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We present the design of MOCCA, a large-area particle detector that is developed for the position- and energy-resolving detection of neutral molecule fragments produced in electron–ion interactions at the Cryogenic Storage Ring at the Max Planck Institute for Nuclear Physics in Heidelberg. The detector is based on metallic magnetic calorimeters and consists of 4096 particle absorbers covering a total detection area of $$44.8\,\mathrm {mm} \times 44.8\,\mathrm {mm}$$ . Groups of four absorbers are thermally coupled to a common paramagnetic temperature sensor where the strength of the thermal link is different for each absorber. This allows attributing a detector event within this group to the corresponding absorber by discriminating the signal rise times. A novel readout scheme further allows reading out all 1024 temperature sensors that are arranged in a $$32 \times 32$$ square array using only $$16+16$$ current-sensing superconducting quantum interference devices. Numerical calculations taking into account a simplified detector model predict an energy resolution of $$\Delta E_\mathrm {FWHM} \le 80\,\mathrm {eV}$$ for all pixels of this detector.

A method is presented to monitor the internal energy distribution of cluster anions via delayed electron detachment by pulsed photoexcitation and demonstrated on Co$_4{}^-$ in an electrostatic ion beam trap. In cryogenic operation, we calibrate the detachment delay to internal energy. By laser frequency scans, at room temperature, we reconstruct the time-dependent internal energy distribution of the clusters. The mean energies of ensembles from a cold and a hot ion source both approach thermal equilibrium. Our data yield a radiative emission law and the absorptivity of the cluster for thermal radiation., Manuscript LaTeX with 6 pages, 4 figures, plus LaTeX supplement with 9 pages, 4 figures and 2 tables. This article has been accepted by Physical Review Letters

Photodetachment thermometry on a beam of OH$^-$ in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional, frequency and time dependent photodetachment spectroscopy over 20 minutes of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the J = 1 natural lifetime (about 193 s) and determine the OH$^-$ rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry., Manuscript LaTeX with 5 pages, 3 figures, and 1 table plus LaTeX supplement with 12 pages, 3 figures and 3 tables. This article has been accepted by Physical Review Letters

We determined absolute rate coefficients for the recurrent fluorescence (RF) process in C6– anions at excitation energies above the adiabatic electron attachment energy of 4.18 eV. The experiment was performed by extracting C6– ions from a sputter ion source and storing them in a bent electrostatic ion beam trap. After 1 s of storage, during which the anions cooled down to temperatures close to room temperature, they were excited by a short laser pulse and the neutralization rate due to vibrational autodetachment (VAD) was measured as a function of time at several wavelengths. Due to the different energy dependence of the two competing decays via the RF and the VAD process, their contributions to the measured total decay rate coefficients could be disentangled. For excitation energies ≲4.6 eV, the decay is found to be dominated by the RF process with decay rate coefficients on the order of 5 × 104 s–1. The result clearly demonstrates the presence of the RF process in C6– and illustrates the importance of ...

We have measured the spectrum of laser photodissociation of OH+ molecular ions to O + H+ and O+ + H fragments for photon energies of 38 100–40 900 cm−1. The OH+ ions were stored as a fast beam (5.50 MeV) in the storage ring TSR for several seconds to achieve rovibrational cooling into the lowest rotations N′′ = 0–11 of the vibrational ground state X3Σ−(v′′ = 0), close to room temperature (≈300 K). The many resonances in the spectra reveal the energies, widths, and O/O+ branching ratios of 44 predissociating quasibound levels (Feshbach resonances) that lie between the fine-structure states of the O fragment and belong to the last, near-threshold vibrational states v′ = 9 and 10 of the A3Π electronic state. For the A3Π0,1 substates, isolated levels with v′ = 11 are observed and attributed to double-well distortions of these curves due to nonadiabatic interactions. Another five isolated levels are assigned to the v′ = 0 and 1 states of the shallow 15Σ− electronic state, borrowing oscillator strength from nearby A3Π levels. Together, the near-threshold levels deliver a new value D0 = 40 253.8(1.1) cm−1 for the dissociation energy of OH+. Through a two-step photodissociation process, 72 levels from the lower bound states A3Π(v′ = 7–8) appear as well and are rotationally analyzed. The level energies are used to construct improved A3Π and 15Σ− Born-Oppenheimer potentials. The totality of the spectral data (energies, widths, intensities, and branching ratios) can provide tight constraints for the potentials and nonadiabatic interactions assumed in future coupled-channel calculations of OH+ photodissociation or of the related charge-exchange reaction O + H+ → O+ + H.

