Publications
A selection from Stockholm University publication database
2007. P. Royen (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 76 (3), 030502
A fast ion beam stored in an ion storage ring will suffer from collisional interaction with the residual gas even in extreme ultrahigh vacuum. Normally these effects are negligible or easy to correct for. For measurements of extremely long radiative lifetimes of metastable levels we have found that correction for collisional effects is very important and usually limits the accuracy. We now suggest that by monitoring the process of collisional excitation of the metastable level of interest the lifetime value could be determined directly with improved accuracy. The method is applied to available data from our recent measurement of the 5d ^{2}D_{5∕2} level in Ba+, yielding a value of 32.0±2.9 s, thus squeezing the error bar by 35%.

2007. Jonas Gurell (et al.). Physical Review A. Atomic, Molecular, and Optical Physics 75 (5), 052506
The two metastable levels 5d ^{2}D_{3∕2} and 5d ^{2}D_{5∕2} in Ba II both show extremely long lifetimes of the order of several tens of seconds each. This has been found both by experiments and by theoretical predictions. The small transition probabilities associated with these two levels make them interesting and challenging for theoreticians as well as for experimentalists. Several calculations and measurements of these two lifetimes have been made previously but discrepancies between the results are present. This article presents values of τ=89.4±15.6 s for the ^{2}D_{3∕2} level and τ=32.0±4.6 s for the ^{2}D_{5∕2} level measured in a beamlaser experiment performed at the ion storage ring CRYRING. These values are supported by our new calculations resulting in τ=82.0 s for the ^{2}D_{3∕2} level and τ=31.6 s for the ^{2}D_{5∕2} level.

2007. P. Lundin (et al.). Physical Review Letters 99, 213001
A laser probing investigation has yielded the lifetimes of the 3s^{2}3p^{4}(^{1}D)3d ^{2}G_{7/2,9/2} metastable doublet states of Ar+. The results, obtained with the CRYRING ion storage ring of Stockholm, are 3.0±0.4 and 2.1±0.1 s, respectively. Comparisons with theoretical values calculated with two independent theoretical approaches, i.e., the pseudorelativistic HartreeFock method and the multiconfiguration BreitPauli approach, have allowed us to establish the unexpected and extraordinary strong contribution of an electric octupole (E3) transition to the ground state, in addition to the M1 decay channels to the 3d ^{2},^{4}F states and the E2 contributions to the 4s ^{2}P, ^{2}D states.

2008. Henrik Hartman (et al.). Astronomy and Astrophysics 480 (2), 575580
Context. In many plasmas, longlived metastable atomic levels are depopulated by collisions (quenched) before they decay radiatively. In lowdensity regions, however, the low collision rate may allow depopulation by electric dipole (E1) forbidden radiative transitions, socalled forbidden lines (mainly M1 and E2 transitions). If the atomic transition data are known, these lines are indicators of physical plasma conditions and used for abundance determination.
Aims. Transition rates can be derived by combining relative intensities between the decay channels, socalled branching fractions (BFs), and the radiative lifetime of the common upper level. We use this approach for forbidden [Sc II] lines, along with new calculations.
Methods. Neither BFs for forbidden lines, nor lifetimes of metastable levels, are easily measured in a laboratory. Therefore, astrophysical BFs measured in Space Telescope Imaging Spectrograph (STIS) spectra of the strontium filament of Eta Carinae are combined with lifetime measurements using a laser probing technique on a stored ionbeam (CRYRING facility, MSL, Stockholm). These quantities are used to derive the absolute transition rates (Avalues). New theoretical transition rates and lifetimes are calulated using the CIV3 code.
Results. We report experimental lifetimes of the Sc II levels 3d^{2 }a^{3}P0,1,2 with lifetimes 1.28, 1.42, and 1.24 s, respectively, and transition rates for lines from these levels down to 3d4s a^{3}D in the region 82708390 Å. These are the most important forbidden [Sc II] transitions. New calculations for lines and metastable lifetimes are also presented, and are in good agreement with the experimental data.

