Towards high precision in-trap laser spectroscopy of highly charged ions

The extremely strong fields that exist around the nuclei of few-electron heavy ions drastically change the properties of the electronic system such as energy level spacings, lifetimes and magnetic moments. In turn, the electrons serve as sensitive probes for nuclear properties such as size, magnetic moment and spatial distribution of charge and magnetization. The energies of their forbidden fine and hyperfine structure transitions strongly depend on the nuclear charge and shift from microwave domains in or close to the optical domain. Thus, they become accessible for laser spectroscopy and its potentially high precision. A number of such measurements have been performed in storage rings and electron beam ion traps, and have yielded results with relative accuracies in the permille region. We present here an experiment under development at GSI Darmstadt which aims to significantly increase the accuracy by employing charged particle traps which allow to slow the ion motion nearly to rest, thus reducing Doppler effects and increasing the possible accuracy to the more than ppm region.     

A laser system for the spectroscopy of highly charged bismuth ions

We present and characterize a laser system for the spectroscopy on highly charged ²⁰⁹Bi⁸²⁺ ions at a wavelength of 243.87?nm. For absolute frequency stabilization, the laser system is locked to a near-infra-red laser stabilized to a rubidium transition line using a transfer cavity based locking scheme. Tuning of the output frequency with high precision is achieved via a tunable rf offset lock. A?sample-and-hold technique gives an extended tuning range of several THz in the UV. This scheme is universally applicable to the stabilization of laser systems at wavelengths not directly accessible to atomic or molecular resonances. We determine the frequency accuracy of the laser system using Doppler-free absorption spectroscopy of Te₂ vapor at 488?nm. Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty of σ _{244 nm}=6.14?MHz (one standard deviation) over six days of operation.     

Precision laser spectroscopy of highly charged ions

Recently, intense beams of highly charged ions have become available at heavy ion cooler rings. The obstacle for producing these highly interesting candidates is the large binding energy of K-shell electrons in heavy systems in excess of 100 keV. One way to remove these electrons is to strip them off by passing the ion through material. In the cooler ring, the ions are cooled to a well defined velocity. At the SIS/ESR complex it is possible to produce, store, and cool highly charged ions up to bare uranium with intensities exceeding 10⁸ atoms in the ring. This opens the door for precision laser spectroscopy of hydrogenlike-heavy ions, e.g.²⁰⁹Bi⁸²⁺, and allows to examine the interaction of the single electron with the large fields of the heavy nucleus, exceeding any artificially produced electric and magnetic fields by orders of magnitude. In the electron cooler the interaction of electrons and highly charged ions otherwise only present in the hottest plasmas can be studied.     

Half-open Penning trap with efficient light collection for precision laser spectroscopy of highly charged ions

We have conceived, built and operated a ’half-open’ cylindrical Penning trap for the confinement and laser spectroscopy of highly charged ions. This trap allows fluorescence detection employing a solid angle which is about one order of magnitude larger than in conventional cylindrical Penning traps. At the same time, the desired electrostatic and magnetostatic properties of a closed-endcap cylindrical Penning trap are preserved in this configuration. We give a detailed account on the design and confinement properties, a characterization of the trap and show first results of light collection with in-trap produced highly charged ions.     

Status of deceleration and laser spectroscopy of highly charged ions at HITRAP

Heavy few-electron ions are relatively simple systems in terms of electron structure and offer unique opportunities to conduct experiments under extremely large electromagnetic fields that exist around their nuclei. However, the preparation of highly charged ions (HCI) has remained the major challenge for experiments. As an extension of the existing GSI accelerator facility, the HITRAP facility was conceived as a multi-stage decelerator for HCI produced at high velocity. It is designed to prepare bunches of around 10⁵ HCI and to deliver them at low energies to various experiments. One of these experiments is SpecTrap, aiming for laser spectroscopy of trapped, cold HCI. We present the latest results on deceleration of ions in a radio-frequency quadrupole, synchrotron cooling of electrons in a trap as a preparation step for the prospective electron cooling of the HCI decelerated in HITRAP, as well as laser cooling of singly charged Mg ions for sympathetic cooling of HCI in SpecTrap.     