An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 $\pm$ 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion) and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm$^{-3}$ is derived, equivalent to a room-temperature pressure below 10$^{-14}$ mbar. Fast atomic, molecular and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams., PDFLaTeX with 17 pages, 16 figures, 2 tables. This article has been accepted by Review of Scientific Instruments. After it is published, it will be found at http://scitation.aip.org/content/aip/journal/rsi

We have studied the photodissociation of CH^{+} in the Cryogenic Storage Ring at ambient temperatures below 10 K. Owing to the extremely high vacuum of the cryogenic environment, we were able to store CH^{+} beams with a kinetic energy of ∼60 keV for several minutes. Using a pulsed laser, we observed Feshbach-type near-threshold photodissociation resonances for the rotational levels J=0-2 of CH^{+}, exclusively. In comparison to updated, state-of-the-art calculations, we find excellent agreement in the relative intensities of the resonances for a given J, and we can extract time-dependent level populations. Thus, we can monitor the spontaneous relaxation of CH^{+} to its lowest rotational states and demonstrate the preparation of an internally cold beam of molecular ions.

The decay of ion-bunches stored in the self-bunching mode of an electrostatic ion beam trap (EIBT) has been investigated as a function of the number of injected ions with the Cryogenic Trap for Fast ion beams (CTF) located at the Max Planck Institute for Nuclear Physics in Heidelberg. By use of the pickup-electrode signal the dependence of the characteristic bunch-spreading time τ, i.e. the time in which the bunch width increases by one third, on the number of injected ions is determined. For F− ions with an initial bunch length of 2.7 μs the spreading times are observed to increase approximately linearly with the number of injected ions and to reach a constant value of ∼10 s for ion numbers larger than 4 × 105.

We report our plasma rate coefficient and branching ratio measurements for dissociative recombination (DR) of D2Cl+ with electrons. The studies were performed in a merged-beams configuration using the TSR heavy-ion storage ring located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Starting with our absolute merged-beams recombination rate coefficient at a collision energy of approximate to 0 eV, we have extracted the cross section and produced a plasma rate coefficient for a translational temperature of approximate to 8 K. Furthermore, extrapolating our cross-section results using the typical low-energy DR behavior, we have generated a plasma rate coefficient for translational temperatures from 5 to 500 K. We find good agreement between our extrapolated results and previous experimental DR studies on D2Cl+. Additionally, we have investigated the three fragmentation channels for DR of D2Cl+. Here we report on the dissociation geometry of the three-body fragmentation channel, the kinetic energy released for each of the three outgoing channels, the molecular internal excitation for the two outgoing channels that produce molecular fragments, and the fragmentation branching ratios for all three channels. Our results, in combination with those of other groups, indicate that any remaining uncertainties in the DR rate coefficient for H2Cl+ appear unlikely to explain the observed discrepancies between the inferred abundances of HCl and H2Cl+ in molecular clouds and predictions from astrochemical models.

We report on our recent studies of dissociative recombination (DR) employing two different fragment imaging detection techniques at the TSR storage ring in Heidelberg, Germany. Principles of an upgraded 3D optical system and the new energy-sensitive multistrip detector (EMU) are explained together with possible applications in reaction dynamics studies. With the EMU imaging detector we succeeded to observe the branching ratios after DR of deuterated hydronium ions D(3)O(+) at energies of 0-0.5 and 4-21 eV. The branching ratios are almost constant at low energies while above 6 eV both oxygen-producing channels O + D + D + D and O + D(2) + D strongly increase and dominate by about 85% at 11 eV. To demonstrate further capabilities of our fragment imaging detectors, we also summarize some of our additional recent studies on DR of molecular ions important for astrophysics as well as for fundamental unimolecular dynamics.

Time dilation is one of the most fascinating aspects of special relativity as it abolishes the notion of absolute time. It was first observed experimentally by Ives and Stilwell in 1938 using the Doppler effect. Here we report on a method, based on fast optical atomic clocks with large, but different Lorentz boosts, that tests relativistic time dilation with unprecedented precision. The approach combines ion storage and cooling with optical frequency counting using a frequency comb. 7Li+ ions are prepared at 6.4% and 3.0% of the speed of light in a storage ring, and their time is read with an accuracy of 2×10−10 using laser saturation spectroscopy. The comparison of the Doppler shifts yields a time dilation measurement represented by a Mansouri–Sexl parameter , consistent with special relativity. This constrains the existence of a preferred cosmological reference frame and CPT- and Lorentz-violating ‘new’ physics beyond the standard model.

