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⁺.

     

Ferrihydrite modification by boron doping

Application of the electrostatic ion storage ring ELISA to studies of clusters and biomolecules is discussed. Ions injected from a plasma source or a sputter source are hot, and at short times the yield of neutrals is usually dominated by decay of metastable ions. We have demonstrated that the decay function is close to a 1/t dependence when the internal energy of the ions is conserved, i.e., when photon emission can be ignored. Deviations from a 1/t distribution therefore gives information about the radiative lifetime or, for larger systems, about the intensity of the emitted radiation. Systematic measurements have been carried out for fullerene anions C _N ^?, for even values of N from 36 to 96, to test a classical dielectric model. Recently we have installed an electrospray ion source with a Paul trap for bunching, which can be used to inject biomolecular ions from solution, and the first experiments on laser spectroscopy of biomolecules have been carried out. Also lifetimes of excited states have been measured for stored biomolecular ions excited by a laser pulse.

     

Preparation of relativistic 7Li+ ion beams for precision experiments at storage rings

Heavy ion storage rings allow for tests of the structure of local space time via the Doppler effect. At the TSR/Heidelberg an experiment with high resolution laser spectroscopy at ⁷Li⁺ is performed. To gain the maximum resolution for saturation spectroscopy new methods of relativistic ion beam preparation and diagnostics have been developed. The laser cooling of the beam allows for precision determination of the mean velocity of the ions. A novel phase synchronous detection scheme, ultimately sensitive to single ions, gives insights into the cooling mechanism and dynamics. With an additional synchronous excitation scheme systematic uncertainties of the test experiment can be drastically reduced. After separation of the ground state ions from the triplet states of ⁷Li⁺ by the combination of laser and electron cooling, a bunched and cooled ensemble of fast moving high precision clocks with minimized perturbations by space charge effects and intra beam scattering is available. This revised version was published online in August 2006 with corrections to the Cover Date.     

Resonance ionization spectroscopy of bismuth at the IGISOL facility

A programme of research has commenced at the IGISOL facility, Jyväskylä, combining the technique of resonance ionization spectroscopy with the development of the highly selective laser ion source trap (LIST). The first element of interest is bismuth, which contains three isomers of multi-quasiparticle states in near-spherical nuclei, namely ²⁰⁷Bi (21/2⁺, 182 μs), ²⁰⁴Bi (10^?, 13 ms) and ²⁰⁴Bi (17⁺, 1.07 ms). A measurement of the optical isomer shift provides a direct comparison of the mean?square charge radii between the isomer and the nuclear ground state. Due to the short isomer lifetimes the spectroscopy will be done either within the ion guide or in a sextupole ion beam guide (SPIG), located after the ion guide and used in the development of the LIST. A mixed dye-Ti:Sapphire laser ionization scheme has been successfully tested for bismuth and first off-line results have been obtained.     

‘Quantum jumps’ observed in the fluorescence of a single ion

The laser-excited resonance fluorescence of a single Ba⁺ ion at 493 nm shows interruptions of macroscopic time durations when the ion drops into a dark state by off-resonant Raman scattering, or by weak excitation of the ²P_3/2 level. We manipulate the ion out of the state by auxiliary laser light and prove that state to be the metastable ²D_5/2 level. Fluorescence from three ions displays an enhanced rate of multiple “jumps” which reveals cooperative ionic interaction with the light.     

Ion storage technique for very long living states: The decay rate of the 5D 3/2 state of Ba II

The metastable 5D _3/2 state of Ba⁺ is used to demonstrate the possibility of the ion storage technique for the measurement of extremely long lived states. The ions are confined in a r.f. quadrupole trap and excited by a strong pulsed tunable dye laser to the 6P _1/2 state which partially decays into the 5D _3/2 state. Sampling of the time development of the ground state population leads to the determination of the 5D _3/2 decay rate. Using He as a buffer gas at a pressure of about 10^?6 mbar to initially store the ions, varying the gas density and extrapolating to zero pressure, we find for the lifetime \(\tau _{D_{3/2} } = 17.5 \pm 4s\) .     

