E. Träbert

Intercombination lines in delayed beam-foil spectra

The intrinsic time resolution of the beam-foil light source has been utilized in order to single out and to identify intercombination lines in the Mg i, Al i, and Si i isoelectronic sequences for ions with nuclear charge Z = 18–30 (Ar, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn). The method is described, and wavelength and lifetime data are presented. For the ions in the Al i and Si i sequences, the level positions of the high–spin term systems (lowest quartet and quintet levels, respectively) have been determined.

Experimental M1 Transition Rates of Coronal Lines from Ar X, Ar XIV, and Ar XV

Transition probabilities of three magnetic dipole (M1) transitions in multiply charged ions of Ar have been measured using the Livermore electron-beam ion trap. Two of the transitions are in the ground con—gurations of Ar XIV (B-like) and Ar IX (F-like), and are associated with the coronal lines at 4412.4 and 5533.4 respectively. The third is in the excited 2s2p con—guration of Be-like Ar XV and produces Ae , the coronal line at 5943.73 Our results for the three atomic level lifetimes are 9.32 ^ 0.12 ms for the Ae . Ar X 2s22p5 level, 9.70 ^ 0.15 ms for the Ar XIV 2s22p level, and 15.0 ^ 0.8 ms for the Ar XV 2P 1@2 2P 3@2 2s2p level. These results diUer signi—cantly from earlier measurements and are the most accurate 3P 2 ones to date. Subject headings: atomic datamethods: laboratory

An unexpectedly low oscillator strength as the origin of the Fe xvii emission problem

Highly charged iron (Fe16+, here referred to as Fe xvii) produces some of the brightest X-ray emission lines from hot astrophysical objects, including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 ångströms. The Fe xvii spectrum is, however, poorly fitted by even the best astrophysical models. A particular problem has been that the intensity of the strongest Fe xvii line is generally weaker than predicted. This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions, fuelling a continuing controversy over whether this discrepancy is caused by incomplete modelling of the plasma environment in these objects or by shortcomings in the treatment of the underlying atomic physics. Here we report the results of an experiment in which a target of iron ions was induced to fluoresce by subjecting it to femtosecond X-ray pulses from a free-electron laser; our aim was to isolate a key aspect of the quantum mechanical description of the line emission. Surprisingly, we find a relative oscillator strength that is unexpectedly low, differing by 3.6σ from the best quantum mechanical calculations. Our measurements suggest that the poor agreement is rooted in the quality of the underlying atomic wavefunctions rather than in insufficient modelling of collisional processes.

Flat-field grating spectrometer for high-resolution soft x-ray and extreme ultraviolet measurements on an electron beam ion trap

A R=44.3 m grazing-incidence grating spectrometer has been implemented on the Livermore electron beam ion traps for high-resolution measurements in the soft x-ray and extreme ultraviolet region spanning from below 10 up to 50 A. The instrument uses a grating with variable line spacing (about 2400 l/mm for a flat field of view. Spectra are recorded with a back-illuminated charge-coupled device detector. The new instrument greatly improves upon the resolution achieved with existing grating spectrometers and complements crystal spectrometers at the shorter wavelengths both in terms of wavelength coverage and polarization independent reflectivity response.

Laboratory Wavelengths of K-Shell Resonance Lines of O V and O VI

We present wavelength measurements of K-shell resonance lines of O V and O VI using the University of California Lawrence Livermore National Laboratory EBIT-I electron beam ion trap. The wavelength accuracy of better than 140 parts per million is sufficient to determine gas outflow velocities of warm absorbers associated with active galactic nuclei to within 40 km s-1 and better. Our measurements confirm that the outflow velocities associated with NGC 5548 and derived from the O V and O VI lines are similar to those derived from the O VII lines. These kinematic measurements make for further evidence that the X-ray and UV absorbers in these systems are truly two manifestations of the same physical outflow.

Experimental and theoretical evaluation of density-sensitive N VI, AR XIV, and FE XXII line ratios

The line ratios of the 2p-3d transitions in the B-like spectra Ar XIV and Fe XXII have been measured using the electron beam ion traps at Livermore. Radiative-collisional model calculations show these line ratios to be sensitive to the electron density in the ranges ne = 10{sup 10} to 10{sup 12} cm{sup -3} and ne = 10{sup 13} to 10{sup 15} cm{sup -3}, respectively. In our experiment, the electron beam density of about 10{sup 11} cm{sup -3} was varied by about a factor of 5. Our data show a density effect for the line doublet in Ar XIV, and good agreement with theory is found. The relative intensity of the Fe XXII doublet shows good agreement with our predicted low density limit. The N VI K-shell spectrum was used to infer the actual electron density in the overlap region of ion cloud and electron beam, and systematic measurements and calculations of this spectrum are presented as well. The Ar XIV and Fe XXII spectra promise to be reliable density diagnostics for stellar coronae, complementing the K-shell diagnostics of helium-like ions.

Measurement of the B+ and Al+ intercombination and Sc12+ forbidden transition rates at a heavy-ion storage ring

Electric-dipole intercombination and forbidden transitions have been optically observed with ions circulating in a storage ring, and atomic lifetimes determined. For the level in the Be-like ion , the measured lifetime of ms corresponds to a transition probability of . This result ties in with experimental work on the neighbouring ion and with recent calculations. For the corresponding level in the Mg-like ion , a lifetime of s (transition rate ) has been measured, corroborating earlier experiments but not the more recent calculations. The transition probability of the M1/E2 transition between the fine-structure levels of the ground state in F-like has been determined as , in agreement with the results of semi-empirically corrected multi-configuration Dirac-Fock calculations.

Beam-Foil Lifetime Studies of Highly Ionized Silicon

Previous beam-foil lifetime studies in the EUV range are extended to cover all n = 2 states of Si VII and Si IX. The lifetime values obtained agree with theoretical data of Nicolaides and Beck. For some states of Si X and Si XI information about initial population ratios of singly and doubly excited states is obtained. After foil excitation, the contribution of cascades from doubly excited states in the n = 2 shell to the intensities of decays from singly excited to ground states amounts to 20-40%.

Laboratory Measurement and Theoretical Modeling of K-shell X-ray Lines from Inner-shell Excited and Ionized Ions of Oxygen

We present high resolution laboratory spectra of K-shell X-ray lines from inner-shell excited and ionized ions of oxygen, obtained with a reflection grating spectrometer on the electron beam ion trap (EBIT-I) at the Lawrence Livermore National Laboratory. Only with a multi-ion model including all major atomic collisional and radiative processes, are we able to identify the observed K-shell transitions of oxygen ions from O III to O VI. The wavelengths and associated errors for some of the strongest transitions are given, taking into account both the experimental and modeling uncertainties. The present data should be useful in identifying the absorption features present in astrophysical sources, such as active galactic nuclei and X-ray binaries. They are also useful in providing benchmarks for the testing of theoretical atomic structure calculations.

Determination of beam-foil lifetimes <100 ps with a grazing-incidence spectrometer

Abstract Lifetime determinations of radiative atomic transitions in the extreme ultraviolet region with the beam-foil technique suffer from the fact that the spectrometer views a section of the beam that corresponds to several decay lengths. A method is presented which allows to measure lifetimes down to 10 ps by taking into account the window function of the spectrometer including the angular dependence of the grating reflectivity.