The negative positronium (Ps–) ion consisting of two electrons and a positron (e+e–e–), represents the simplest three-body system with a bound state. Its constituents are stable, point-like particles, and it is essentially free from perturbations by strong interaction effects. Together with the rather unique mass ratio, these properties make the positronium ion an interesting object for studying the quantum-mechanical three-body problem. We present a new determination of the decay rate of Ps–, using a beam-foil method and a stripping-based detection technique. The measured value of Γ = 2.089(15) ns–1 is a factor of six times more precise than the previous experimental result, and there is excellent agreement both with the latter and with the theoretical value. With the new high-intensity positron source NEPOMUC at the FRM-II research reactor in Munich being available, a further improvement in precision seems possible. Moreover, the high flux of low-energy positrons at this facility brings other properties of this exotic system within reach of experiments. The prospects for such investigations are discussed.PACS No.: 36.10Dr

Absolute photoabsorption cross sections of negatively charged tetra-atomic aluminum clusters have been measured for photon energies between 1.8 and 2.7 eV. The experiment used the depletion technique in combination with an electrostatic ion-beam trap, in which ${{\mathrm{Al}}_{4}}^{\ensuremath{-}}$ ions produced in a sputter ion source were stored for 90 ms before being subjected to a short laser pulse. Moreover, the competition between one-atom fragmentation and electron emission of the laser-excited ${{\mathrm{Al}}_{4}}^{\ensuremath{-}}$ has been measured. These measurements show that fragmentation dominates electron emission at all photon energies below the electron attachment energy of $\ensuremath{\sim}2.2$ eV, even though the fragmentation energy is expected to be 10%--20% higher than the electron attachment energy. These findings, when taken together with the delayed-electron and fragmentation yields observed in a previous measurement [O. Aviv et al., Phys. Rev. A 83, 023201 (2011)], can be well explained within the statistical phase-space theory for unimolecular decays assuming the ${{\mathrm{Al}}_{4}}^{\ensuremath{-}}$ ions to be rotationally hot. The analysis permits the determination of the adiabatic electron detachment energy of ${{\mathrm{Al}}_{4}}^{\ensuremath{-}}$ to be ${E}_{\text{ad}}=(2.18\ifmmode\pm\else\textpm\fi{}0.02)$ eV and the one-atom fragmentation energy to be ${D}_{0}=(2.34\ifmmode\pm\else\textpm\fi{}0.05)$ eV. Moreover, two direct $s$-wave ionization channels are observed with threshold energies of $(2.18\ifmmode\pm\else\textpm\fi{}0.02)$ eV and $(2.45\ifmmode\pm\else\textpm\fi{}0.02)$ eV.

Measurements on the energetic structure of the dissociative recombination rate coefficient in the millielectronvolt range are described for ions produced in the lowest rotational levels by collisional cooling and stored as a fast beam in the magnetic storage ring TSR (Test Storage Ring). The observed resonant structure is consistent with that found previously at the storage ring facility CRYRING in Stockholm, Sweden; theoretical predictions yield good agreement on the overall size of the rate coefficient, but do not reproduce the detailed structure. First studies on the nuclear spin symmetry influencing the lowest level populations show a small effect different from the theoretical predictions. Heating processes in the residual gas and by collisions with energetic electrons, as well as cooling owing to interaction with cold electrons, were observed in long-time storage experiments, using the low-energy dissociative recombination rate coefficient as a probe, and their consistency with the recent cold measurements is discussed.

The fragmentation of LiH2 - anions after electron impact was investigated at the heavy-ion storage ring TSR. The main reaction channel was found to be electron detachment followed by a breakup into LiH + H. In the first ms after production of the molecular ions in a cesium sputtering ion source, additional contributions were observed in the Li + H2 and Li- + H2 channels, hinting at an initial population of a short-lived state of the anion. To gain a better understanding of the mechanisms underlying the observed behavior of the system, ab initio calculations of relevant potential energy surfaces were performed at selected geometries. The experimental findings are discussed in the light of these calculations.

The Doppler-free laser-spectroscopic frequency measurement of Doppler-shifted optical lines in forward and backward direction of a fast ion beam permits a sensitive test of the relativistic Doppler-formula and, hence, the relativistic time dilation factor γ SR=(1−ν 2/c 2)−1/2. An experiment on metastable 7Li+, stored at a velocity of ν=0.064c in the Heidelberg heavy-ion storage ring TSR, has confirmed time dilation with unprecedented accuracy. Latest tests at two different ion-velocities (ν=0.03c and ν=0.064c) will enhance these measurements. An improved version of this experiment will be carried out at the experimental storage ring (ESR) at the Gesellschaft fur Schwerionenforschung (GSI) in Darmstadt. The ESR permits 7Li+ to be stored at ν=0.33c which promises an improvement of the sensitivity to deviations from γ SRby an order of magnitude. A first test at the ESR has shown the feasibility for this kind of experiment.