Soft x-ray laser spectroscopy on trapped highly charged ions at FLASH

In a proof-of-principle experiment, we demonstrate high-resolution resonant laser excitation in the soft x-ray region at 48.6 eV of the 2 (2)S(1/2) to 2 (2)P(1/2) transition of Li-like Fe23+ ions trapped in an electron beam ion trap by using ultrabrilliant light from Free Electron Laser in Hamburg (FLASH). High precision spectroscopic studies of highly charged ions at this and upcoming x-ray lasers with an expected accuracy gain up to a factor of a thousand, become possible with our technique, thus potentially yielding fundamental insights, e.g., into basic aspects of QED.     

Absolute dielectronic recombination rate coefficients of highly charged ions at the storage ring CSRm and CSRe

Dielectronic recombination(DR) is one of the dominant electron–ion recombination mechanisms for most highly charged ions(HCIs) in cosmic plasmas, and thus, it determines the charge state distribution and ionization balance therein.To reliably interpret spectra from cosmic sources and model the astrophysical plasmas, precise DR rate coefficients are required to build up an accurate understanding of the ionization balance of the sources. The main cooler storage ring(CSRm) and the experimental cool...     

Laser spectroscopy methods for probing highly charged ions at GSI

We describe two opposite and partly complementary experimental approaches for performing high-precision laser spectroscopy of dipole-forbidden transitions in highly charged ions. We report on the wavelength determination of the ground state hyperfine transitions in hydrogen-like and lithium-like bismuth ions confined in the experimental storage ring at GSI. Direct comparison of the experimental results with theoretical predictions reveals an agreement of the specific hyperfine-structure splitting difference $\Delta ^{\prime }E$ within the 1- σ confidence interval of the experimental value. Additionally, we discuss an experimental strategy based on ion manipulation and cooling in a cylindrical open-endcap Penning trap to further increase the precision of the previous measurement. Trapping and laser cooling of external produced singly charged magnesium ions is demonstrated. This represents a first step towards sympathetic cooling of simultaneously confined ion species in order to perform laser spectroscopy measurements on highly charged ions nearly at rest. These measurements will offer new prospects in the field of laser-based tests of quantum electrodynamics in strong electric and magnetic fields.     

Rate predictions for two photon spectroscopy of highly charged ions through laser induced recombination

Laser nitriding is used for the fast and easy production of nitride coatings on iron and alloys. Here, first results of the laser nitriding process applied to stainless steel are reported. The laser treatment led to the appearance of additional lines in the Mössbauer spectra, which are attributed to γ-Fe(N) produced by the laser nitriding process. The Mössbauer results are discussed in connection with the results obtained from X-ray diffraction and resonant nuclear reaction analysis. Furthermore, the results of isochronical annealing treatments of laser nitrided iron are reported.     

Laser spectroscopy for QED tests in highly charged ions

An overview on laser spectroscopic work on highly charged ions (HCI) currently performed or in preparation at GSI is given. This includes laser spectroscopy on HCI in ion traps at the HITRAP facility and at the experimental storage ring (ESR).     

High-precision metrology of highly charged ions via relativistic resonance fluorescence

Resonance fluorescence of laser-driven highly charged ions is investigated with regard to precisely measuring atomic properties. For this purpose an ab initio approach based on the Dirac equation is employed that allows for studying relativistic ions. These systems provide a sensitive means to test correlated relativistic dynamics, quantum electrodynamic phenomena and nuclear effects by applying x-ray lasers. We show how the narrowing of sidebands in the x-ray fluorescence spectrum by interference due to an additional optical driving can be exploited to determine atomic dipole or multipole moments to unprecedented accuracy.     

