High precision x-ray spectroscopy on highly charged argon ions

High-precision wavelength measurements on the $1s2s\ ^3S_1 \rightarrow 1s^2\ ^1S_0$ ($z$'') and $1s2p\ ^1P_1 \rightarrow 1s^2\ ^1S_0$ ($w$'') transitions in Ar$^{16+}$ as well as of the Lyman-$\alpha_1$ transition in Cl$^{16+}$ with respect to the Lyman-$\alpha_1$ transition in Ar$^{17+}$ were carried out, using a new flat crystal spectrometer installed at the Heidelberg electron beam ion trap (HD-EBIT). A novel, highly accurate technique of Bragg-angle determination was developed, employing two beams of visible light reflected on the x-ray crystal to mark the x-ray reflection position. The need for collimating entrance slits causing unacceptable x-ray flux losses is thereby avoided. Relative uncertainties of better than $\Delta \lambda / \lambda < 10^{-5}$ were achieved in the measurement of all three lines. The measured Cl$^{16+}$ Lyman-$\alpha_1$ transition energy is in perfect agreement with the theoretical prediction and previous experimental work, however $4$ times more accurate. The $z$ transition energy, never measured before in argon, agrees with predictions within its error bar. The $w$ transition energy, while in perfect agreement with but two times more accurate than earlier experimental results, disagrees with by $2\sigma$ with predictions, pointing at their possible incompleteness

Progress at the Heidelberg EBIT

Two years after the relocation of the Heidelberg EBIT, several experiments are already in operation. Spectroscopic measurements in the optical region have delivered the most precise reported wavelengths for highly charged ions, in the case of the forbidden transitions of Ar XIV and Ar XV. The lifetimes of the metastable levels involved in those transitions has been determined with an error of less than 0.2%. A new, fully automatized x-ray crystal spectrometer allows systematic measurements with very high precision and reproducibility. Absolute measurements of the Lyman series of H-like ions are currently underway. Dielectronic recombination studies have yielded information on rare processes, as two-electron-one photon transitions in Ar16+, or the interference effects between dielectronic and radiative recombination in Hg77+. The apparatus can now operate at electron beam currents of more than 500 mA, and energies up to 100 keV. A further beam energy increase is planned in the near future. Ions can be extracted from the trap and transported to external experiments. Up to 4 x 107 Ar16+ ions per second can be delivered to a 1 cm diameter target at 10 m distance. Charge-exchange experiments with U64+ colliding with a cold He atomic beam have been carried out, as well as experiments aiming at the optimization of the charge state distribution of the extracted via dielectronic recombination. Two new EBITs, currently in advanced state of construction in Heidelberg, will be used for experiments at the VUV free electron laser at TESLA (Hamburg) and for the charge breeding of short-lived radioactive isotopes at the TRIUMF ISAC facility

Benchmarking High-Field Few-Electron Correlation and QED Contributions in Hg⁷⁵⁺ to Hg⁷⁸⁺ Ions. I. Experiment

The photorecombination of highly charged few-electron mercury ions Hg75+ to Hg78+ has been explored with the Heidelberg electron beam ion trap. By monitoring the emitted x rays (65-76 keV) and scanning the electron beam energy (45-54 keV) over the KLL dielectronic recombination (DR) region, the energies of state-selected DR resonances were determined to within ±4 eV (relative) and ±14 eV (absolute). At this level of experimental accuracy, it becomes possible to make a detailed comparison to various theoretical approaches and methods, all of which include quantum electrodynamic (QED) effects and finite nuclear size contributions (for a 1s electron, these effects can be as large as 160 and 50 eV, respectively). In He-like Hg78+, a good agreement between the experimental results and the calculations has been found. However, for the capture into Li-, Be-, and B-like ions, significant discrepancies have been observed for specific levels. The discrepancies suggest the need for further theoretical and experimental studies with other heavy ions along these isoelectronic sequences

Relativistic Electron Correlation, Quantum Electrodynamics, and the Lifetime of the 1s²2s²2p²P\u3csup\u3eo\u3c/sup\u3e\u3csub\u3e3/2\u3c/sub\u3e Level in Boronlike Argon

The lifetime of the Ar13+ 1s22s22p2Po3/2 metastable level was determined at the Heidelberg Electron Beam Ion Trap to be 9.573(4)(5)ms(stat)(syst). The accuracy level of one per thousand makes this measurement sensitive to quantum electrodynamic effects like the electron anomalous magnetic moment (EAMM) and to relativistic electron-electron correlation effects like the frequency-dependent Breit interaction. Theoretical predictions, adjusted for the EAMM, cluster about a lifetime that is approximately 3σ shorter than our experimental result

Exploring Relativistic Many-Body Recoil Effects in Highly Charged Ions

The relativistic recoil effect has been the object of experimental investigations using highly charged ions at the Heidelberg electron beam ion trap. Its scaling with the nuclear charge Z boosts its contribution to a measurable level in the magnetic-dipole (M1) transitions of B- and Be-like Ar ions. The isotope shifts of 36Ar versus 40Ar have been detected with sub-ppm accuracy, and the recoil effect contribution was extracted from the 1s22s22p 2P1/2-2P3/2 transition in Ar13+ and the 1s22s2p 3P1-3P2 transition in Ar14+. The experimental isotope shifts of 0.00123(6) nm (Ar13+) and 0.00120(10) nm (Ar14+) are in agreement with our present predictions of 0.00123(5) nm (Ar13+) and 0.00122(5) nm (Ar14+) based on the total relativistic recoil operator, confirming that a thorough understanding of correlated relativistic electron dynamics is necessary even in a region of intermediate nuclear charges

Zeeman Splitting and g Factor of the 1s²2s²2p ²P\u3csub\u3e3/2\u3c/sub\u3e and ²P\u3csub\u3e1/2\u3c/sub\u3e Levels in Ar¹³⁺

The Zeeman line components of the magnetic-dipole (M1) 1s2 2s2 2 P1/2 -2P3/2 transition in boronlike Ar13+ were experimentally resolved by high-precision emission spectroscopy using the Heidelberg electron beam ion trap. We determined the gyromagnetic (g) factors of the ground and first-excited levels to be g1/2 =0.663(7) and g3/2 =1.333(2), respectively. This corresponds to a measurement of the g factor of a relativistic electron in a bound non- S state of a multielectron ion with a 1.5 parts-per-thousand accuracy. The results are compared to theoretical calculations by means of the configuration interaction Dirac-Fock-Sturmian method including electron correlation effects and additional quantum electrodynamic corrections. Our measurements show that the classical Landé g factor formula is sufficiently accurate to the present level of accuracy in few-electron ions of medium nuclear charge number Z

Single-Electron Capture in keV Ar\u3csup\u3e15+...18+\u3c/sup\u3e + He Collisions

Single-electron capture in 14 keV q-1 Ar15+...18++He collisions is investigated both experimentally and theoretically. Partial cross sections and projectile scattering angle dependencies have been deduced from the target ion recoil momenta measured by the COLTRIMS technique. The comparison with close-coupling results obtained from a two-centre extension of the basis generator method yields good overall agreement, demonstrating the applicability of close-coupling calculations to collision systems involving highly charged ions in charge states up to 18+