We have measured resonance strengths and energies for dielectronic recombination (DR) of beryllium-like Fe XXIII forming boron-like Fe XXII via N = 2 -> N' = 2 and N = 2 -> N' = 3 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max Planck Institute for Nuclear Physics (MPI-K) in Heidelberg, Germany. We have also calculated these resonance strengths and energies using three independent, perturbative, state-of-the-art theoretical techniques: the multiconfiguration Breit-Pauli (MCBP) method, the multiconfiguration Dirac-Fock (MCDF) method, and the Flexible Atomic Code (FAC). Overall reasonable agreement is found between our experimental results and these theoretical calculations. We have used our measurements to produce a Maxwellian-averaged DR rate coefficient for Fe XXIII. Our experimentally derived rate coefficient is estimated to be accurate to better that approximate to 20%. At temperatures where Fe XXIII is predicted to form in both photo-ionized and electron-ionized gas, we find mixed agreement between our experimental rate coefficient and previously published rate coefficients. We find good agreement at these temperatures between the experimentally derived rate coefficient and our MCBP, MCDF, and FAC results.

The one-neutron transfer reaction Li-9 + H-2 -> Li-8 + t has been investigated in an inverse kinematics experiment by bombarding a deuterated polypropylene target with a 2.36 MeV/u Li-9 beam from the post-accelerator REX-ISOLDE at CERN. Excitation energies in Li-8 as well as angular distributions of the tritons were obtained and spectroscopic factors deduced. (c) 2006 Elsevier B.V. All rights reserved.

Dielectronic recombination (DR) of zinc-like platinum ions was measured at the Heidelberg heavy-ion storage-ring TSR. Pronounced differences were observed in the DR resonance structure of the isotopes 194Pt and 198Pt on the one hand and of the isotope 195Pt on the other hand. The differences are ascribed to the presence of a metastable 4s4p 3P0 component in the 194Pt48+ and 198Pt48+ ion beams. The hyperfine quenching of this component in the 195Pt48+ ion beam manifests itself by ‘missing’ DR resonances associated with strong excitations of ions in the metastable 4s4p 3P0 state.

In order to improve the geometrical characterization of molecular ions using the Coulomb Explosion Imaging method, we discuss the need to include wake effects in a full description of the interactions felt by the swift dissociated atomic ions moving through thin foils. Comparison of simulations and experimental results for the structure of diatomic (HD+) and triatomic (CH2+) molecular ions is significantly improved when including the polarization effects induced in the target by the passage of swift ions. This work was supported by the Spanish Ministerio de Ciencia y Tecnología (projects HA2001-0052 and BFM2003-04457-C02-01/02), the DAAD in the framework of the Acciones Integradas Program 2002/03 and the German-Israel Foundation for Scientific Research (GIF) under Contract No. I-707-55.7/2001. SHA thanks the Fundación Cajamurcia for a Postdoctoral Research Grant.

The laser-spectroscopic frequency measurement of Doppler-shifted optical lines in the forward and backward directions of a fast ion beam permits a sensitive test of the relativistic Doppler formula and, hence, the relativistic time-dilation factor γSR = 1/[Formula: see text]. An experiment on 7Li+, stored at a velocity of v = 0.065c in the Heidelberg heavy-ion storage ring TSR, has confirmed time dilation with unprecedented accuracy limiting deviations to below 2.2 × 10–7. Ongoing improvements on the experimental setup will further tighten this limit.PACS Nos.: 03.30.+p, 06.30.Ft, and 42.62.Fi

A great number of theoretical papers have been published dealing with the negative positronium ion Ps–. On the other hand, experimental data on this purely leptonic three-body system (e+e–e–) is very limited. Apart from a first observation, a lifetime measurement with an accuracy of 4% has been published. We have built a set-up to produce Ps– making use of moderated positrons from a 22Na source, and we are presently running an experiment to improve on its decay rate. This paper discusses the status of the project as well as the possibilities of extending these investigations to other properties of Ps–, the latter becoming possible using the NEPOMUC positron source at the FRM II reactor in Munich.PACS No.: 36.10.Dr

In studies of the rate coefficient of the dissociative recombination of H3+ and its isotopomers, the rovibrational excitation of the molecular ions was found to play an important role, in particular when employing the technique of heavy-ion storage rings. The dependence of the DR rate on rotational excitation was investigated in recent experiments at the Test Storage Ring TSR in Heidelberg through time-resolved measurements on D2H+ and H3+ over long storage times. For both molecules, an influence of rotational excitation on the DR rate was observed. The level of excitation in turn was found to be dominated by radiative coupling to the surrounding 300 K background for D2H+. In the case of H3+, a strong influence of electron collisions on the excitation level was found, whereas an additional influence of collisions with residual gas in the storage ring cannot be excluded.

