Relativistic MeV photoelectrons from the single atom response of argon to a 10 19 W/cm2 laser field

We present photoelectron measurements from argon ionization at 10(19) W/cm(2). Photoelectrons with energies above 400 keV, including a 1.2 MeV cutoff, are in quantitative agreement with a semiclassical, relativistic 3D ionization model that includes a nonparaxial laser field. L-shell photoelectrons have energies and momentum dominated by the field, including the acceleration out of the focus. Yields and angular distributions at 60 keV come from valence shell ionization by strong fields where rescattering and atomic processes determine photoelectron final states.     

Explicit designs of spin chains for perfect quantum state transfer

For the process of electron-electron (e-e) bremsstrahlung the momentum and energy distributions of the recoiling electrons are calculated in the laboratory frame. In order to get the differential cross section and the photon spectrum for target electrons which are bound to an atom, these formulae are multiplied by the incoherent scattering function and numerically integrated over the recoil energy. The effect of atomic binding is most pronounced at low energies of the incident electrons and for target atoms of high atomic numbers. The results are compared to those of previous calculations.     

Atomic analogue of photonuclear configurational splitting

It is shown that the polarized bremsstrahlung (PB) emitted upon the scattering of a relativistic charged particle’s Coulomb field by atomic electrons yields direct information on the electronic structure of a medium. Indeed, in the X-ray photon energy range 1–10 keV, PB exhibits collective features and the PB intensity sharply increases. At higher photon energies, the coherence of shell electrons rapidly collapses, which is accompanied by a drastic decrease in the PB intensity; hence, the position of the PB spectrum kink allows estimation of the atomic size.     

Influence of electron diffraction on measured energy-resolved momentum densities in single-crystalline silicon

Electron momentum spectroscopy is used to determine the spectral function of silicon single crystals. In these experiments 50 keV electrons impinge on a self-supporting thin silicon film and scattered and ejected electrons emerging from this sample with energies near 25 keV are detected in coincidence. Diffraction effects are present that give rise to additional structures in the measured spectral momentum densities. Spectra for a specific momentum value can be obtained at different orientations of the crystal relative to the analysers. By comparing these spectra for which the measured momentum density is the same, but the diffraction conditions of the incoming and outgoing electron trajectories differ, one can distinguish between features due to diffraction of the incoming and/or outgoing electrons, and those due to the electronic structure of the target itself.     

The bremsstrahlung induced by 0.3–2 keV electron scattering by Ar atoms

The differential spectra of a bremsstrahlung resulting from a 0.3–2 keV electron scattering by Ar atoms are studied. Photon energies within the ultrasoft X-ray band from 124 to 190.8 eV, which is characterized by the low dynamic polarizability of the Ar atom, are considered. For the entire spectrum of photon energies (124–190.8 eV), the intensity of the bremsstrahlung differential spectra first grows with an increase in the electron energy from 0.3 to 0.7 keV and then decreases as the electron energy increases from 0.7 to 2 keV. The increase in intensity is directly proportional, and the decrease is inversely proportional to the square root of the energy of the scattered electrons. Within the context of a "low-energy" approximation, the increase in the number of photons with the electron energy is due to the contribution of the atomic excitation and ionization channels being available during the bremsstrahlung process.     

Ab initio study of valence correlation satellites in photoelectron spectra of neon

Ab initio calculations of energies and widths of the valence correlation satellites in the binding energy region between 48 and 99 eV of the photoelectron spectrum of neon are presented. The results are compared with the high-resolution, synchrotron radiation excited valence photoelectron spectra recorded by Kikas et al. [J. Electron Spectrosc. Relat. Phenom. 77 (1996) 241] using photon energies of 96 and 169 eV and with previous synchrotron radiation and conventional radiation studies. Our calculations allow a deeper insight in the regions where a bunch of states, different in spin and angular momentum, are concentrated, and add considerable information on assignment and width of several transitions. The theoretical spectrum calculated in the region between 74 and 94 eV compares quite well with the experimental one.     

Monte Carlo calculation of electron spectra produced in water phantoms by 20 keV-1 GeV photons

Abstract

A Monte Carlo calculation for the energy spectra of electrons and positrons produced in infinite and semiinfinite water phantoms by photons ranging in energies from 20 keV to 1 GeV are presented. The dominant processes considered are the photoelectric effect, Auger effect, Compton effect, pair, and triplet production. Bremsstrahlung produced by electrons and positrons with energies greater than 1 MeV is also included. The effect of multiple Compton scattering, not considered in the earlier calculation, for the infinite phantom for photon energies higher than 2 MeV has been incorporated. For a semi-infinite phantom, multiple Compton scattering and backscattering through the top are considered. The results are compared with the earlier calculations for the first-collision spectrum. It is found that the inclusion of multiple Compton scattering significantly increases the average number of electrons/cm³ per photon/cm² at all energies considered whereas bremsstrahlung reduces the number of high energy electrons and produces more low energy electrons in the spectrum.     

