Distribution Of Emitted Electrons Research Articles

Demonstration of full polarization control of soft X-ray pulses with Apple X undulators at SwissFEL using recoil ion momentum spectroscopy.

The ability to freely control the polarization of X-rays enables measurement techniques relying on circular or linear dichroism, which have become indispensable tools for characterizing the properties of chiral molecules or magnetic structures. Therefore, the demand for polarization control in X-ray free-electron lasers is increasing to enable polarization-sensitive dynamical studies on ultrafast time scales. The soft X-ray branch Athos of SwissFEL was designed with the aim of providing freely adjustable and arbitrary polarization by building its undulator solely from modules of the novel Apple X type. In this paper, the magnetic model of the linear inclined and circular Apple X polarization schemes are studied. The polarization is characterized by measuring the angular electron emission distributions of helium for various polarizations using cold target recoil ion momentum spectroscopy. The generation of fully linear polarized light of arbitrary angle, as well as elliptical polarizations of varying degree, are demonstrated.

Doubly differential cross sections for ionization in proton collisions with atomic hydrogen: Energy and angular distribution of emitted electrons

Doubly differential cross sections for ionization in proton collisions with atomic hydrogen: Energy and angular distribution of emitted electrons

Theoretical triple differential cross sections for ionization of N 2 and CH 4 molecules by electron impact

We present the angular distribution of electron emission by calculating the triple differential cross sections for (e, 2e) process on N 2 and CH 4 by using the three-body formalism of molecular first Born approximation (MFBA), two-Coulomb wave (M2CW), and three-Coulomb wave (M3CW) models, respectively. In these models, a Coulomb distorted wave is considered for the motion of the incident electron. We have considered the continuum-continuum correlation effect by choosing the final state as the three-Coulomb and two-Coulomb wave functions in M3CW and M2CW models, whereas the ejected electron is affected by a single centre field of the target in the MFBA model. In the M2CW model, the interaction between scattered electron-residual target ion has been described as a plane wave. The distinguishing feature among the three models has been noted in the TDCS as a strong binary peak with and without a recoil peak for several electron emission energies at fixed scattering angle. In the case of N 2 molecule, the TDCS shows oscillatory behaviour with the variation of the electron emission angle. The positions of the binary peak obtained by our theoretical models are well established by the experimental findings, but a large deviation is found in the region of the recoil peak. The contributions of TDCS for different molecular orbitals of the molecules to the spectrum of angular distributions at different electron emission energies have also been analyzed. Finally, a comparison is made with the measurement in coplanar asymmetry geometry. Overall, good agreement was found between experiments and M3CW theory.

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.

Excitation dynamics in molecule resolved by internuclear distance driven by the strong laser field.

Rydberg-state excitation of stretched model molecules subjected to near-infrared intense laser fields has been investigated based on a fully quantum model (QM) proposed recently and the numerical solutions of time-dependent Schrödinger equation (TDSE). Given the good agreement between QM and TDSE, it is found that, as the molecules are stretched, the electron tends to be trapped into low-lying Rydberg-states after its ionization from the core, which can be attributed to the shift of the ionization moments corresponding to maximum excitation populations. Moreover, the n-distribution is broadened for molecules with increasing internuclear distance, which results from the change of momentum distribution of emitted electrons. Analysis indicates that both of the above phenomena are closely related to the interference effect of electronic wave packets emitted from different nuclei. Our study provides a more comprehensive understanding of the molecular excitation in intense laser fields, as well as a means of possible applications to related experimental observations.

Ionization of uracil molecule in collisions with 2.5-MeV/u Si12+ ions

The energy and angular distribution of electron emission from a RNA base molecule, uracil (C 4 H 4 N 2 O 2), are investigated in collisions with 2.5-MeV/u Si12+ ions under high perturbation strength. The absolute double differential cross sections (DDCS) are measured using electron spectroscopy for emission angles between 20° to 160° in the electron energy range 1-620 eV. The single differential cross sections (SDCS) are evaluated by integrating the DDCS over emission angles or energies. The measured cross sections (DDCS and SDCS) are compared with the state-of-the-art continuum distorted wave-eikonal initial state (CDW-EIS) theoretical model. The DDCS obtained by the CDW-EIS is found to provide better agreement with the measurements in the backward angles. With an increase in perturbation strength from 0.5 to 1.19 a.u., the DDCS is found to increase by 10 times in comparison to the earlier reported uracil data for 3.5-MeV/u bare C-ions [Phys. Rev. A 87, 032716 (2013)]. The significant enhancement in the TCS is found for the total cross section in case of Si12+ ions over that predicted by a scaling law as derived for lower charge state projectiles. The forward-backward angular asymmetry is found to increase monotonically with the velocity of emitted electrons and shows very good agreement with the model. For a comparative study, the DDCS is also measured for oxygen molecule using same ion impact, which is found to be about ten times lower than that for the uracil. The forward-backward angular asymmetry for oxygen is found to be almost the same as uracil.

Optical measurement of the work function and the field reduction factor of metallic needle tips.