Spektren und Winkelverteilungen der Photoelektronen von Atomen und Molekülen

The photoelectron energy spectra produced by the impact of 584 Å photons on the gases Kr, H₂, N₂, O₂, CO, NO and several alkanes are reported. The angular distribution of photoelectrons corresponding to the simultaneous formation of specific electronic states of the ion have been measured in the range 30°—130°. A preference for electron ejection in the direction of light propagation is shown in the formation of the electronic ground states of NO⁺ and O ₂ ⁺ . Both involve the ejection of an electron from aπ _g orbital. The onset energies for the various electronic states have been obtained with a resolution of ca. 40 meV. The relative transition probabilities for formation of various electronic states in a given molecular ion, as well as the Franck-Condon factors within specific states have been obtained. Similar experiments with the alkanes (methane, ethane, propane, and n-butane) reveal directly the existence of widely separated electron states (or grouping of states) in the ion, as well as the probability of forming these states. This energy distribution function, of vital importance to the study of the unimolecular decay of ions, could only be inferred previously by use of a questionable assumption.     

Theoretical investigations of the SH and LiS cations

Reliable theoretical data on spectroscopy and spin-orbit matrix elements are computed for the lowest electronic states of SH⁺ and LiS⁺ ions. Accurate spectroscopic predictions for their excited electronic states are given. For SH⁺, polarization minima at large internuclear distances are located in addition to the strongly bound electronic states already known. For LiS⁺, our calculations confirm that the electronic ground state of this ion is of ³Σ⁻ species and reveal the existence of a ¹Δ state presenting a potential well as deep as the potential of the ground state. Moreover, the LiS⁺ electronic excited states potential energy curves possess shallow potential minima in the molecular region and at long-internuclear distances. Generally, these shallow minima may be populated during low energy collisions between the corresponding atomic fragments. Finally, spin-orbit calculations have allowed giving accurate determinations of the spin-orbit splittings for these cations and elucidation of the predissociation mechanisms of SH⁺ leading to the formation of the S⁺ and H species in their electronic ground states. Accordingly, long-lived SH⁺ ions can be found in the X³Σ⁻, a¹Δ and b¹Σ⁺ electronic states and the rovibrational levels of LiS⁺ in its electronically excited and ground states should be weakly perturbed.     

Excitation of bands of the first negative system of the N 2 + ion in collisions of N+ and O+ ions with N2 molecules

Absolute values of the excitation cross sections of the (0,0) bands [for O⁺(⁴ S), O⁺(² P)-N₂ pairs] and the (0,0), (0,1), (1,2), and (2,3) bands [for N⁺(³ P)-N₂ pairs] of the first negative system of the N ₂ ⁺ ion have been measured in collisions with nitrogen molecules of nitrogen and oxygen ions in the ground state and in a metastable state in the interval of ion energies 1–10 keV. The process of excitation of the (0,0) band of the first negative system of the N ₂ ⁺ ion by oxygen ions in the metastable ² P state is of a quasi-resonant character. The presence in the beam of ions in metastable states was monitored by measuring the excitation efficiency of the (0,0) band λ3914 Å of the N ₂ ⁺ ion in different operating regimes of the highfrequency ion source. For N⁺ ions in the ³ P ground state, as the collision frequency is decreased the relative vibrational population of the v′=1 and v′=2 levels of the B ²Σ _u ⁺ state of the N ₂ ⁺ ion is observed to deviate strongly from the value calculated in the Franck-Condon model.     

Metastable Decomposition of Ar3+ Cluster Ions Into Ar2+ and Ar+

In this note we present the first account of a study of metastable (unimolecular decay) and collision-induced dissociation of Ar₃+ cluster ions using an experimental setup consisting of a molecular beam-electron impact ion source and a double focussing (reversed Nier Johnson geometry) mass spectrometer. The existence of the following metastable decay processes (accessible by our sampling time window) could be demonstrated: Ar₃⁺* → Ar₂+ and Ar₃⁺* → Ar⁺. The processes were studied as a function of electron impact energy. The present results are of importance in order to provide some guidance for the development of appropriate theoretical models for the dissociation of cluster ions.     

