H.-J. Kluge

A new cooling technique for heavy ions in a Penning trap

A new cooling technique for heavy ions stored in a Penning trap has been developed. The axial and cyclotron motions are cooled by buffer gas collisions. The outward radial diffusion caused by the buffer gas is counteracted by an azimuthal quadrupole rf field at the sum frequency of the magnetron and cyclotron motions. A mass selectivity of 500 in the cooling is achieved while the axial energy distribution is observed to be in equilibrium with the buffer gas temperature (T = 300 K).

Quadrupole excitation of stored ion motion at the true cyclotron frequency

Abstract The motion of an ion in a Penning trap has been investigated in the presence of an azimuthal quadrupole radio frequency field and a damping force provided by buffer gas collisions. Analytical expressions are derived which describe the line shape of the cyclotron resonance as well as the properties of the mass-selective cooling mechanism for heavy ions. Excellent agreement is observed between theoretical results and experimental data obtained with the tandem Penning trap mass spectromer ISOLTRAP at ISOLDE (CERN).

External-ion accumulation in a Penning trap with quadrupole excitation assisted buffer gas cooling

Abstract A pulsed ion beam from an external source is injected into a Penning trap and accumulated by repeatedly lowering during ion capture to prevent the ions already captured from escaping. For the same reason the newly captured ions have to be cooled, which achieved by buffer gas collisions. To prevent radial on loss, the ions are exposed to azimuthal quadrupole excitation. By choosing the appropriate frequency (range) this method (selective quadrupole excitation assisted capture and centering (SQUEACE) allows a mass selection during the capture process and leads to a centering of those ions in the Penning trap. The multiple ion bunch capture results in a significant improvement in signal-to-noise ratio and a decrease in experiment duration.

A resonance ionization mass spectrometer as an analytical instrument for trace analysis

Abstract An instrument for trace analysis based on resonance ionization mass spectroscopy has been developed. It consists of three tunable pulsed dye lasers pumped by one or two copper vapour lasers and a time-of-flight mass spectrometer. The properties of the laser system, the quartz fibres that are used for the transport of the laser beams, the time-of-flight spectrometer, the atomic beam sources and the data acquisition and laser control electronics are described in detail. A mass resolution of 2700 was obtained with gadolinium evaporated from an oven. The detection efficiency after evaporation from a filament was measured to be a few times 10 −6 for plutonium as well as for technetium at a very low background. Measurements of the isotopic ratios of plutonium samples yielded a satisfactory agreement with mass spectrometric data. Examples are given for some spectroscopic applications of the instrument such as the investigation of autoionizing states.

A Penning trap mass spectrometer for the study of cluster ions

A Penning trap system has been set up for storing and investigating cluster ions over time ranges from microseconds up to minutes. This enables studies of cluster reactions with extremely low cross sections and the observation of their time dependence in a new regime. The ions are created externally by laser vaporization, cooled by adiabatic expansion of a supersonic beam, and injected into the Penning trap. Detection of reaction products is achieved by combining the advantages of two complementary approaches, viz. the high resolution of Fourier transform mass spectrometry and the high sensitivity of single‐ion counting with a time‐of‐flight mass spectrometer. The performance of the apparatus is illustrated by results of recent cluster experiments.

Quadrupole-detection FT-ICR mass spectrometry☆

Abstract A new detection scheme for Fourier transform-ion cyclotron resonance (FT-ICR) studies is proposed and initial experimental results are presented. While in conventional FT-ICR the difference of the transient signals from two opposite electrodes yields a signal at the reduced cyclotron frequency v + , which depends on the applied trapping voltage and space charge, it is also possible to use the difference of the sum of these signals and the sum of a similar set of electrodes perpendicular to the first. In this way resonances are observed at the harmonic of the reduced cyclotron frequency, 2· ν + , as well as at ( v + + v - ). The latter is interesting in respect of accurate mass measurements, since ( v + + v - ) does not depend on the trapping voltage.

Ground-state properties of neutron-deficient platinum isotopes

The hyperfine structure splitting and the isotope shift in the λ=266 nm transition of Pt isotopes within the mass range 183 ≦A≦ 198 have been determined by Resonance Ionization Mass Spectroscopy (RIMS) in combination with Pulsed-Laser Induced Desorption (PLID). The Pt isotopes were obtained at the on-line isotope separator ISOLDE-3/CERN as daugthers of the primarily produced Hg isotopes. Magnetic moments, quadrupole moments, and changes in the mean-square charge radii are deduced and compared with results of a particle-triaxial rotor model and mean field calculations. Good agreement with experimental data (including nuclear level schemes and transition probabilities) can only be obtained if triaxial shape is admitted. The calculations yield a smooth transition in the shape of odd-A Pt nuclei from a slightly deformed, nearly oblate195Pt via triaxial197-187Pt to a strongly deformed nearly prolate177Pt.

Ramsey technique applied in a Penning trap mass spectrometer

Abstract The Ramsey method has been applied in an experiment aiming for accurate mass determination of unstable isotopes. The ion motion in a Penning trap has been excited with time-separated oscillatory fields and Ramsey fringes were observed in the case of dipole and quadrupole excitation. The experimental resonances are in good agreement with theory. Further applications of the technique are discussed.

Parametric‐mode‐excitation/dipole‐mode‐detection Fourier‐transform–ion‐cyclotron‐resonance spectrometry

A new excitation/detection scheme for Fourier‐transform–ion‐cyclotron‐resonance (FT‐ICR) studies is described. The ions are created in the center of a hyperbolic Penning trap and excited to a large magnetron radius. Parametric‐mode excitation is used to drive the cyclotron motion. The ions are detected in the conventional dipole‐mode detection scheme by use of a segmented ring electrode. Since the resonance frequencies of excitation (ν+−ν−) and detection (ν+, the reduced cyclotron frequency) differ by the magnetron frequency (ν−), this method allows the simultaneous excitation and detection of the ion motion.

Trapped metal cluster ions

An overview is given of experiments with stored metal cluster ions in a Penning trap system. The setup allows axial injection of clusters produced in an external source and a time-of-flight mass analysis of the reaction products after axial ejection. The systems options include the selection of stored ions, the manipulation of their orbits, addition of reactant and buffer gases and axial optical access for laser spectroscopic studies. As described by various examples, investigations have been made with respect to the development of trapping techniques and the characterization of metal clusters in terms of their physical and chemical properties.