Strong Coulomb Field Research Articles

Photon angular distribution of radiative electron capture into high-Z projectiles: A brief review

With the revolutionary development in the design of heavy-ion cooler storage rings and detectors, increasing interest is being focused on ion-atom collisions, in which the projectiles with a velocity comparable to the speed of light and the electronic transitions in inner shells of high-[Formula: see text] ions are involved. For this purpose, a fundamental interaction mechanism between a photon and an atomic electron in the presence of a strong Coulomb field is reviewed from the perspective of radiative electron capture into highly-charged ions. This is followed by the discussions of photon-electron interactions beyond the dipole approximation, which can be unambiguously visible in the photon angular distributions. In addition, the interference between the leading electric dipole transition [Formula: see text] and the magnetic multipole transition [Formula: see text] for the case of Lyman-[Formula: see text] radiation in hydrogen-like projectiles is also discussed.

Measurement of prompt D0 and [formula omitted] meson azimuthal anisotropy and search for strong electric fields in PbPb collisions at [formula omitted

The strong Coulomb field created in ultrarelativistic heavy ion collisions is expected to produce a rapidity-dependent difference (Δv2) in the second Fourier coefficient of the azimuthal distribution (elliptic flow, v2) between D0 (u‾c) and D‾0 (uc‾) mesons. Motivated by the search for evidence of this field, the CMS detector at the LHC is used to perform the first measurement of Δv2. The rapidity-averaged value is found to be 〈Δv2〉=0.001±0.001(stat)±0.003(syst) in PbPb collisions at sNN=5.02TeV. In addition, the influence of the collision geometry is explored by measuring the D0 and D‾0mesons v2 and triangular flow coefficient (v3) as functions of rapidity, transverse momentum (pT), and event centrality (a measure of the overlap of the two Pb nuclei). A clear centrality dependence of prompt D0 meson v2 values is observed, while the v3 is largely independent of centrality. These trends are consistent with expectations of flow driven by the initial-state geometry.

Atomic physics at the HIAF under extreme conditions: QED effects from strong Coulomb fields to beyond Schwinger limits

The high-intensity heavy ion accelerator facility (HIAF) is one of the major large-scale research facilities at the nuclear science center planned for Huizhou City in Guangdong Province. HIAF and its experimental terminals can be used to realize unprecedented research progresses in nuclear physics, atomic physics, materials, high-energy-density physics, and applications related to heavy ion beams such as cancer therapy, material modification, and radiobiology. In addition, they will provide excellent possibilities for atomic physics to investigate the quantum electrodynamics (QED) effects from strong electromagnetic fields to super-critical electromagnetic fields beyond the Schwinger limit. Starting from modern atomic spectroscopy based on the accelerator technology and in combination with the beam characteristics of the HIAF, this paper discusses the cutting-edge research topics that could be performed at HIAF and the upgrades to HIAF that could be implemented in the near future. The research fields include the QED effects in super-strong fields, parity nonconservation in highly charged ions, few-body processes in relativistic collisions, and testing the locality of matter waves of massive particles. This paper also discusses the possibility to perform laser spectroscopy with respect to radioactive ions. Furthermore, we believe that the latest high harmonic extreme ultraviolet (XUV) laser technology can be combined with HIAF for realizing high-precision measurements.

Electron-positron pair creation in the electric fields generated by micro-bubble implosions

We show that electron-positron pair production from the vacuum is possible via the strong Coulomb fields generated by micro-bubble implosions induced by ultra-high intensity lasers. Even in the case where the Coulomb fields are lower than the pair creation threshold, externally injected high energy electrons or photons could be used to generate pairs.

Probing cosmic rays with Fe Kα line structures generated by multiple ionization process

Abstract Supernova remnants (SNRs) have been regarded as major acceleration sites of Galactic cosmic rays. Recent X-ray studies revealed neutral Fe Kα line emission from dense gas in the vicinity of some SNRs, which can be best interpreted as K-shell ionization of Fe atoms in the gas by sub-relativistic particles accelerated in the SNRs. In this Letter, we propose a novel method of constraining the composition of particles accelerated in SNRs, which is currently unknown. When energetic heavy ions collide with target atoms, their strong Coulomb field can easily cause simultaneous ejection of multiple inner-shell electrons of the target. This results in shifts in characteristic X-ray line energies, forming distinctive spectral structures. Detection of such structures in the neutral Fe Kα line strongly supports the particle ionization scenario, and furthermore provides direct evidence of heavy ions in the accelerated particles. We construct a model for the Fe Kα line structures by various projectile ions utilizing atomic-collision data.