Ion storage and trapping techniques in connection with efficient internal state control and diagnostic for molecular ions offer the potential of providing rate coefficients for the dissociative recombination of H3+ for well-defined initial rotational states, required for understanding the role of this ion in the interstellar medium and other cold environments. Information on the vibrational and rotational excitation in stored H3+ ion beams, as obtained from experiments at the ion storage ring TSR, is reviewed. In addition, the arrangement of the TSR injector trap, using buffer gas cooling in a cryogenic radiofrequency multipole structure to inject pulses of internally cold H3+ ions into the TSR via a high-energy accelerator, is outlined. An account is given of tests towards the in-situ diagnostic of rotational level populations, where laser transitions between low-lying rovibrational levels could be detected in dilute H3+ ion ensembles using chemical probing in the radiofrequency multipole ion trap.

To study electron–ion interactions at the Heidelberg heavy-ion storage ring, electron beams with low-energy spreads and dc-currents of milliamperes are desired. Measurements of the photoelectron energy distribution showed that electron beams with energy spreads of 5–8 meV can be obtained from GaAs photocathodes, cooled to about LN 2 -temperature. However, in order to get milliamperes beam currents, the laser illumination has to be increased up to 1 W, causing substantial cathode heating. The presented new electron gun design based on sapphire-substrate transmission-mode photocathodes, cooled by LN 2 , stabilizes the GaAs bulk temperature under 1 W laser illumination at about 95 K and thereby provides the prerequisites for an electron gun being operated at milliampere-currents with low-energy spreads.

An ultra cold electron source was developed for the storage ring TSR in Heidelberg to study electron-ion interactions with high energy resolution. The heart of the source is a GaAs photocathode which emits electrons with energy spreads below 10 meV. Photoemitted electrons are extracted in the space charge mode and then undergo adiabatic magnetic expansion and adiabatic acceleration to obtain an ultra cold electron beam which is overlapped with the stored ion beam in a straight section of the ring. In the first recombination measurements on HD+ unprecedented energy resolution for low-energy resonances was found, demonstrating a transverse and longitudinal temperature of the electron beam of about 0.5 meV and 0.02 meV, respectively.

The dissociative recombination (DR) of the helium dimer 3He4He+ has been investigated at the heavy-ion Test Storage Ring (TSR) in Heidelberg at relative energies up to 40 eV. The vibrational level population in the stored ion beam was shown to be non-thermal with a fraction of 0.1–1% in higher vibrational states, resulting mainly from vibrational excitation in collisions with the residual gas species. The temporal evolution of the DR rate during storage showed evidence for an electron induced rotational de-excitation from the vibrational ground state. After haracterizing the evolution of the rovibrational population of the stored ions, the zero energy DR rate coefficient was extracted from the measurement to be αv=0 DR (0) = (7.3±2.1)×10−10 cm3/s, and the DR reaction from the vibrational ground state was seen to proceed mainly to the 1s2s 1S and 1s2p3P atomic limits. For v ≥ 3, the DR reaction has a rate coefficient ≥ 2×10−7 cm3/s and leads primarily to atomic fragments with n ≥ 3. The energy dependent rate coefficient displays several prominent structures.

A novel cryogenic electrostatic storage ring is planned to be built at the Max-Planck Institute for Nuclear Physics in Heidelberg. The machine is expected to operate at low temperatures (∼ 2K) and to store beams with kinetic energies between 20 to 300 keV. An electron target based on cooled photocathode technology will serve as a major tool for the study of reactions between molecular ions and electrons. Moreover, atomic beams can be merged and crossed with the stored ion beams allowing for atom molecularion collision studies at very low up to high relative energies. The proposed experimental program, centered around the physics of cold molecular ions, is shortly outlined.

A source of ultra-cold magnetically guided DC electron beams with a transmission mode GaAs-photocathode has been developed. The mA currents and the required stability are obtained by using state-of-the-art technology of photocathode fabrication and cleaning in UHV. To optimize energy spreads from photocathodes, studies of the complete (longitudinal and transverse) energy distribution of the magnetized electron beam were performed. They allowed us to get a detailed look on the energy loss and momentum scattering of the electrons at the interface and finally, to obtain narrow electron energy spreads (below 10 meV ) from the photocathode.

