Sven Mannervik

First storage of ion beams in the Double Electrostatic Ion-Ring Experiment: DESIREE

We report on the first storage of ion beams in the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. We have produced beams of atomic carbon anions and small carbon anion molecules (C(n)(-), n = 1, 2, 3, 4) in a sputter ion source. The ion beams were accelerated to 10 keV kinetic energy and stored in an electrostatic ion storage ring enclosed in a vacuum chamber at 13 K. For 10 keV C2 (-) molecular anions we measure the residual-gas limited beam storage lifetime to be 448 s ± 18 s with two independent detector systems. Using the measured storage lifetimes we estimate that the residual gas pressure is in the 10(-14) mbar range. When high current ion beams are injected, the number of stored particles does not follow a single exponential decay law as would be expected for stored particles lost solely due to electron detachment in collision with the residual-gas. Instead, we observe a faster initial decay rate, which we ascribe to the effect of the space charge of the ion beam on the storage capacity.

Lifetimes for Excited Levels in Si I-Si IV

The beam-foil method has been used to measure lifetimes for six levels in Si I. The results are compared with emission data and theoretical values. Improved experimental values are also given for a number of previously measured levels in Si II-Si IV.

The Quartet System of Li I

The doubly-excited system of quarters in Li I have been studied by the beam-foil technique using optical detection in the VUV. The absolute term value of the lowest quartet 1s2s2p 4P0 has been determined to be 57.4113 ± 0.0006 eV which is much more accurate than earlier experimental results. We have also made the first experimental determination of the term energy of the 1s2p3p 4P state by observing its decay to the 1s2s2p 4P0 state by emission of a spectral line at 1680.2 A. In addition, eight new observations of quartet transitions are reported as well as some lifetimes. The most recent experimental results are summarized and compared to calculations.

Lifetimes in Mg I and Mg II; Beam-Foil Measurements and Numerical Coulomb-Approximation Calculations

Lifetime measurements using the beam-foil technique have been made for 10 levels in Mg I and for 11 levels in Mg II. Lifetimes have been calculated for the same levels using the numerical Coulomb-approximation (NCA) method, and the results are compared with the experimental data. Whereas good agreement exists between theory and experiment for Mg II levels and for some of the low lying Mg I levels, there are large disagreements for some higher lying levels in Mg I. Results from NCA calculations on ionized elements in the Mg I sequence are compared with experimental data. In some but not every case the NCA results agree with experimental data, as for Mg I.

Transitions in Multiply Excited FVI and FVII

The transitions 1s2s2p2 5P-1s2p3 5S0 in F VI and 1s2s2p 4P0-1s2p2 4P in F VII have been investigated using the method of beam-foil spectroscopy. The fine-structure separations have been determined for the 1s2s2p2 5P term in F VI and for both 4P terms in F VII. The results are compared with recent theoretical calculations of fine structure, based on relativistic wavefunctions, and with previous experimental data. Lifetime measurements are reported for the 1s2p2 4P5/2 level and the 1s2p3 5S20 level.

Theoretical potential curves for the A 2Π and X 2Σ+ states of NO2+ and an experimental search for the A–X transition

Abstract Potential curves for the X 2 Σ + and A 2 Π states of NO 2+ have been calculated using large ANO basis sets including g functions and CASSCF/second-order ACPF methods. On the basis of these calculations resonant photodissociation of NO 2+ via the A 2 Π−X 2 Σ + transition has been searched for by means of a collinear fast ion beam laser apparatus. The failure to observe the A–X transition is discussed.

The FERRUM Project: laboratory-measured transition probabilities for Cr II

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 m ...Aims. We measure transition probabilities for Cr ii transitions from the z HJ , z DJ , y FJ , and y GJ levels in the energy range 63 000 to 68 000 cm−1. Methods. Radiative lifetimes were measured using time-resolved laser-induced fluorescence from a laser-produced plasma. In addition, branching fractions were determined from intensity-calibrated 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 laboratory-measured transition probabilities are compared to the values from semi-empirical calculations and laboratory measurements in the literature.

A technique for lifetime measurements of long-lived atomic states utilising laser excitation of stored ion beams

Abstract A laser probing technique for atomic lifetime measurements of metastable states using an ion storage ring is presented. The method benefits from the high spectral resolution of conventional fast ion beam laser spectroscopy, and is applicable to metastable systems in lowly charged ions with lifetimes in the 10 ms–1 s regime. Due to strong probing mechanisms, the technique can be applied to ion beams with low metastable fractions. A comprehensive description of the laser probing technique is given, with examples from recent experiments on singly charged calcium, strontium and xenon, and future prospects are discussed.

Vibrational cooling of D2+ in an ion storage ring as revealed by dissociative recombination measurements

It is demonstrated that a substantial degree of vibrational cooling of an infrared inactive molecule can be achieved by means of storage in an ion cooler ring. The cross section for dissociative recombination of D2+ has been measured in the storage ring CRYRING using 6 MeV amu-1 ions and variable storage times prior to data taking. By using a simple model for describing the recombination process, it is inferred that 68% of the D2+ ions remaining in the ring after 22.5 s populate nu =0. It is proposed that the removal of vibrational excitation contained in the ions injected into the ring is due to collisions in the rest gas. The rate coefficient measured for 22.5 s storage time, alpha (T=300 K)=2.3*10-8 cm3 s-1 is in very good agreement with theoretical predictions.

Optical studies of multiply excited states

Optical studies of multiply-excited states are reviewed with emphasis on emission spectroscopy. From optical measurements, properties such as excitation energies, lifetimes and autoionization widths can be determined with high accuracy, which constitutes a challenge for modern computational methods. This article mainly covers work on two-, three- and four-electron systems, but also sodium-like quartet systems. Furthermore, some comments are given on bound multiply-excited states in negative ions. Fine structure effects on transition wavelengths and lifetimes (autoionization) are discussed. In particular, the most recent experimental and theoretical studies of multiply-excited states are covered. Some remaining problems, which require further attention, are discussed in more detail.