Sub-optical-cycle electron dynamics of NO molecules: the effect of strong laser field and Coulomb field

The sub-cycle bound- and free-electron dynamics and the Coulomb effect during the strong field ionization of NO molecules are studied by using a combination of phase-controlled two-color femtosecond laser fields and photoelectron imaging. The AC–Stark effect and ponderomotive potential is identified and tracked by measuring the energy shift of phase-dependent photoelectron spectra under different asymmetric laser fields. The Coulomb effects of molecules during ionization are studied by measuring the energy-dependent phase change and the cusp-like transverse-momentum distributions of the ionized electrons. The results show an obvious sub-cycle time scale response of ionized electrons in NO molecular ionization. The present method provides new ways to measure the electronic dynamics of molecules in strong field ionization.

Non-Born-Oppenheimer Molecular Dynamics Observed by Coherent Nuclear Wave Packets.

The reaction dynamics of a photochemical reaction is typically described by reaction coordinates based on the Born-Oppenheimer (BO) approximation. A strong interaction between electrons and nuclei, conventionally occurring at conical intersections, however, breaks the BO approximation and has major consequences for the efficiency of a photochemical reaction. Despite its importance, related studies into the non-BO dynamics are scarce. Here, we investigate the non-BO dynamics of excited-state intramolecular proton transfer (ESIPT) occurring in 10-hydroxybenzo[h]quinoline (HBQ). Two coherent vibrational modes at 237 and 794 cm-1 representing molecular dynamics on a diabatic surface in HBQ are identified by a wave packet analysis based on a transient absorption measurement with a time resolution of 11 fs and with a density functional theory-based model calculation. It is also revealed that the strong Coulomb field effect in HBQ leads to the completion of ESIPT within about two cycles of the OH stretching mode. The work paves the way for time-domain studies of molecular dynamics beyond the BO approximation in other photochemical reactions.

Spontaneous coherent cyclotron emission from photo-injector electron bunches: superradiation and two-frequency regime

Short dense electron bunches are attractive from the viewpoint to use them for the implementation of powerful and compact radiation source based on the spontaneous mechanism of radiation. The regime of spontaneous radiation is realized, when the bunch phase size is smaller than 2π. Modern photo-injectors insure formation of mildly relativistic dense electron bunches with duration less than 1 ps. Such bunches are “ready” to radiate waves at terahertz frequencies in the spontaneous regime. However, the repulsion of particles caused by a strong Coulomb field inside the dense electron bunch strictly limits the duration of the radiation process due to the increase in the bunch length. We propose two mechanism of cyclotron radiation to solve this problem. The first one is the regime of group synchronism of particles with the radiated wave; in this case compensation of Coulomb repulsion in the phase space takes place. The second proposed solution is to use compression of bunch by radiated electromagnetic fields.

Spontaneous superradiant sub-THz coherent cyclotron emission from a short dense electron bunch

Short dense electron bunches produced by modern photoinjectors are attractive from the viewpoint of the realization of powerful and effective sources of subterahertz radiation based on the spontaneous coherent mechanism of emission. This type of emission is realized if the effective phase size of the bunch with respect to the radiated wave is small enough. Therefore, the repulsion of particles caused by a strong Coulomb field inside the dense electron bunch strictly limits the duration of the radiation process due to the increase in the bunch length. We show that this problem can be solved by using the cyclotron mechanism of the spontaneous radiation due to the effect of compensation of the Coulomb repulsion in the phase space.

High-energy direct photoelectron spectroscopy in strong-field ionization

Recently, in a strong Coulomb field regime of tunneling ionization an unexpected large enhancement of photoelectron spectra due to the Coulomb field of the atomic core has been identified by numerical solution of time-dependent Schr\"odinger equation [Phys. Rev. Lett. \textbf{117}, 243003 (2016)] in the upper energy range of the tunnel-ionized direct electrons. We investigate the origin of the enhancement employing a classical theory with Monte Carlo simulations of trajectories, and a quantum theory of Coulomb-corrected strong field approximation based on the generalized eikonal approximation for the continuum electron. Although the quantum effects at recollisions with a small impact parameter yield an overall enhancement of the spectrum relative to the classical prediction, the high energy enhancement itself is shown to have a classical nature and is due to momentum space bunching of photoelectrons released not far from the peak of the laser field. The bunching is caused by a large and nonuniform, with respect to the ionization time, Coulomb momentum transfer at the ionization tunnel exit.