At low relative velocities the transverse cooling rates of fast stored ions by cold electrons are usually smaller than the longitudinal ones. By dispersive electron cooling, however, it is possible to increase the horizontal cooling rate on the expense of the longitudinal rate. The cooling scheme requires a horizontal gradient of the longitudinal cooling force, which was achieved by displacing the electron beam relative to the ion beam, and a finite dispersion in the ion–electron interaction region. Moreover, electron cooling of very hot ion beams is usually rather slow. Two different methods were investigated to study the longitudinal cooling times. The first method uses an induction accelerator to push the ions into the effective velocity range of the longitudinal electron cooling force. In the second method the mean electron velocity is swept over the velocity distribution of the ion beam in order to decrease the collection time.

Despite numerous theoretical papers on the properties of the negative positronium ion Ps−, which reflect the great interest in this fundamental system consisting of three leptons (e+e−e−), so far only two measurements have been performed twenty years ago. They established the existence of Ps− and measured its lifetime to an accuracy of 4%. We have built a set-up to produce Ps− making use of moderated positrons from a 22Na source, and we are presently preparing experiments to improve on its decay rate. The status of the experiment is discussed together with the possibilities to extend these investigations to other fundamental properties of Ps− which are becoming accessible using the new positron source at the FRM II reactor in Munich.

Progress in the experimental techniques used to investigate superdeformed fission isomers in the actinides allowed for detailed spectroscopic results of collective properties as well as for the identification of the rotational structure of multiphonon vibrational excitations. A novel approach could be established to determine the depth of the second potential well.

At low relative velocities, the transverse cooling rates of fast stored ions by cold electrons are usually smaller than the longitudinal ones. By dispersive electron cooling, however, it is possible to transfer heat from the horizontal into the longitudinal degree of freedom, thereby increasing the horizontal cooling rate on the expense of the longitudinal rate. The cooling scheme requires a horizontal gradient of the longitudinal cooling force which can be achieved by displacing the electron beam relative to the ion beam, and a finite dispersion in the ion–electron interaction region. Operating the heavy ion test storage ring TSR at a moderate dispersion of D S =1.81 m , we investigated the dispersive electron cooling process and found good agreement with the expectations from fundamental considerations.

The dynamics of the fragmentation process following the recombination with low energetic free electrons was studied for H 3 + and D 3 + using a two-dimensional imaging technique. The kinematical correlation of the atomic fragments in the three-body fragmentation channel, as well as the vibrational excitation distribution of the molecular fragments in the two-body channel, could be determined. In addition, a considerable rotational excitation of the H 3 + and D 3 + ions circulating in the storage ring TSR was observed even after storage time of more than 1 min. In contrast, the heteronuclear D 2 H + isotopomer possessing an electric dipole moment does not exhibit any internal excitations after 10 s of storage.

We have measured dielectronic recombination (DR) resonance strengths and energies for carbon-like Fe XXI forming Fe XX and for boron-like Fe XXII forming Fe XXI via N = 2 → N' = 2 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany. We have also calculated these resonance strengths and energies using three independent, state-of-the-art perturbative techniques: a multiconfiguration Breit-Pauli (MCBP) method using the code AUTOSTRUCTURE, a multiconfiguration Dirac-Fock (MCDF) method, and a relativistic configuration interaction method using the Flexible Atomic Code (FAC). Overall, reasonable agreement is found between our experimental results and our theoretical calculations. The most notable discrepancies tend to occur for relative collision energies 3 eV. We have used our measured 2 → 2 results to produce Maxwellian-averaged rate coefficients for Fe XXI and Fe XXII. Our experimentally derived rate coefficients are estimated to be accurate to better than ≈20% both for Fe XXI at kBTe > 0.5 eV and for Fe XXII at kBTe > 0.001 eV. For these results, we provide fits that are accurate to better than 0.5% for Fe XXI at 0.001 eV ≤ kBTe ≤ 10,000 eV and for Fe XXII at 0.02 eV kBTe ≤ 10,000 eV. Our fitted rate coefficients are suitable for ionization balance calculations involving Fe XXI and Fe XXII in photoionized plasmas. Previous published Burgess formula and LS-coupling calculations are in poor agreement with our experimentally derived rate coefficients. None of these published calculations reliably reproduce the magnitude or temperature dependence of our experimental results. Our previously published Fe XXI MCDF results are in good agreement with our experimental results for kBTe 0.07 eV. For both ions in this temperature range our new MCBP, MCDF, and FAC results are in excellent agreement with our experimentally derived rate coefficient.