Article Metastable levels in ScII2008. Peter Lundin (et al.). Physica scripta. T 78 (1), 015301
The lifetime of the metastable level 3d^{2} ^{1}G_{4} in a singly charged scandium has been experimentally investigated at the ion storage ring CRYRING. A laser probing technique has been used for the studies. We also report calculated lifetimes and transition rates for 10 metastable levels in Sc II, including the 3d^{2} ^{1}G_{4} state, by calculations using configuration interaction wave functions. The lifetime of the ^{1}G_{4} level is experimentally determined to be τ_{exp}=2.64±0.18 s and is in good agreement with the theoretical calculation τ_{th}=2.84 s.

2008. Patrick Palmeri (et al.). Journal of Physics B 41 (12)
This paper presents new theoretical lifetimes of metastable levels in singly ionized titanium, Ti II. Along with the lifetimes, transition probabilities for several decay channels from these metastable levels are presented. The calculations are supported by experimental lifetime determinations of the 3d^{3} b ^{2}D_{5/2} and 3d^{2}(^{3}P)4s b ^{2}P_{3/2} levels along with revised values of the previously published lifetimes of the 3d^{2}(^{3}P)4s b ^{4}P_{5/2} and 3d^{2}(^{3}P)4s b ^{2}P_{1/2} levels originating partly from a reanalysis utilizing a recently developed method applied on the previously recorded data and partly from new measurements. The presented theoretical investigation of lifetimes of metastable levels in Ti II shows that the HFR calculations are in general compatible with measurements performed using the ion storage ring CRYRING of Stockholm University. The transition probabilities of forbidden lines derived from the new lifetime values will be useful for the diagnostics of low density laboratory or astrophysical plasmas, particularly those encountered in the strontium filament found in the ejecta of η Carinae.

2008. Jonas Gurell, Sven Mannervik, Ahmed AlKhalili.
Measurements of atomic and ionic data has been an active research area since the beginning of spectroscopic studies. The experimental data deduced from thesestudies are of great interest for our understanding of fundamental as well as applied physics. The metastability of certain energy levels has been used toexplain phenomena in widely different fields ranging from the principles of the laser to observations of forbidden spectral lines originating fromastrophysical objects. Eventhough measurements of radiative lifetimes have been carried out for many decades new interesting results are still found in theexplanation of phenomena observed in these studies. The technological development in the field of atomic physics has provided researchers with several toolswhich have opened up completely new possibilities in recent years. Atoms and ions can now be stored for long times which allows studies of extremely weakprocesses. In this thesis results from studies of weak radiative decays of metastable levels are presented. The measurements were carried out utilizing a laserprobing technique together with ions stored in the ion storage ring CRYRING at the Manne Siegbahn laboratory in Stockholm, Sweden. The longest radiativelifetime measured so far in a storage ring, 89 s in singly ionized barium, is presented along with a completely new method for determining extremelylong radiative lifetimes. The thesis also includes new interesting results from lifetime measurements in argon which revealed the unexpected importance of an E3 decay channel. This is to the best of our knowledge the first ever observation of an E3 transition rate of this magnitude in a singlycharged ion. All lifetime measurements are presented with complementary calculations by our collaborators at Université de MonsHainaut and Université de Liège, Belgium. The studies of BaII are also part of a collaboration with the Institute of solid state physics of the Bulgarian Academy of Sciences.

