Parity forbidden radiative transitions from metastable levels are observed in spectra of low-density astrophysical plasmas. These lines are used as probes of the physical conditions, made possible due to the long lifetime of their upper level. In a joint effort, the FERRUM project aims to obtain new and accurate atomic data for the iron-group elements, and part of this project concerns forbidden lines. The radiative lifetimes of the metastable energy levels 3 d4(a 3 D)4 s c4 D 5/2 and 3 d4(a 3 D)4 s c4 D 7/2 of singly ionized chromium have been measured. The experiment has been performed at the ion storage ring CRYRING. We employed a laser-probing technique developed for measuring long lifetimes. In this article, we present the lifetimes of these levels to be t5/2= 1.28(16) s and t7/2= 1.37(7) s, respectively. A comparison with previous theoretical work shows good agreement and the result is discussed in a theoretical context.

Context. Accurate transition probabilities for forbidden lines are important diagnostic parameters for low-density 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)(D-3)4s b D-4(1/2) metastable levels in Fe II and experimental transition probabilities for the forbidden transitions 3d(7) a F-4(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 D-4(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 F-4(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.

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 eta Carinae.

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.

The lifetime of the metastable level 3d2 1G4 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 3d2 1G4 state, by calculations using configuration interaction wave functions. The lifetime of the 1G4 level is experimentally determined to be τexp=2.64±0.18 s and is in good agreement with the theoretical calculation τth=2.84 s.

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 ??? = 89.4 +/- 15.6 s for the D-2(3/2) level and ??? = 32.0 +/- 4.6 s for the D-2(5/2) level measured in a beam-laser experiment performed at the ion storage ring CRYRING. These values are supported by our new calculations resulting in ???= 82.0 s for the D-2(3/2) level and ???= 31.6 s for the D-2(5/2) level.

A laser probing investigation has yielded the lifetimes of the 3s(2)3p(4)(1D)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 Hartree-Fock method and the multiconfiguration Breit-Pauli 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 2P, 2D states.

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 D-2(5/2) level in Ba+, yielding a value of 32.0 +/- 2.9 s, thus squeezing the error bar by 35%.

The difficulty associated with an accurate determination of transition rates for forbidden lines in lowly ionized heavy elements is illustrated in the case of Nd II. We have investigated the radiative decay of the low-lying metastable levels in Nd+ including the two levels \(4f^4(^5{\rm I})5d \ ^6\)K11/2 and \(4f^4(^5{\rm I})5d \ ^6\)I13/2. In these two particular cases, using different theoretical approaches, we find that the decay is dominated by the M1 channels but that the E2 contributions are of the same order of magnitude. These levels have also been studied experimentally by lifetime measurements with the heavy ion storage ring CRYRING of Stockholm University. The difficulties encountered when performing such experiments are underlined and discussed.

We present a technique for measuring the radiative lifetimes of metastable states of negative ions that involves the use of a heavy-ion storage ring. The method has been applied to investigate the radiative decay of the np(3) P-2(1/2) levels of Te-(n=5) and Se-(n=4) and the 3p(3) D-2 state of Si- for which the J=3/2 and 5/2 levels were unresolved. All of these states are metastable and decay primarily by emission of E2 and M1 radiation. Multi Configuration Dirac-Hartree-Fock calculations of rates for the transitions in Te- and Se- yielded lifetimes of 0.45 s and 4.7 s, respectively. The measured values agree well with these predicted values. In the case of the D-2 state of Si-, however, our measurement was only able to set a lower limit on the lifetime. The upper limit of the lifetime that can be measured with our apparatus is set by how long the ions can be stored in the ring, a limit determined by the rate of collisional detachment. Our lower limit of 1 min for the lifetime of the D-2 state is consistent with both the calculated lifetimes of 162 s for the D-2(3/2) level and 27.3 h for the D-2(5/2) level reported by O'Malley and Beck and 14.5 h and 12.5 h, respectively, from our Breit-Pauli calculations.