For the investigation of electron-ion collisions at the Test Storage Ring with a much enhanced energy resolution a cold electron source with cryogenic GaAs(Cs,O) is being developed. To ensure optimal performance of GaAs photocathodes the two-dimensional energy distributions of emitted electrons are studied at room and low (90 K) temperature. The photocathode performance data promise electron beams of several 10 mA/cm2 with an energy spread ⩽ 10 meV.

The frequency measurement of Doppler-shifted optical lines of ions circulating in a storage ring at high speed permits a sensitive test of the relativistic Doppler-formula and, hence, the time dilation factor γSR of special relativity. Previous measurements at the storage ring TSR with 7Li+ at v = 0.065c gave a new, improved limit, but were hampered by the large observed linewidth, exceeding the natural width 15-fold. Recently we have identified the broadening to be caused by velocity-changing processes in the storage ring. Saturation spectroscopy has proven to be largely immune against these effects and has yielded linewidths only a few MHz larger than the natural one. This is the major ingredient for an improved test of γSR, which is now under way.

We have measured and theoretically analyzed a photodissociation spectrum of the CH+ molecular ion in which most observed energy levels lie within the fine-structure splitting of the C+ fragment and predissociate, and where the observed irregular line shapes and dipole-forbidden transitions indicate that nonadiabatic interactions lead to multichannel dynamics. The molecules were prepared in low rotational levels J″=0–9 of the vibrational ground state X 1Σ+ (v″=0) by storing a CH+ beam at 7.1 MeV in the heavy-ion storage ring TSR for up to 30 s, which was sufficient for the ions to rovibrationally thermalize to room temperature by spontaneous infrared emission. The internally cold molecules were irradiated with a dye laser at photon energies between 31 600–33 400 cm−1, and the resulting C+ fragments were counted with a particle detector. The photodissociation cross section displays the numerous Feshbach resonances between the two C+ fine-structure states predicted by theory for low rotation. The data are an...

For the determination of the first ionization potential of actinium, 227Ac was electrodeposited on a Ta backing and covered with ~1 μm Zr. From this filament, Ac atoms were evaporated at ≥ 1250 °C. By resonant excitation with UV light of 388.67 nm and subsequent excitation with light of ca. 568 nm, Ac was ionized in an external electrical field. By determining the ionization thresholds as a function of the electrical field strength and by extrapolation to zero field strength, the first ionization potential of 43398(3) cm−1 = 5.3807(3) eV was measured.About 1 ng of 255Fm, half life 20.1 h, was prepared at ORNL by milking from 255Es produced in the High Flux Isotope Reactor and shipped to Mainz. Here, a fraction of 2.7 · 1010 atoms was electrodeposited on a Ta backing and covered with ~ 1 μm Ti. From this filament, Fm atoms were evaporated at ~ 1000 °C in the buffer gas of an optical cell and resonantly excited and ionized with two beams of an excimer-dye-laser combination. The ions were identified by mass-...

We have measured resonance strengths and energies for dielectronic recombination (DR) of Fe XIX forming Fe XVIII via N = 2 → N' = 2 and N = 2 → N' = 3 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also calculated these resonance strengths and energies using two independent, state-of-the-art techniques: the perturbative multiconfiguration Breit-Pauli (MCBP) and multiconfiguration Dirac-Fock (MCDF) methods. Overall, reasonable agreement is found between our experimental results and theoretical calculations. The most notable discrepancies are for the 3l3l' resonances. The calculated MCBP and MCDF resonance strengths for the n = 3 complex lie, respectively, ≈47% and ≈31% above the measured values. These discrepancies are larger than the estimated 20% total experimental uncertainty in our measurements. We have used our measured 2 → 2 and 2 → 3 results to produce a Maxwellian-averaged rate coefficient for DR of Fe XIX. Our experimentally derived rate coefficient is estimated to be good to better than ≈20% for kBTe ≥ 1 eV. Fe XIX is predicted to form in photoionized and collisionally ionized cosmic plasmas at kBTe 1 eV. Hence, our rate coefficient is suitable for use in ionization balance calculations of these plasmas. Previously published theoretical DR rate coefficients are in poor agreement with our experimental results. None of these published calculations reliably reproduce the magnitude or temperature dependence of the experimentally derived rate coefficient. Our MCBP and MCDF results agree with our experimental rate coefficient to within ≈20%.