Alpha-like four nucleon correlations in superfluid phases of atomic nuclei

α-like four-nucleon correlations are included in the structure of superfluid ground and low-lying excited states of atomic nuclei within a BCS-like approach. New metastable superfluid and normal states are predicted. These states could be associated with some of the recently discovered I^π = 0⁺ states in different regions of atomic nuclei. A new type of elementary excitations may be constructed on these metastable states in the same way as those constructed on the BCS superfluid ground states. The region of superfluid cold nuclei is enlarged due to the fact that the neutron and proton superfluidity can mutually be induced via the α-like four-nucleon ineractions. This type of correlations lead to a further enhancement of the probabilities of the favoured α-clusterization processes (such as α-decay or α-transfer reactions), two-nucleon transfer reactions and other clusterization processes such as e.g. the heavy cluster decay.     

Structure of excited states in100Mo and102Mo from spectroscopy using18O induced reactions

The lifetimes have been determined for the 2⁺, 0^+～ and 4⁺ states in¹⁰⁰Mo and¹⁰²Mo using the recoil-distance Doppler-shift method. The states have been excited in¹⁰⁰Mo by Coulomb excitation and in¹⁰²Mo by the two-neutron transfer reaction induced by¹⁸O ions on a¹⁰⁰Mo target. The study of the excitation function for the elastic and inelastic scattering on the ground and first excited 2⁺ state in¹⁰⁰Mo at beam energies between 20 and 61 MeV shows that 40 MeV is the highest incident energy for pure Coulomb excitation. Above this energy nuclear absorption sets in and nuclear scattering contributes to the excitation of the 2⁺ state of¹⁰⁰Mo. From the lifetimes of the 2⁺ and 4⁺ states deformation parameters of ¦β¦= 0.21 and ¦β¦=0.31 for¹⁰⁰Mo and¹⁰²Mo respectively were deduced. The 0^+′ levels are not shape isomeric states, as suggested earlier, but they decay by enhancedE2 transitions to the first 2⁺ states. From a comparison with similar states in other transitional nuclei it is suggested that they are band heads forβ vibrational bands.     

3d D lifetime in laser cooled Ca : Influence of cooling laser power

We have measured the lifetime of the metastable 3D _5/2 level in Ca⁺ using the “quantum jump" technique on a single stored and laser cooled ion in a linear Paul trap. We found a linear dependence of the measured decay rate on the power of the laser which repumps the ions from the long lived 3D _3/2 level. This can be explained by off-resonant depletion of the 3D _5/2 level. The proper lifetime of this level is obtained by a linear extrapolation of the measured lifetime to zero laser power. We obtain 1100(18) ms in agreement with theoretical calculations. The observed systematic change of the decay rate resolves discrepancies between earlier experiments in which this effect had not been considered. Measurements on a linear chain of 10 laser cooled ions showed unexpected frequent coincidences of quantum jumps within our observation time of 20 ms. This indicates a so far unexplained correlation between the ions in the chain at large distances. Received 3 March 1999     

Dye laser saturation spectroscopy of the 23 S 1-23 P transition in6, 7Li+ ions

The metastable 2³ S ₁ state and the short lived 2³ P states of the helium-like^{6, 7}Li⁺ ion spectra have been investigated by dye laser saturation spectroscopy in a low-energy Li⁺ ion-beam and fluorescence light detection. The hyperfine structure splittings of all the levels, the 2³ P fine structure intervals and the isotope shift of the 2³ S ₁-2³ P transitions have been measured. These measurements were made by application of a specially constructed tunable dye laser system capable of single-mode laser scans over more than 60 GHz.     