In the spectrum of an emission-line region ejected from the massive star Eta Carinae, called the strontium filament (SrF), forbidden lines from many elements, in particular [Sr II] and [Ti II], are observed. These lines are strong in this specific region and valuable for plasma diagnostics. Forbidden lines are not easily produced in laboratory light sources and the atomic parameters for these lines can thus not be measured in a straightforward way. We use a combination of laboratory and astrophysical measurements to determine transition probabilities for the [Ti II] lines. Lifetimes for metastable levels in Ti II are measured using a laser probing technique on a stored ion beam at CRYRING, MSL, Stockholm. Branching fractions from some of these levels are derived from Hubble Space Telescope/STIS spectra of the SrF. The astrophysical branching fractions are combined with the experimental lifetimes to determine absolute transition probabilities. We report lifetimes for the Ti II levels b(4)P(3/2), b(2)P(1/2), c(2)D(3/2) and c(2)D(5/2), in the range 0.29-17 s, and transition probabilities for eight parity-forbidden lines from the levels c(2)D(3/2) and c(2)D(5/2), along with uncertainty estimates.

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.

We have investigated the radiative decay of the metastable level 5d(4)D(7/2) in Xe+. Theoretically we find the decay to be heavily dominated by an M2 transition and not by M1/E2 transitions. Lifetime measurements of 5d(4)D(7/2) in a storage ring are difficult since magnetic mixing of the metastable with a short-lived level quenches its population. Decay rates were determined at different magnetic field strengths (B) in order to allow a nonlinear extrapolation to B=0. The experimental lifetime of 2.4 +/- 0.8 s was in agreement with the calculated value, but much smaller than previously estimated.

An experimental investigation of the radiative lifetime of the metastable 4s24p4(3P)4d4D7/2 level in Kr II shows an unusual situation regarding the importance of an M2 depopulation channel. While the first order M1 and E2 channels are expected to contribute in a dominant way to the decay, the experimental result, obtained using a laser probing technique on a stored ion beam, tau = 0.57+/-0.03 s, is far too short to be due to these channels according to our relativistic multiconfiguration Dirac-Fock calculation. Only if second order contributions to the decay branches (including essentially the M2 contribution) are taken into account in the calculations could the unexpected short lifetime be explained.

We report on the first experimental study of the lifetime of a bound excited state of a negative ion. A new experimental technique was developed and used to measure the radiative lifetime of the 5p(5) (2)P(1/2) level of Te-. The experiment was performed in a magnetic storage ring, where a laser beam was applied along one of the straight sections. In the experiment the population of the excited J=1/2 level was probed each time the Te- ions passed through the laser field. A decay curve was built up by sampling the population of the excited level of the Te- ions as a function of time after injection into the ring. A multiconfiguration Dirac-Hartree-Fock calculation was performed in conjunction with the experiment. The calculation yielded a radiative lifetime of 0.45 s, in excellent agreement with the measured value of 0.42(5) s.

Lifetime measurements of the metastable 3d D-2(3/2) level in singly charged calcium were performed using the newly developed violet diode laser operated in an external cavity arrangement. The laser was employed on stored ions at the CRYRING facility to optically pump the resonance line at 397 nm. In combination with laser probing at 866 nm this detection scheme gave the possibility to record the lifetime of this level. which has a radiative lifetime of about 1 s.

We report on a calculation of five lifetimes of metastable levels in Ar II, obtained with a relativistic Hartree-Fock method in which most of the intravalence correlation is represented within a configuration interaction scheme while core-valence correlation is described by a core-polarization model potential with a core-penetration corrective term. The quality of the calculation has been assessed through an experimental determination of the radiative lifetime of the metastable 3d 4F9/2 level. The experiment was performed with a laser probing technique on a stored ion beam at the CRYRING of Stockholm.

Stark measurements of the dipole moments in excited states of hafnium mono-sulfide, 180Hf32S, and hafnium mono-oxide, 180Hf16O, have been performed. The molecules were produced using a laser ablation source combined with a time-of-flight mass spectrometer, where a static electric field was applied. From the first order Stark effect splitting the permanent electric dipole moments, μ, were determined to be 3.416(23) D and 3.478(25) D for the HfS D1Πv=0 and v=1 states, respectively, and 2.665(29) D for the HfO b3Π1v=0 state. The e- and f-parity mixing due to the Stark effect was examined. Field-free low temperature spectra for the (0,0), (1,0), and (2,0) bands of the D1Π–X1Σ+ transition of HfS and the (0,0) band of the b3Π1–X1Σ+ transition of HfO were recorded and analyzed. Term-value calculations were performed, including most previously reported spectra, and potential energy curves and sets of molecular parameters were extracted.

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.