Vibrational relaxation and isomerization of internally excited deuterated formyl and isoformyl cations has been investigated on the time scale of 2 ms to 12 s using the nearly interaction-free environment of an ion storage ring. De-excitation of the v2 bending modes of DCO+ and DOC+ due to spontaneous radiative transitions was observed as a function of the storage time by measuring their foil-induced Coulomb explosion using three-dimensional coincident fragment imaging. No isomerization of low-lying vibrational levels of DOC+ ions was observed on the time scales considered. By comparing the Coulomb explosion data to molecular bond angle distributions obtained from vibrational wave function calculations, the time evolution of the mean v2 population is deduced for both isomers. The stored DOC+ ions are found to thermalize with the 300 K black-body radiation, while relaxation of the DCO+ bending vibrations was found to require considerably longer times, in agreement with a predicted very small transition mom...

MINIBALL a highly efficient, position sensitive γ -spectrometer is under construction for upcoming radioactive-beam experiments. In contrast to high spin arrays the total full energy peak efficiency and not the resolving power is optimized with a factor of two larger efficiency for γ -ray energies up to 10 MeV and for low to medium γ -ray multiplicity events. Cluster detectors, incorporating three or four Ge crystals, will be employed to provide high flexibility and to accomodate the various demands of the experiments.

The REX-ISOLDE experiment which is presently under construction at CERN is intended to investigate exotic, very neutron-rich, radioactive nuclei. A linear accelerator will deliver radioactive beams which are produced by the isotope separator ISOLDE, with energies between 0.85 and 2.2 MeV/u. The linac will consist of an RFQ accelerator, an interdigital H-structure (IH) and three 7-gap-resonators for variable final energy. The 7-gap-resonators are being operated at a frequency of f =101.28 MHz with a pulsed power of P =90 kW (duty cycle 1:10). After the fabrication of the power type resonators, high-power tests were performed in order to prove the design voltage of 1.74 MV at 90 kW rf power. These tests were carried out with a 32 S beam and showed that the 7-gap-resonators exceed their design voltage; voltages between 1.77 and 1.88 MV were reached. In this paper the status of the 7-gap-resonator accelerator and the results of the high-power test are reported.

We have investigated electron capture by NH+ resulting in dissociative recombination (DR). The impact energies studied of ~4–12 eV extend over the range below the two lowest predicted NH+ dissociative states in the Franck–Condon (FC) region of the ion. Our focus has been on the final state populations of the resulting N and H atoms. The neutral DR fragments are detected downstream of a merged electron and ion beam interaction zone in the TSR storage ring, which is located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Transverse fragment distances were measured on a recently developed high count-rate imaging detector. The distance distributions enabled a detailed tracking of the final state populations as a function of the electron collision energy. These can be correlated with doubly excited neutral states in the FC region of the ion. At low electron energy of ~5 eV, the atomic product final levels are nitrogen Rydberg states together with ground-state hydrogen. In a small electron energy interval near 7 eV, a significant part of the final state population forms hydrogen Rydberg atoms with nitrogen atoms in the first excited (2D) term, showing the effect of Rydberg doubly excited states below the predicted 2 2Π ionic potential. The distance distributions above ~10 eV are compatible with nitrogen Rydberg states correlating to the doubly excited Rydberg state manifold below the ionic 2 4 Σ − level.

We present the concluding result from an Ives-Stilwell-type time dilation experiment using 7Li+ ions confined at a velocity of β=v/c=0.338 in the storage ring ESR at Darmstadt. A Λ-type three-level system within the hyperfine structure of the 7Li+3S1 →3P2 line is driven by two laser beams aligned parallel and antiparallel relative to the ion beam. The lasers' Doppler shifted frequencies required for resonance are measured with an accuracy of

We have investigated the autodetachment of electrons from rovibrationally hot SF${}_{6}{}^{\ensuremath{-}}$ anions using a cryogenic ion-beam trap. Extremely low residual gas densities of ${10}^{4}$ cm${}^{\ensuremath{-}3}$ provided undisturbed observation of the neutralization rates due to vibrational autodetachment (VAD) over almost five orders of magnitude and over times up to 100 ms. We successfully explain our experimental decay curves using statistical rate theory combined with electron attachment data and vibrational frequencies calculated for a ${C}_{4v}$-distorted SF${}_{6}{}^{\ensuremath{-}}$. The unprecedented sensitivity of the experiment to the decay constants at the VAD threshold allows us to infer from the data the adiabatic electron affinity of SF${}_{6}$ to be $(0.91\ifmmode\pm\else\textpm\fi{}0.07)$ eV and to confirm the recently predicted ${C}_{4v}$ symmetry of SF${}_{6}{}^{\ensuremath{-}}$.

We give a brief overview of time dilation tests using high-resolution laser spectroscopy at heavy-ion storage rings. We reflect on the various methods used to eliminate the first-order Doppler effect and on the pitfalls encountered, and comment on possible extensions at future facilities providing relativistic heavy ion beams at $\gamma \gg 1$.