Energy dependence of photon-inducedL shell x-ray intensity ratios in Ta and W

TheL shell x-ray intensity ratios have been measured for the elements Ta and W by photoionization ofL shell electrons in the photon energy region 14⩽E⩽44 keV. The experimental results are compared with those calculated at the photon energies used in the present measurements. The measured values show fairly good agreement with the calculated values within the experimental uncertainties.     

Chiral multi-electron emission

In this report we review recent progress in the understanding of the role of chirality in the multi-electron emission. A brief account of the chiral single-electron photoemission is given. In this case the chirality of the experimental set-up is brought about by an initial orientation of the target or/and by specifying a certain projection of the photoelectron spin. The dependence of the photoelectron spectrum on the chirality of the experiment is probed by changing the initial orientation of the target or by inverting the photoelectron spin projection. In a further section we envisage the direct transition of chiral electron pairs from an isotropic bound initial state into a double-continuum state following the absorption of a circularly polarised photon. We work out the necessary conditions under which the spectrum of the correlated photoelectron pair shows a chiral character, i.e. a dependence on the chirality of the exciting photon. The magnitude and the general behaviour of the chiral effects are estimated from simple analytical models and more elaborate numerical methods are presented for a more quantitative predictions. As a further example for the chiral multi-electron emission we study the photoelectron Auger-electron coincidence spectrum. The Auger hole is created by ionising a randomly oriented target by a circular polarised photon. We investigate how the helicity the photon is transferred to the emitted photoelectron pair. The theoretical findings are analysed and interpreted in light of recent experiments. In a final section we focus on the emission of correlated electrons where the initial state is already oriented, e.g. via optical pumping by circularly polarised light. The initial orientation of the atom is transferred to the continuum states following the ionisation of the target by low-energy electrons. We formulate and analyse the theoretical concepts for the transition of the screw sense of the initially bound atomic electron to the continuum electron pair. Numerical methods for the calculations of the cross-sections for the electron-impact ionisation of oriented atoms are presented and their results are contrasted against recent experimental data.     

High-momentum analysis in Doppler spectroscopy

Unique information about the chemical vicinity of positron annihilation sites is provided by the contribution of high electron momenta to the Doppler spectrum, since this momentum range is characteristic for the annihilation with core electrons and hence element specific. However, the corresponding energy region in the spectrum is overlaid by a huge background caused by the annihilation radiation itself and the Compton spectrum of other gamma lines having an energy above 511 keV. Usually these backgrounds are reduced by measuring both annihilation quanta in coincidence.By mathematically analyzing the background contributions, we open another possibility to obtain the high-momentum region employing one single germanium detector. A necessary precondition is employing either background-free positron beams or a low-background positron source, e.g. ⁶⁸Ge, instead of the widely used positron emitter ²²Na. The ⁶⁸Ge-source emits positrons with an endpoint energy of about 1.9 MeV, where as the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low.When analyzing spectra from metals and semiconductors according to the described background subtraction, the same information contained in the momentum range up to 35 × 10⁻³m₀c or beyond can be extracted, as if the spectra were measured employing a coincidence setup with two Ge-detectors.     

Effective attenuation length dependence on photoelectron kinetic energy for Au from 1 keV to 15 keV

Hard X-ray PhotoElectron Spectroscopy (HAXPES) is an extremely powerful tool for the electronic, compositional, and chemical characterization of bulk materials and buried interfaces. Its success is based on the dramatic increase of the electron effective attenuation length (EAL) with increasing photoelectron kinetic energy. EALs are well established for electrons with kinetic energies up to several keV (below 3 keV). However, few data are available for kinetic energies up to 15 keV. In the present study we have determined the EAL dependency on kinetic energy for gold from 1 keV up to 15 keV. Two different approaches have been used. The first approach consists of following the signal rate from a core level for a fixed kinetic energy as a function of overlayer thickness (overlayer method). The second approach consists of following the signal rate from a core level as a function of the incident photon energy, i.e., electron kinetic energy, for a fixed overlayer thickness (depth profile method). An EAL dependency of EAL (nm) = 0.022 × E_kin (eV)^0.627 has been obtained from both methods. Hence, the EAL, for gold, is 4.7, 7.3 and 9.4 nm for 5 keV, 10 keV and 15 keV electron kinetic energies, respectively. A comparison between the experimental data and the EALs predicted by practical expressions available in the literature is also performed.     