2009. Jonas Gurell (et al.). Physica Scripta 79 (3), 035306
We recorded spectra of Ho II emitted by hollowcathode lamps, using the ultraviolet Fourier transform spectrometer (FTS) at Lund Observatory and the 2m FTS at the National Institute of Standards and Technology. The combined wavenumber coverage spans the interval from 8000 cm^{1} to 49600 cm^{1}(12500 Å to 2016 Å). We measured 303 lines in these spectra and used them to determine energies and hyperfine structure constants of 100 levels. Ofthese, 41 energy levels have not been reported previously and hyperfine structure constants have not been reported previously for 83 levels. Comparisonsbetween the hyperfine structure constants and energy levels presented in this article and those previously determined from other studies are included anddiscrepancies are discussed. A theoretical study of the oddparity levels leads to hyperfine structure parameters and to predictions for unknown excited levelsof the groundstate configuration.

Article The FERRUM project2009. Jonas Gurell (et al.). Astronomy and Astrophysics 508 (1), 525529
Context. Accurate transition probabilities for forbidden lines are important diagnostic parameters for lowdensity astrophysical plasmas. In this paper we present experimental atomic data for forbidden [Fe II] transitions that are observed as strong features in astrophysical spectra. Aims. We measure lifetimes for the 3d(6)((3)G)4s a (4)G(11/2) and 3d(6)((3)D)4s b (4)D(1/2) metastable levels in Fe II and experimental transition probabilities for the forbidden transitions 3d(7) a (4)F(7/2,9/2)3d(6)((3)G)4s a (4)G(11/2). Methods. The lifetimes were measured at the ion storage ring facility CRYRING using a laser probing technique. Astrophysical branching fractions were obtained from spectra of Eta Carinae, obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. The lifetimes and branching fractions were combined to yield absolute transition probabilities. Results. The lifetimes of the a (4)G(11/2) and the b (4)D(1/2) levels have been measured and have the following values, tau = 0.75 +/ 0.10 s and tau = 0.54 +/ 0.03 s respectively. Furthermore, we have determined the transition probabilities for two forbidden transitions of a (4)F(7/2,9/2)a (4)G(11/2) at 4243.97 and 4346.85 angstrom. Both the lifetimes and the transition probabilities are compared to calculated values in the literature.

Article The FERRUM project2010. Jonas Gurell (et al.). Astronomy and Astrophysics 511 (A68)
Aims: We measure transition probabilities for Cr II transitions from the z ^4H_J, z ^2D_J, y ^4F_J, and y ^4G_J levels in the energy range 63000 to 68000 cm^{1}.
Methods:Radiative lifetimes were measured using timeresolved laserinduced fluorescence from a laserproduced plasma. In addition, branching fractions were determined from intensitycalibrated spectra recorded with a UV Fourier transform spectrometer. The branching fractions and radiative lifetimes were combined to yield accurate transition probabilities and oscillator strengths.
Results: We present laboratory measured transition probabilities for 145 Cr II lines and radiative lifetimes for 14 Cr II levels. The laboratorymeasured transition probabilities are compared to the values from semiempirical calculations and laboratory measurements in the literature.

Thesis (Doc) High Resolution Spectroscopy of Singly Charged Ions2010. Jonas Gurell, Sven Mannervik, Richard Holt.
This thesis presents measurements of fundamental properties of singly charged ionic systems including spectral wavelengths, lifetimes of excited states and branching fractions with the main focus on time resolved laser spectroscopy of stored ions providing lifetime measurements of metastable states. The results of these measurements have been used to determine energy levels and transition probabilities of the studied systems. The included experimental data are compared with results from calculations which provides evaluations of different theoretical models. The presented results have been applied by others to the field of atomic astrophysics in order to deduce electron densities and elemental abundances in ejecta of the supermassive star eta Carinae and have also been followed by additional work of theoreticians interested in comparisons with laboratory data. The thesis is a result of several collaborations in which Stockholm University has been providing lifetime measurements of metastable states, Lund Observatory and the National Institute of Standards and Technology have been providing wavelength and branching fraction measurements, Lund Laser Centre has been providing lifetime measurements of shortlived states and calculations have been performed by theoretical physicists from Université de MonsHainaut, Université de Liège, Queen's University of Belfast and Laboratoire Aimé Cotton.