Guiding-center motion for electrons in strong laser fields

We consider the dynamics of electrons in combined strong laser and Coulomb fields. Under a timescale separation condition, we reduce this dynamics to a guiding-center framework. More precisely, we derive a hierarchy of models for the guiding-center dynamics based on averaging over the fast motion of the electron using Lie transforms. The reduced models we obtain describe well the different ionization channels, in particular, the conditions under which an electron is rescattered by the ionic core or is directly ionized. The comparison between these models highlights the models which are best suited for a qualitative and quantitative agreement with the parent dynamics.

Capturing Photoelectron Motion with Guiding Centers.

Electron motion in combined strong laser and Coulomb fields is central to laser-matter interactions. By mapping this problem onto the motion of a guiding center, we derive a reduced model which naturally embeds important Coulomb effects such as focusing and asymmetry, and clearly distinguishes direct versus rescattered electron ionization processes. We demonstrate the power of this tool by unraveling the bifurcation in photoelectron momentum distributions seen in experiments.

High-precision X-ray spectroscopy of highly-charged ions at the experimental storage ring using silicon microcalorimeters

X-ray spectroscopy on highly charged heavy ions provides a sensitive test of quantum electrodynamics in very strong Coulomb fields. One limitation of the current accuracy of such experiments is the energy resolution of available X-ray detectors for energies up to 100keV. To improve this accuracy, a novel detector concept, namely the concept of microcalorimeters, is exploited for this kind of measurements. The microcalorimeters used in the present experiments consist of silicon thermometers, ensuring a high dynamic range, and of absorbers made of high-Z material to provide high X-ray absorption efficiency. Recently, besides an earlier used detector, a new compact detector design, housed in a new dry cryostat equipped with a pulse tube cooler, was applied at a test beamtime at the experimental storage ring (ESR) of the GSI facility in Darmstadt. A U89+ beam at 75MeV/u and a 124Xe54+ beam at various beam energies, both interacting with an internal gas-jet target, were used in different cycles. This test was an important benchmark for designing a larger array with an improved lateral sensitivity and statistical accuracy.

Precise determination of the 1s Lamb shift in hydrogen-like lead and gold using microcalorimeters

Quantum electrodynamics in very strong Coulomb fields is one scope which has not yet been tested experimentally with sufficient accuracy to really determine whether the perturbative approach is valid. One sensitive test is the determination of the 1s Lamb shift in highly-charged very heavy ions. The 1s Lamb shift of hydrogen-like lead (Pb81+) and gold (Au78+) has been determined using the novel detector concept of silicon microcalorimeters for the detection of hard x-rays. The results of eV for lead and eV for gold are within the error bars in good agreement with theoretical predictions. To our knowledge, for hydrogen-like lead, this represents the most accurate determination of the 1s Lamb shift.

Systematic Vibration Studies on a Cryogen-Free $$^3$$ 3 He/ $$^4$$ 4 He Dilution Refrigerator for X-ray Spectroscopy at Storage Rings

High-precision X-ray spectroscopy of highly charged ions at storage rings provides a sensitive test of quantum electrodynamics in strong Coulomb fields. To increase the precision of such experiments, silicon microcalorimeters have already been applied successfully. To minimize the interruption of beam times due to maintenance, a new cryogen-free \(^3\)He/\(^4\)He dilution refrigerator has been designed and is under commissioning. However, in cryogen-free systems microphonic noise due to vibrations contributes considerably to the overall noise and may limit the detector energy resolution. Therefore, we report on systematic vibration studies on a cryogen-free \(^3\)He/\(^4\)He dilution refrigerator which is specially adapted for experiments at storage rings.

Spin-orbit interaction in bremsstrahlung and its effect on the electron motion in a strong Coulomb field

Linear polarization of bremsstrahlung x rays produced in collisions of longitudinally polarized 2.1-MeV electrons with gold atoms was studied using the Compton scattering technique. We observed that the angle of x-ray polarization is strongly correlated with the incoming electron polarization. This correlation reveals the dominance of the spin-orbit interaction in bremsstrahlung and indicates a striking effect of the electron spin on the electron motion in a strong Coulomb field. The results confirm the validity of the theoretical predictions in a computationally challenging energy regime.

