Radiative Electron Capture Process Research Articles

Enhancement of Spontaneous Photon Emission in Inverse Photoemission Transitions in Semiconductor Quantum Dots.

Inverse photoemission (IPE) is a radiative electron capture process where an electron is transiently captured in the conduction band (CB) followed by intraband de-excitation and spontaneous photon emission. IPE in quantum dots (QDs) bypasses optical selection rules for populating the CB and provides insights into the capacity for electron capture in the CB, the propensity for spontaneous photon emission, intraband transition energies where both initial and final states are in the CB, and the generation of photons with frequencies lower than the bandgap. Here, we demonstrate using time-dependent perturbation theory that judicious application of electric fields can significantly enhance the IPE transition in QDs. For a series of CdSe, CdS, PbSe, and PbS QDs, we present evidence of field-induced enhancement of IPE intensities (188% for Cd54Se54), field-dependent control of emitted photon frequencies (Δω = 0.73 eV for Cd54Se54), and enhancement of light-matter interaction using directed Stark fields (103% for Cd54Se54).

Towards a determination of absolute cross sections for projectile excitation of hydrogen‐like uranium in collisions with neutral atoms

Recently, the contribution of the generalized Breit interaction to electron impact ionization was identified for the first time in a high‐Z system, namely, hydrogen‐like uranium. This study employed a measurement of the relative population of the j = 1/2 and j = 3/2 states of the L shell by projectile excitation in collision of U91+ with hydrogen and nitrogen targets. However, for a rigorous test of ion–atom collision theory, also the absolute excitation cross sections are of great importance. In the present work, we report on our efforts to extend the previous study to a determination of the absolute projectile excitation cross sections by normalization to the well‐known radiative electron capture process.

Radiative electron capture as a tunable source of highly linearly polarized x rays

The radiative electron capture (REC) into the K shell of bare Xe ions colliding with a hydrogen gas target has been investigated. In this study, the degree of linear polarization of the K-REC radiation was measured and compared with rigorous relativistic calculations as well as with the previous results recorded for U92+. Owing to the improved detector technology, a significant gain in precision of the present polarization measurement is achieved compared to the previously published results. The obtained data confirms that for medium-Z ions such as Xe, the REC process is a source of highly polarized x rays which can easily be tuned with respect to the degree of linear polarization and the photon energy. We argue, in particular, that for relatively low energies the photons emitted under large angles are almost fully linear polarized.Received 22 February 2019DOI:https://doi.org/10.1103/PhysRevA.99.052702Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)PropertiesPolarizationTechniquesCompton scatteringRadiation detectorsAccelerators & BeamsAtomic, Molecular & Optical

Disentangling the bulk and exit surface contribution on projectile-charge-state evolution

Charge state evolution of the projectile ions while traversing through the solid target medium has been explored using the radiative electron capture process. The measured centroid energies of the convoluted radiative electron capture peak structures have been used to determine the mean K-shell binding energies and mean charge state of the projectile ions. It has been observed that the mean charge states of present measurements are lower than the earlier measurements done using the characteristic ${\mathrm{K}}_{\ensuremath{\alpha}}$ x-ray transitions. The difference is due to the capture of target electrons in the inner-shell vacancies, created during the collision process, of projectile ions. Further, the measured mean charge states are compared with the empirical predictions. A significant discrepancy between experimental and theoretical values has been observed, which is attributed to the multielectron capture by projectile ions due to nonradiative electron capture process from the exit surface while exiting from the foil. The significant variation between mean charge state values obtained from different tools provides a clear indication of the dynamic nature of the charge-changing mechanism at different regions (entrance surface, bulk, and exit surface) of the ion-solid interaction. The present results can be used to validate the departure between the theory and experiment on the charge state dependent stopping powers.