Quintessential parameters for needle tip-based electron sources are the work function, the tip apex radius, and the field reduction factor. They determine the static emission properties and strongly influence laser-triggered photoemission experiments at these needle tips. We present a simple method based on photoemission with two different commonly available continuous-wave laser diodes to determine both parameters in situ. We demonstrate our technique at tungsten needle tips. In a first application, use the method to in situ monitor changes of the emitter caused by illumination with strong femtosecond laser pulses. After illumination, we observe an increase in the work function caused by laser-induced changes to the apex of the tip. These changes are reversible upon field evaporation and are accompanied by a change in the spatial electron emission distribution. We believe that this simple in situ work function determination technique is applicable to any metal and in many experimental settings.

Doubly differential ionization in proton-helium collisions at intermediate energies: Energy distribution of emitted electrons as a function of scattered-projectile angle

Doubly differential ionization in proton-helium collisions at intermediate energies: Energy distribution of emitted electrons as a function of scattered-projectile angle

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.

K-LL Auger electron angular distribution and absolute cross section measurement upon keV energy electron impact ionization of CH[formula omitted], N[formula omitted] and Ne

We report on the measurement of absolute cross sections and angular distribution of electron emission in the K-LL Auger region, upon electron impact ionization of molecular (CH4 and N2) and atomic (Ne) targets. Differential cross sections in the K-LL Auger energy range were measured at 6 keV and 8 keV impact energies. The angular distribution of the K-LL Auger yield is shown to be isotropic for all the three targets. Total K-LL Auger emission cross sections were estimated by integrating the measured differential cross sections over the entire energy and angular range. The estimated cross sections are in good agreement with the theoretical estimates.

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{}.

Design of modular multi-channel electron spectrometers for application in laser matter interaction experiments at Prague Asterix Laser System

This paper describes design, development, and implementation of a multi-channel magnetic electron spectrometer for the application in laser-plasma interaction experiments carried out at the Prague Asterix Laser System. Modular design of the spectrometer allows the setup in variable configurations to evaluate the angular distribution of hot electron emission. The angular array configuration of the electron spectrometers consists of 16 channels mounted around the target. The modules incorporate a plastic electron collimator designed to suppress the secondary radiation by absorbing the wide angle scattered electrons and photons inside the collimator. The compact model of the spectrometer measures electron energies in the range from 50keV to 1.5MeV using ferrite magnets and from 250keV to 5MeV using stronger neodymium magnets. An extended model of the spectrometer increases the measured energy range up to 21MeV or 35MeV using ferrite or neodymium magnets, respectively. Position to energy calibration was obtained using the particle tracking simulations. The experimental results show the measured angularly resolved electron energy distribution functions from interaction with solid targets. The angular distribution of hot electron temperature, the total flux, and the maximum electron energy show a directional dependence. The measured values of these quantities increase toward the target normal. For a copper target, the average amount of measured electron flux is 1.36 × 1011, which corresponds to the total charge of about 21nC.

Coulomb blockade in field electron emission from carbon nanotubes

We report the observation of Coulomb blockade in electron field emission (FE) from single-wall carbon nanotubes (SWCNTs), which is manifested as pronounced steps in the FE current–voltage curves and oscillatory variations in the energy distribution of emitted electrons. The appearance of the Coulomb blockade is explained by the formation of nanoscale protrusions at the apexes of SWCNTs due to the electric field-assisted surface diffusion of adsorbates and carbon adatoms. The proposed adsorbate-assisted FE mechanism is substantially different from the well-known resonant tunneling associated with discrete electronic states of adsorbed atoms. The simulations based on the Coulomb blockade theory are in excellent agreement with the experimental results. The SWCNT field emitters controlled by the Coulomb blockade effect are expected to be used to develop on-demand coherent single-electron sources for advanced vacuum nanoelectronic devices.

Controlling the Spatial and Momentum Distributions of Plasmonic Carriers: Volume vs Surface Effects.

Spatial and momentum distributions of excited charge carriers in nanoplasmonic systems depend sensitively on optical excitation parameters and nanoscale geometry, which therefore control the efficiency and functionality of plasmon-enhanced catalysts, photovoltaics, and nanocathodes. Growing appreciation over the past decade for the different roles of volume- vs surface-mediated excitation in such systems has underscored the need for explicit separation and quantification of these pathways. Toward these ends, we utilize angle-resolved photoelectron velocity map imaging to distinguish these processes in gold nanorods of different aspect ratios down to the spherical limit. Despite coupling to the longitudinal surface plasmon, we find that resonantly excited nanorods always exhibit transverse (sideways) multiphoton photoemission distributions due to photoexcitation within volume field enhancement regions rather than at the tip hot spots. This behavior is accurately reproduced via ballistic Monte Carlo modeling, establishing that volume-excited electrons primarily escape through the nanorod sides. Furthermore, we demonstrate optical control over the photoelectron angular distributions via a screening-induced transition from volume (transverse/side) to surface (longitudinal/tip) photoemission with red detuning of the excitation laser. Frequency-dependent cross sections are separately quantified for these mechanisms by comparison with theoretical calculations, combining volume and surface velocity-resolved photoemission modeling. Based on these results, we identify nanomaterial-specific contributions to the photoemission cross sections and offer general nanoplasmonic design principles for controlling photoexcitation/emission distributions via geometry- and frequency-dependent tuning of the volume vs surface fields.

