pi})$ resonances of the HCl molecule. The orientation of the core hole is concluded from the overall intensity distribution. The final states of resonant Auger transitions are identified with the aid of angular distribution that together with the final-state energy splittings, reveals the \ensuremath{\sigma} or \ensuremath{\pi} nature of the excited electron also in the intermediate state.