Fast calculator for X-ray emission due to Radiative Recombination and Radiative Electron Capture in relativistic heavy-ion atom collisions

In experiments with highly charged, fast heavy ions the Radiative Recombination (RR) and Radiative Electron Capture (REC) processes have significant cross sections in an energy range of up to a few GeV/u. They are some of the most important charge changing processes in collisions of heavy ions with atoms and electrons, leading to the emission of a photon along with the formation of the ground and excited atomic states. Hence, for the understanding and planning of experiments, in particular for X-ray spectroscopy studies, at accelerator ring facilities, such as FAIR, it is crucial to have a good knowledge of these cross sections and the associated radiation characteristics. In the frame of this work a fast calculator, named RECAL, for the RR and REC process is presented and its capabilities are demonstrated with the analysis of a recently conducted experiment at the Experimental Storage Ring (ESR) at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. A method is presented to determine unknown X-ray emission cross sections via normalization of the recorded spectra to REC cross sections calculated by RECAL.

Theoretical studies on the radiative electron capture and subsequent radiative decay in the collision of Xe53+ ions with neutral Xe

The radiative electron capture (REC) and subsequent radiative decay of initial hydrogen-like Xe52+ ions are studied in the collision of Xe53+ with Xe atom at a projectile energy of 197 MeV/u within the framework of the multiconfiguration Dirac-Fock method and the density matrix theory. We calculate the differential and total cross sections as well as the REC photon energies for REC to the 1snp1/2, 3/2 Jf=1 (n=2-5) levels of finally helium-like Xe53+ ions. Moreover, the transition energies and rates of the subsequent 1snp3/2 Jf=1 1s2 Jd = 0 decay as well as the angular distribution and linear polarization of the associated characteristic photons are also calculated. It is found that the REC photons are remarkably anisotropic. Through the analysis of the REC angular distribution characteristics, we find that the different configurations of the REC angular distribution are similar in quality, and they all have a peak at the 90. That is to say, the REC process can more easily occur in the direction perpendicular to the incident direction of the projectile ions. In addition, while the characteristic photons from the subsequent 1snp3/2 Jf=11s2 Jd= 0 radiative decay of Xe52+ ions exhibit an anisotropic angular distribution and strong linear polarization, their counterparts from the 1snp1/2 Jf = 1 1s2 Jd = 0 decay are almost isotropic and linearly unpolarized. The angular distribution and linear polarization of the radiation photon decay from the (1s np1/2,3/2 Jf = 0) states to the ground state（1s2 Jd = 0）both reach a maximum value at the 90, their characteristics are similar to those of the REC photons, that is to say, the deexcited process can more easily occur in the direction perpendicular to the incident direction of the projectile ions, and in this direction the decay photons have much larger polarization degree.

Influence of quantum interference on the polarization and angular distribution of x-ray radiation following electron-impact excitation of highly charged H-like and He-like ions

Detailed calculations are carried out for the polarization properties and angular distribution of the x-ray photoemission of highly charged H-like and He-like ions following electron-impact excitation using a fully relativistic distorted-wave method. The contributions of the magnetic quadrupole ($M2$) to the subsequent characteristic x-ray emission are investigated systematically. For the Lyman-${\ensuremath{\alpha}}_{1}$ $(2{p}_{3/2}\ensuremath{\rightarrow}1{s}_{1/2}$) radiation of H-like ions, it is shown that the interference between the leading electric dipole ($E1$) decay and the much weaker $M2$ decay may significantly reduce the polarization and slightly increase the angular distribution of the emitted x-rays, these characteristics are very similar to the conclusions regarding the radiative electron capture process [G. Weber et al., Phys. Rev. Lett. 105, 243002 (2010)]. However, for the $2{s}_{1/2}2{p}_{3/2}(J=1)\ensuremath{\rightarrow}1{s}_{1/2}2{s}_{1/2}$($J=1$) radiation of He-like ions, the $E1$-$M2$ interference leads to an obvious increase in polarization and a large decrease in the angular distribution of the x-ray photoemission, which is contrary to the Lyman-${\ensuremath{\alpha}}_{1}$ line of H-like ions. And all these characters strongly depend on the atomic number and the incident electron energy.

Polarization and angular correlation studies of X-rays emitted in relativistic ion-atom collisions

Particle and photon polarization phenomena occurring in collisions of relativistic ions with matter have recently attracted particular interest. Investigations of the emitted characteristic x-ray and radiative electron capture radiation has been found to be a versatile tool for probing our present understanding of the dynamics of particles in extreme electromagnetic fields. Owing to the progress in x-ray detector technology, in addition, accurate measurements of the linear polarization for hard x-ray photons as well as the determination of the polarization plane became possible. This new diagnostic tool enables one today to derive information about the polarization of the ion beams from the photon polarization features of the radiative electron capture process.