Continuous angular control over anisotropic photoemission from isotropic gold nanoshells.

A variety of applications rely on the efficient generation of hot carriers within metal nanoparticles and charge transfer to surrounding molecules or materials. The optimization of such processes requires a detailed understanding of excited carrier spatial, temporal, and momentum distributions, which also leads to opportunities for active optical control over hot carrier dynamics on nanometer and femtosecond scales. Such capabilities are emerging in nanoplasmonic systems and typically rely on tuning optical polarization and/or frequency to selectively excite one or more discrete hot spots defined by the particle geometry. Here, we introduce a unique case in which hot electron excitation and emission distributions can instead be continuously controlled via linear laser polarization in the azimuthal plane of a gold nanoshell supported on a substrate. In this configuration, it is the laser field that breaks the azimuthal symmetry of the supported nanoshell and determines the plasmonic field distribution. Using angle-resolved photoelectron velocity map imaging, we find that the hot electrons are predominantly emitted orthogonal to the nanoshell dipolar surface plasmon resonance axis defined by the laser polarization. Furthermore, such anisotropic emission is only observed for nanoshells, while solid gold nanospheres are found to be isotropic emitters. We show that all of these effects are recapitulated via simulation of the plasmonic electric field distributions within the nanoparticle volume and ballistic Monte Carlo modeling of the hot electron dynamics. These results demonstrate a highly predictive level of understanding of the underlying physics and possibilities for ultrafast spatiotemporal control over hot carrier dynamics.

Tomography of photoelectron distributions produced through strong-field photodetachment of Ag−

We have applied a tomographic imaging method to recreate the full three-dimensional distribution of photoelectrons produced in a strong-field photodetachment process. The method is general and can be applied to any laser polarization. This stands in contrast to traditional imaging inversion methods, such as Abel inversion, which require prior knowledge of the symmetries of the electron distribution and are therefore limited to experiments using linear or circular polarization. Our method is also useful in a situation where linear polarization is used, since it compensates for detector inhomogeneities by spreading the information on a larger detector surface. In addition, it facilitates a method to detect polarization defects. Measurements were made for photodetachment of ${\mathrm{Ag}}^{\ensuremath{-}}$ at laser wavelengths of 1310 and 2055 nm, and were found to agree well with simulations in the strong field approximation. The data in the 1310 nm case revealed an unexpected asymmetry in the plane in which the laser polarization axis is rotated. Using a quasistationary quasienergy state model, a residual elliptical polarization of $\ensuremath{\varepsilon}=0.21\ifmmode\pm\else\textpm\fi{}0.01$ consistently explains the observed asymmetry. We conclude that the method described in this paper has the potential to be applied in experiments where a more complete characterization of electron emission distributions is required.

Metal–ceramic cathode for nanosecond electron accelerators

The technology for producing metal ceramic (MC) plates is described. It was shown that application of the MC cathode allows one to increase the beam power by a factor ∼1.5 and the current by a factor of 2, compared to metal–dielectric cathodes. It has been found that the emission properties of the MC cathode can be controlled by changing the composition of the MC plate as well as by changing the geometric form of the MC plate.Namely, the rectangular MC plate produces a uniform potential distribution on its surface and it results in faster ignition of the surface flash-over which uniformly covers the cathode surface. This leads to a more uniform electron emission distribution. The latter results in significantly higher (>2 times, up to 20 A/ns) rate of the current increase. In addition, no visible damage of the MC surface was found after 106 shots indicating a potentially long lifetime of such type of cathodes.

Electron emission from CH4 molecules in collisions with fast bare C ions

We present the energy and angular distributions of electron emission from a ${\mathrm{CH}}_{4}$ molecule in collisions with fast bare C ions with energies 3.5 and 5.5 MeV/u. The absolute double differential cross sections (DDCS) are measured for the ejected electrons having energies from 11 eV to 330 eV for $3.5\ensuremath{-}\mathrm{MeV}/\mathrm{u}$ projectiles and from 5 eV to 330 eV for 5.5 MeV/u bare C ions. The emission is measured in the angular range from ${20}^{\ensuremath{\circ}}$ to ${160}^{\ensuremath{\circ}}$. The forward-backward angular asymmetry, the single differential cross sections (SDCS), and the total cross section are deduced from the measured DDCS values. The energy and angular distributions of the DDCS and SDCS are compared with those calculated using a theoretical model based on the prior form of the continuum distorted wave--eikonal initial state (CDW-EIS) approximation. The dynamics of the process is considered within the CDW-EIS approximation, while the initial orbitals of the molecular target are represented using the complete neglect of the differential overlap approximation. The calculations are found to be in very good agreement with the measured cross sections. The angle dependence of the carbon K-LL Auger emission and the total Auger emission cross section are also derived for both projectile energies.

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.