Iodine laser production of highly charged Ta ions

The results of systematic studies of multiply charged Ta ion production with the fundamental frequency of an iodine laser (=1.315m), and its 2nd (0.657m) and 3rd (0.438m) harmonics are summarized and discussed. Short laser pulse (350 ps) and a focus spot diameter of 100m allowed for the laser power densities in the range of 5×10¹³–1.5×10¹⁵ W/cm². Corpuscular diagnostics were based on time-of-flight methods; two types of ion collectors and a cylindrical electrostatic ion energy analyzer were used. The Ta ions with charge state up to 55+ were registered in the distance of 210 cm; the maximum amplitude of the signal of a high energy ion group was found to belong to the ions with the charge state around 43+, depending on the laser power density. The ion energy distribution was measured for all three wavelengths, however, in a different energy range; the maximum registered ion energy was 8.8 MeV. The occurrence of highly charged ions in the far expansion zone is discussed in view of the mechanism of charge distribution freezing during two-temperature isothermal plasma expansion.The work was performed in a partial fulfillment of the research grant project No. A1010525 sponsored by the Academy of Sciences of the Czech Republic and grant project No. 202/95/0039 sponsored by the Grant Agency of the Czech Republic.     

Numerical investigations on resistive cooling of trapped highly charged ions

The investigation of simple atomic systems in extreme electromagnetic fields with highest precision demands intense beams of heavy and highly charged ions to be decelerated and cooled to extremely low energies for long-time observation. Resistive cooling, i.e., the electronic dissipation of energy of a stored ion cloud on an external impedance, seems to be a good candidate for in-trap cooling of highly charged ions and has been chosen for the upcoming HITRAP (Highly charged Ion TRAP) facility at GSI, Darmstadt. Nevertheless, resistive cooling of large ensembles of ions confined in a Penning trap has never been thoroughly studied until now and the understanding of this highly nonlinear phenomenon is far from complete. Through the use of systematic simulations we give a proof of the feasibility of resistive cooling of large numbers of highly charged ions as well as the interpretation of some specific features observed experimentally.     

Preparing a laser cooled plasma for stopping highly charged ions

We present a new cooling scheme for the preparation of highly charged ions for future in-trap precision experiments. A plasma of laser cooled ²⁴Mg⁺ ions trapped in a 3D harmonic confinement potential is used as a stopping medium for the highly charged ions. We focus on the dynamic evolution of the plasma, determining suitable cooling conditions for fast recooling of the ²⁴Mg⁺ ions. The results of a realistic parallel simulation of the complete stopping process presented here indicate that a small, constant detuning of the laser frequency is sufficient for subsequent recooling of the plasma, thus maintaining the stability of the plasma.     

高剥离态Au、U离子电离势的相对论计算

采用全相对论量子力学GRASP 2程序平均能级AL模型,考虑到核的有限体积效应、Breit和QED效应,系统地计算了金和铀的高剥离态的部分离子态的电离势,将所得的计算结果和文献报道的实验值进行比较.结果表明,考虑高剥离态原子的高阶修正后,高剥离态离子的电离势的计算值与实验数据之间的误差基本消除.     

Atomic spectroscopy of trapped, highly charged, heavy ions

For spectroscopy, the electron beam ion trap (EBIT) is of special interest, because it provides a cloud of highly charged ions that is confined to a small volume, at very low particle density, at a relatively low temperature, and without any net velocity in the laboratory rest frame. These conditions are favourable for observations at high spectral resolution and wavelength accuracy. Examples from recent work at Livermore comprise extreme ultraviolet and soft-X-ray spectra. A time-resolving multi-pixel microcalorimeter furthermore permits the study of time-dependent plasma phenomena and atomic lifetimes. Lifetime measurements at a heavy-ion storage ring can be combined with EBIT measurements to clarify isoelectronic behaviour.     

Excitation and ionization cross sections of hydrogen-like atoms in collisions with relativistic highly charged ions

The relativistic eikonal approximation and a matching procedure are used to describe excitation and ionization of hydrogen-like atoms from an arbitrary discrete energy state by the impact of a highly charged relativistic bare ion. Bethe-type formulas are derived that are asymptotically valid in the limits of v → c and Z ? 1, where v is the relative collision velocity, c is the speed of light, and Z is the ion charge.