Incoherent X-radiation produced by relativistic electrons in crystals

Differential and integral features of incoherent X-radiation, induced by relativistic electrons in crystals, are studied for observation angles θ_γ several times greater than γ^-1, where γ is the projectile Lorentz factor. The existence of sharp maxima and a minimum of the five-folded incoherent differential cross-section as a function of the final electron angles, and a dip minimum when the cross-section is taken as a function of the photon energies, is demonstrated. At near backward observation angles the three-folded cross-section shows a maximum in the region of several keV photon energies. The obtained results allow us to optimize the conditions for coincidence experiments, minimizing the incoherent contribution to the total radiation yield, and helping to analyse results of finite-size detector experiments with crystal targets. Received: 2 July 2001 / Accepted: 26 November 2001     

Photoelectron imaging of helium droplets

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, with less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.     

Photodouble ionization dynamics for fixed-in-space H2

The Coulomb explosion of the hydrogen molecule, after absorption of a 76 eV photon, has been studied by momentum imaging the two electrons and the two protons. Absolute fully differential cross sections of high statistical quality are obtained. A subset of the overall data, namely, equal electron-energy sharing, is used to investigate the effects of molecular orientation on the photoelectron angular distribution. Departures from the first-order helium-like model are evident in detection geometries where electron-electron correlation is "frozen."     

Interpretation of intensities in electron-momentum and photoelectron spectroscopiesag

Relative intensities for the photoelectron reaction on atoms and molecules are not related to structure calculations in the same way as those for the noncoplanar symmetric (e, 2e) reaction. The photoelectron dipole matrix element is dependent on recoil momentum only through the unique relationship of recoil momentum to the photon energy and is much harder to calculate for chemically-interesting momenta. Relative intensities for binary (e, 2e) reactions are independent of total energy at high enough energies and strongly dependent on symmetry and recoil momentum, for which an intensity profile can be measured for values starting at zero. In comparing with structure calculations, binary (e, 2e) intensities for low recoil momentum may be compared directly with pole strengths in calculations of the one-electron Green's function. In the case of states within a single symmetry manifold the relative intensities will be independent of recoil momentum up to some maximum, usually at least a few atomic units.     

Pitch Angle Distribution Evolution of Energetic Electrons by Whistler-Mode Chorus

We develop a two-dimensional momentum and pitch angle code to solve the typical Fokker-Planck equation which governs wave-particle interaction in space plasmas. We carry out detailed calculations of momentum and pitch angle diffusion coefficients, and temporal evolution of pitch angle distribution for a band of chorus frequency distributed over a standard Gaussian spectrum particularly in the heart of the Earth＇s radiation belt L = 4.5, where peaks of the electron phase space density are observed. We find that the Whistler-mode chorus can produce significant acceleration of electrons at large pitch angles, and can enhance the phase space density for energies of 0.5 - 1 MeV by a factor of 10 or above after about 24h. This result can account for observation of significant enhancement in flux of energetic electrons during the recovery phase of a geomagnetic storm.     

Relative electron inelastic mean free paths for diamond and graphite at 8 keV and intrinsic contributions to the energy-loss

We used hard X-ray photoelectron spectroscopy (HAXPES) with 8 keV X-rays to investigate the 1s emission of carbon. We recorded spectra extending from the peak of the C 1s electrons (“elastic” line) to electrons with up to 110 eV energy-loss. Using two samples side by side, we could compare the inelastic mean free paths (IMFPs) of the electrons of almost 8 keV in diamond and graphite and find them to be practically identical despite about 50% difference in densities. Published extrapolations of their IMFP calculations at lower energies are in good agreement with this result. We show that information from the almost structureless region of overlapping multiple extrinsic energy-losses can be used to quantify the fraction of photoelectrons experiencing intrinsic energy-losses (those due to the sudden creation of the hole). We find that this fraction is 58% of the primary excited C 1s electrons for diamond and is practically the same for graphite. This is at first sight an unexpected result since hole-screening should differ in a semimetal from that in an insulator. The observation can be accounted for by dynamic screening in contrast to static screening.     

Experimental retrieval of target structure information from laser-induced rescattered photoelectron momentum distributions

We have measured two-dimensional photoelectron momentum spectra of Ne, Ar, and Xe generated by 800-nm, 100-fs laser pulses and succeeded in identifying the spectral ridge region (back-rescattered ridges) which marks the location of the returning electrons that have been backscattered at their maximum kinetic energies. We demonstrate that the structural information, in particular the differential elastic scattering cross sections of the target ion by free electrons, can be accurately extracted from the intensity distributions of photoelectrons on the ridges, thus effecting a first step toward laser-induced self-imaging of the target, with unprecedented spatial and temporal resolutions.     

Precise energy of the 9.4 keV gamma transition observed in the <Superscript>83</Superscript>Rb decay

The energy of the 9.4 keV γ-transition observed in the ⁸³Rb decay was established to be 9405.8(3) eV. This energy value was obtained from photon spectrometry measurements of the differences in the energies of closely spaced lines. The result allows one to determine more precisely the energy of conversion electrons of the 9.4 keV transition, which represent a unique tool for energy calibration of the tritium beta spectrum and systematic measurements in the KATRIN neutrino mass determination experiment.