Lifetime of metastable fluorine atoms

For laser collimation of neutral F atoms, a resonance transition cycle between the metastable and the upper excited states (3s⁴ P _5/2?3p⁴ D ⁰ _7/2) can be used as a two-level closed system. We have determined the lifetime of the metastable state (3s⁴ P _5/2) in F atoms by measuring the decay curve of the fluorescence intensity as a function of distance from the plasma source. Combining the measured velocity of F radicals from the Doppler shift of the fluorescence peak, we have obtained the lifetime of the F metastable state as 3.7±0.5 μs. With this short metastable lifetime of F radicals, the simple Doppler cooling method using spontaneous light force is not practical for laser collimation of F radicals. Use of stimulated light force may be necessary to collimate F radical beams in a short distance. Received: 4 July 2000 / Published online: 13 September 2000     

Lifetime measurements of metastable states in Fe+

The lifetime of two metastable levels in Fe+ has been measured by laser probing of a stored ion beam. In the dense spectrum of Fe+, the metastable levels a (6)S(5/2) and b (4)D(7/2) were selected and their lifetimes were determined to be 230 +/- 30 and 530 +/- 30 ms, respectively. The lifetimes are compared with previous theoretical results. Metastable lifetime measurements of Fe+ are of great importance for interpretation of spectra from astronomical objects. The present experiment opens for the possibilities to investigate lifetimes of metastable states in complex atomic ions, which have, so far, been unexplored.     

Lifetime measurements of metastable states of Au,Bi, Cd,Mg, Pb,and Sr in an acetylene/air flame by laser-enhanced ionization spectrometry

The collisional dominated lifetimes of 9 metastable states of Au, Bi, Cd, Mg. Pb, and Sr in an acetylene/air flame have been measured by the step-wise delayed laser-enhanced ionization spectrometry technique. First, the dependency of the lifetime of the metastable 6p ^{2 3} P ₁ state in Pb upon both flame composition and height in the flame were studied. It was found that the lifetime was significantly shorter, both for lean as well as for rich flames, as compared to a stoichiometric composition. The lifetime was furthermore found to be dependent of the height in the flame with a maximum around 10 mm. The lifetimes were not significantly affected by the electron concentration in the flame. The lifetimes for the other metastable states were determined for one given flame composition ( 0.9 _stoich) and one given height (20 mm) only. The measured lifetimes were found to be between 5 ns and 3.1 s. The results show clearly that the lifetimes of metastable states are significantly shortened in the flame (as compared to natural lifetimes). The results also indicate that the metastable states which are of the same parity as the ground configuration (Au, Bi and Pb) in general have longer (collisional) lifetimes (85 ns to 3.1 s) than those which are forbidden to decay to the ground configuration only by violation of spin conservation (Cd, Mg and Sr) (5–33 ns).     

Lifetime measurements in166Er

The lifetimes of five states in the ground band, from spin 6⁺ up to spin 14⁺, and of all even states in the gamma band up to spin 12⁺, have been measured in¹⁶⁶Er using the recoil distance method. The reduced electric quadrupole transition probabilities have been determined from the measured lifetimes using previously measured branching ratios, and the mixing between the ground band and theγ-band has been studied. The transitional electric quadrupole moments for the ground band and theγ-band have been deduced and are discussed.     

ESD and IID study of CO on nickel

Electron and ion induced desorption has been used to study release of ionic species from CO on nickel. O⁺, CO⁺, and O^? are found to be released by electron impact. Thermal desorption spectra indicate the presence of two binding states. The O⁺ species is observed to be released from state 1, the state of higher binding energy, while state 2, a more loosely bound state is the source of CO⁺. O^? species appear to be released from both states. The cluster ions NiCO⁺ and Ni₂CO⁺ released by argon ion impact were observed to originate from state 2.     

Investigation on luminescence properties of Nd ions in alkaline-earth titanium phosphate glasses

Nd³⁺ ions doped alkaline-earth titanium phosphate glasses were prepared by using the conventional melt quenching method. Absorption spectra were recorded and oscillator strengths of the transitions were calculated using area under the absorption bands. The energy level analysis was carried out by using free-ion Hamiltonian model. The three host dependent Judd-Ofelt intensity parameters, Ω_λ (λ = 2, 4, and 6) were used to elucidate the structure of glassy matrix around Nd³⁺ ion as well as to determine the ⁴F_3/2 metastable state radiative properties such as radiative transition probabilities, branching ratios and radiative lifetimes. The decay curves of all the three glasses show single exponential behavior. The discrepancy between the experimental and calculated lifetimes of emitting level was attributed to multiphonon relaxation. The achieved high quantum efficiencies for the ⁴F_3/2 level indicate efficient laser emission at 1.06 μm in these glass systems.