Precise determination of the 1s Lamb Shift in hydrogen-like heavy ions at the ESR storage ring using microcalorimeters

The precise determination of the energy of the Lyman α1 and α2 lines in hydrogen-like heavy ions provides a sensitive test of quantum electrodynamics in very strong Coulomb fields. To improve the precision of such experiments, the new detector concept of microcalorimeters, which detect the temperature change of an absorber after an incoming particle or photon has deposited its energy as heat, is now exploited. The microcalorimeters for x-rays used in these experiments consist of arrays of silicon thermometers and x-ray absorbers made of high-Z material. With such detectors, a relative energy resolution of about 1 per mille is obtained in the energy regime of 50–100 keV. Two successful measurement campaigns to determine the 1s Lamb Shift in Pb81+ and Au78+ have been completed: a prototype array has been applied successfully for the determination of the 1s Lamb Shift of Pb81+ at the ESR storage ring at GSI in a first test experiment. Based on the results of this test, a full array with 32 pixels has been equipped and has recently been applied to determine the 1s Lamb Shift in Au78+ ions. The energy of the Lyman-α1 line agrees within error bars well with theoretical predictions. The obtained accuracy is already comparable to the best accuracy obtained with conventional germanium detectors for hydrogen-like uranium.

Lamb shift for states withj=1/2

The Lamb shift in the light hydrogenlike atoms is evaluated for the state-dependent part of the $S$ states $({n}^{3}\mathrm{\ensuremath{\Delta}}{E}_{n}\ensuremath{-}\mathrm{\ensuremath{\Delta}}{E}_{1})/{n}^{3}$ and ${P}_{1/2}$ states by means of a recently proposed relativistic generalization of multipole expansion. We obtain the results for $Z=1--5$ and $n=2--10$, where $Z$ is the charge of the nucleus and $n$ is the principal quantum number of the state under consideration. The results are in an excellent agreement with the results obtained by means of $Z\ensuremath{\alpha}$ expansion truncated after the $\ensuremath{\alpha}{(Z\ensuremath{\alpha})}^{6}$ term. The state-dependent part of the $S$ states for $n=2--4$ is evaluated also for the case of moderately strong Coulomb fields, $Z=10--50$. It is argued that the present results for the state-dependent part of the $S$ states for low $Z$ are the most accurate results given so far in the literature. In the case of hydrogen the uncertainty is significantly less than 1 Hz. Finally, the extrapolation of the results to infinite $n$ is performed.

Observation of Coherence in the Time-Reversed Relativistic Photoelectric Effect

The photoelectric effect has been studied in the regime of hard x rays and strong Coulomb fields via its time-reversed process of radiative recombination (RR). In the experiment, the relativistic electrons recombined into the 2p_{3/2} excited state of hydrogenlike uranium ions, and both the RR x rays and the subsequently emitted characteristic x rays were detected in coincidence. This allowed us to observe the coherence between the magnetic substates in a highly charged ion and to identify the contribution of the spin-orbit interaction to the RR process.

New results on Coulomb effects in meson production in relativistic heavy ion collisions

We propose a new method of investigating the space-time evolution of meson production in heavy ion collisions, by making use of spectator-induced electromagnetic (“Coulomb”) effects. The presence of two nuclear remnants (“spectator systems”) in the non-central collision generates a strong Coulomb field, which modifies the trajectories of charged final state hadrons. This results in charge-dependent azimuthal anisotropies in final state meson emission. In our approach, this effect can be computed numerically by means of a high-statistics Monte Carlo simulation, using the distance between the meson formation zone and the spectator system as free parameter. Our simulation correctly describes the electromagnetic effect on azimuthal anisotropies observed for π + and π − mesons in Au+Au collisions at lower RHIC energy, known from data recently reported by the STAR Collaboration. Similarly to our earlier studies of spectator-induced electromagnetic effects, also in the present study we find that these effects offer sensitivity to the position of the meson formation zone with respect to the spectator system. Therefore, we conclude that they can serve as a new tool to investigate the space-time evolution of meson production, and the dynamics of the heavy ion collision.