Radiative Electron Capture Studied for Bare, Decelerated Uranium Ions

By applying the deceleration technique for bare uranium ions at the ESR storage ring we studied the Radiative Electron Capture process in collisions with low-Z target atoms. This allowed us to extend the current information about the time-reversed photoionization process of highly-charged heavy ions to much lower energies than those accessible for neutral heavy elements in the direct reaction channel. The angular differential data obtained prove theoretical predictions that at high-Z higher-order multipole contributions and magnetic corrections play an important role even at energies close to the threshold.

The influence of QED on the Radiative Electron Capture process in Highly Charged Ions

The influence of quantum electrodynamics (QED) to the process of radiative electron capture (REC) is investigated in the region of highly relativistic projectile energies by modifying the wave functions according to the Uehling potential. Exact relativistic calculations are employed for the radiative recombination. The deviation from non-QED calculations is found to amount to up to 2% in U92+ for a projectile energy of 10 GeV/u and forward emission angles of the emitted photon.

Radiative electron capture by metallic clusters

We have investigated the radiative electron capture process in collisions of electrons with metal clusters, at an electron energy sufficient for excitation of the giant collective resonance in the target cluster. We have demonstrated that accounting for the dynamical polarizability of the cluster plays an essential role giving rise to a new kind of giant resonance in the cross section for radiative capture.

Radiative electron capture studied in relativistic heavy-ion-atom collisions.

The process of radiative electron capture (REC) in relativistic collisions of high-Z ions with low-Z gaseous and solid targets is studied experimentally and theoretically. The observed x-ray spectra are analyzed with respect to photon angular distributions as well as to total K-REC cross sections. The experimental results for angle-differential cross sections are well reproduced by exact relativistic calculations which yield significant deviations from standard ${\mathrm{sin}}^{2}$\ensuremath{\theta} distributions. Total cross sections for K-REC are shown to follow a simple scaling rule obtained from exact relativistic calculations as well as from a nonrelativistic dipole approximation. The agreement between these different theoretical approaches must be regarded as fortuitous, but it lends support to the use of the nonrelativistic approach for practical purposes.

Energy loss dependence of X-rays emitted by titanium ions channeled in a thin gold single crystal

X-rays generated by fully stripped titanium ions channeled along the 〈100〉 axis of a thin gold single crystal have been measured in coincidence with charge-changed ions as a function of ion energy loss. X-rays from the process of radiative electron capture are used to derive mean target electron densities for three values of mean energy loss. These densities are compared with densities calculated from a free-electron gas model for stopping. The results are used in a model calculation for Lyman series X-ray emission yields which includes effects of strong competition from secondary electron impact excitation and ionization of excited-state hydrogenic ions formed by electron capture in the crystal.

Polarisational bremsstrahlung in the scattering of a multicharged ion

Polarisational bremsstrahlung in the high-energy collision between an atom of nuclear charge Z2 and an ion of charge Z1>>Z2 is studied. The elastic polarisation bremsstrahlung is shown to dominate in the absolute radiation spectrum in a wide region of the photon frequencies. Simple expressions for the absolute radiation spectrum are obtained in the range of the photon frequencies, which is located below the region spanned by the process of radiative electron capture.

Threshold effects in ultrasoft X-ray emission

The N4,5(4d) spectra of the atoms of Rare-Earth elements in metals and oxides have been examined under monoenergetic electron beam-excitation with energies varying in the region containing 4d thresholds. The full pattern of the radiative decay of the states of Rare-Earth atoms with a vacancy in the 4d shell have been obtained. Besides, the process of radiative electron capture into free 4f states (inverse photoemission, IPE) has been studied. Attention has been paid to the results of the competition between the capture and 4d shell excitation. The results obtained have been used for the analysis of the structure of 4d appearance potential spectra.

Angular distribution and linear-polarization of X-rays induced by radiative electron capture process

By using the Born approximation, calculations are made for photon angular distributions of the radiative electron capture to the L shell and for linear polarization correlations of photons induced by the radiative electron capture to the K shell for relativistic incident velocities. The angular distributions have minimum value in the forward directions and maximum values in the backward directions relative to 90°. This behavior becomes significant with increasing velocity. The polarization correlation of the emitted photon represents the crossover feature in the case of relativistically high energy impacts.

