P. Schef

The FERRUM Project: Experimental Transition Probabilities of [Fe II] and Astrophysical Applications

We report on experimental transition probabilities for thirteen forbidden (Fe II) lines originating from three dierent metastable Feii levels. Radiative lifetimes have been measured of two metastable states by applying a laser probing technique on a stored ion beam. Branching ratios for the radiative decay channels, i.e. M1 and E2 transitions, are derived from observed intensity ratios of forbidden lines in astrophysical spectra and compared with theoretical data. The lifetimes and branching ratios are combined to derive absolute transition probabilities, A-values. We present the first experimental lifetime values for the two Fe II levels a 4 G9=2 and b 2 H11=2 and A-values for 13 forbidden transitions from a 6 S5=2, a 4 G9=2 and b 4 D7=2 in the optical region. A discrepancy between the measured and calculated values of the lifetime for the b 2 H11=2 level is discussed in terms of level mixing. We have used the code CIV3 to calculate transition probabilities of the a 6 D-a 6 S transitions. We have also studied observational branching ratios for lines from 5 other metastable Fe II levels and compared them to calculated values. A consistency in the deviation between calibrated observational intensity ratios and theoretical branching ratios for lines in a wider wavelength region supports the use of (Fe II) lines for determination of reddening.

The FERRUM project: an extremely long radiative lifetime in Ti II measured in an ion storage ring

We have extended the laser probing technique at the CRYRING storage ring to measurement of the extremely long lifetime (28 s) of the metastable 3d2(3P)4s b 4P5/2 level in Ti II. The result obtained demonstrates the power of this method for investigation of such long-lived levels. This is the first experimental lifetime investigation of metastable states in Ti II.

The FERRUM project: Experimental and theoretical transition rates of forbidden [Sc II] lines and radiative lifetimes of metastable Sc II levels

Context. In many plasmas, long-lived metastable atomic levels are depopulated by collisions (quenched) before they decay radiatively. In low-density regions, however, the low collision rate may allow depopulation by electric dipole (E1) forbidden radiative transitions, so-called 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, so-called 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 ion-beam (CRYRING facility, MSL, Stockholm). These quantities are used to derive the absolute transition rates (A-values). 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)P(0,1,2) 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 8270-8390 angstrom. 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.

Precise hyperfine structure measurements of La II utilizing the laser and rf double resonance technique

The hyperfine structure of the 5d 6s b1D2 level in La II has been measured utilizing the laser and radio-frequency double resonance (LRDR) technique in a collinear geometry. Magnetic dipole and electric quadrupole hyperfine constants have been extracted and the results have been compared with previous measurements. With the present technique drastically improved accuracy has been obtained.

Lifetime Measurements by Laser Probing in CRYRING – Recent Experimental Developments

A laser probing technique for lifetime measurements of long-lived metastable levels has been developed at the ion storage ring CRYRING. The method has been in use for about eight years and it has been applied to various ions. Levels with radiative lifetimes from a few milliseconds up to 28 s have been measured. The method has been continuously developed and this article is focused on recent experimental developments of the method in order to widen its applicability and to increase its efficiency and accuracy.

Experimental Oscillator Strengths for Forbidden Lines in Complex Spectra

In dilute astrophysical plasmas, such as planetary nebulae and H ii regions, strong lines appear that only have weak, if any, correspondence to the spectrum of laboratory light sources. Some of these are parity forbidden lines, i.e. emission from long-lived metastable states that cannot decay via normal electric dipole (E1) routes but only via the slower M1 and E2 transitions. The long lifetime of the upper levels and their sensitivity to collisions make the lines good diagnostics of the emitting plasma, but then the transition probability, or A-value, of these lines must be known. We report on a technique to experimentally determine A-values for forbidden lines using the method of combining the lifetime of the upper level with the branching fractions for the different decay channels. The lifetime is measured using the laser probing technique (LPT) on a stored ion beam. Since these lines are rarely produced in laboratory plasmas, we use astrophysical spectra to determine the branching fractions (BF ). The lifetime and the BF then give the A-value, which is one quantity needed for modeling the spectrum of the plasma emitting the forbidden lines. The present measurements are performed within the FERRUM project, an international collaboration producing and evaluating transition probabilities for iron group element lines of astrophysical importance [Johansson, P. I. S. et al., Physica Scripta T100, 71 (2002).]

Laser-probing measurements and calculations of lifetimes of the 5d {sup 2}D{sub 3} at {sub {approx}}{sub sol{approx}} at {sub 2} and 5d {sup 2}D{sub 5} at {sub {approx}}{sub sol{approx}} at {sub 2} metastable levels in Ba II

The two metastable levels 5d {sup 2}D{sub 3} at {sub {approx}}{sub sol{approx}} at {sub 2} and 5d {sup 2}D{sub 5} at {sub {approx}}{sub sol{approx}} at {sub 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 {tau}=89.4{+-}15.6 s for the {sup 2}D{sub 3} at {sub {approx}}{sub sol{approx}} at {sub 2} level and {tau}=32.0{+-}4.6 s for the {sup 2}D{sub 5} at {sub {approx}}{sub sol{approx}} at {sub 2} level measured in a beam-laser experiment performed at the ion storage ring CRYRING. These values are supported by our new calculations resulting in {tau}=82.0 s for the {sup 2}D{sub 3} at {sub {approx}}{sub sol{approx}} at {sub 2} level and {tau}=31.6 s for the {sup 2}D{sub 5} at {sub {approx}}{sub sol{approx}} at {sub 2} level.The two metastable levels 5d 2D3∕2 and 5d 2D5∕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 ...

Studies of Lifetimes in an Ion Storage Ring Using Laser Technique

The laser-probing method for lifetime measurements of metastable levels, performed by applying the Fast Ion Beam Laser (FIBLAS) method to ions stored in a storage ring, has been developed by the Stockholm group. Recently, we have applied this method to lifetime measurements of close lying metastable levels. In this paper we discuss experimental studies of ions with complex structure and present the first experimentally obtained lifetimes of selected metastable levels in complex systems as Fe+, Eu+ and La+.