Charge equilibrium of relativistic uranium ions in matter

Calculations are made for the electron capture and loss cross sections and the charge state distributions of uranium ions at energies of 200 MeV/nucleon, 437 MeV/nucleon and 962 MeV/nucleon colliding with low- Z and high- Z targets. The charge states of the projectile ions are determined from the equilibrium condition between electron loss from the ion and electron capture from a target atom into the projectile ion. The electron loss cross sections are formulated using the relativistic Born approximation. The electron capture process is considered to be non-radiative electron capture (non-REC) and radiative electron capture (REC) in the relativistic impact velocity range. The non-REC cross sections are calculated by Eichler's formula based on the eikonal theory, while for the evaluation of the relativistic REC process the contributions of internal e +e − pair creation and successive e +e − pair annihilation processes are taken into account in addition to the usual REC photon emission process.

Angular-distribution and linear-polarization correlation of photons induced by the relativistic radiative electron-capture process.

By using the Born approximation, calculations are made for photon angular distributions of the radiative electron capture to the L shell and for linear polarization correlations of photons induced by the radiative electron capture to the K shell for relativistic incident velocities. The angular distributions have minimum value in the forward directions and maximum values in the backward directions relative to 90/sup 0/. This behavior becomes significant with an increase of velocity. The polarization correlation of the emitted photon represents the crossover feature in the case of relativistically high-energy impacts.

Cross sections of relativistic radiative electron capture by use of the strong-potential Born calculation.

The relativistically extended strong-potential Born (SPB) formalism is applied to the radiative electron capture process caused by the bombardment of a heavy and highly stripped charged particle with relativistically high velocity. The results are compared with those by use of nonrelativistic SPB calculations and with those by use of the relativistic Born calculation (Sauter's formula), which includes no distortion effects between a heavy projectile ion and an active electron. Even if the strong distortion effects are taken into consideration, the shapes of photon angular distributions in the laboratory frame still nearly depend on sin/sup 2/theta/sub L/(theta/sub L/ is the angle of the emitted photon) in the vicinity of the angle of 90/sup 0/, which is the same as the results by use of Sauter's formula. The higher the charge of a projectile ion becomes, however, the greater the discrepancy between the angular shape of our results and that of Sauter's becomes at both smaller and larger angles than at 90/sup 0/. As is expected, the magnitudes of the differential and the total cross sections are drastically influenced by the distortion effects ascribable to a large charge of a heavy projectile ion such as U/sup 92+/. Our results are in goodmore » agreement with recent experiments. In addition, the Coulomb off-shell factor introduced by the SPB theory is found playing important roles in the case of the relativistic radiative electron capture process because the results calculated by using the relativistic impulse approximation are too underestimated.« less

Projectile-charge dependence of quasi-free-electron bremsstrahlung

Continuum x rays induced by bombardments of a Be target with 20.14-MeV/amu protons and /sup 3/He/sup 2 +/ ions have been measured with a Si(Li) detector in the direction of 90/sup 0/ with respect to the incident beam. Difference in the x-ray--production cross sections multiplied by the x-ray energy h..omega.. )h..omega.. x (sigma/sub h/(h..omega..)/4-sigma/sub p/(h..omega..))) and ratio of the cross sections R(h..omega..) (equivalentsigma/sub h/(h..omega..)/4sigma/sub p/(h..omega..)) for the proton and /sup 3/He/sup 2 +/-ion impact, where sigma/sub h/(h..omega..) and sigma/sub p/(h..omega..) are the x-ray--production cross sections for proton and /sup 3/He/sup 2 +/-ion impact, respectively, were obtained as a function of the x-ray energy. Both the difference and the ratio show peaks in the region where the x-ray energy is equal to the relative kinetic energy between the projectile and the inner-shell electron to be scattered by the projectile. From the comparison with a theory of quasi-free-electron bremsstrahlung based on the impulse approximation, it is found that this peak corresponds to the maximum of the velocity-distribution function of the inner-shell electron. Furthermore, the contribution of radiative electron-capture process in the case of proton and /sup 3/He/sup 2 +/ impact is clearly found.