L. Schweikhard

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).

ISOLTRAP: a tandem Penning trap system for accurate on-line mass determination of short-lived isotopes

The tandem Penning trap mass spectrometer ISOLTRAP has been set up at the on-line mass separator ISOLDE at CERN/Geneva for accurate mass measurements of short-lived nuclei with T12 ≥ 1 s. The mass measurement is performed via the determination of the cyclotron frequency of an ion in a magnetic field. The design of the spectrometer matches the particular requirements for on-line mass measurements on short-lived isotopes. With the ISOLTRAP spectrometer masses of more than 70 radioactive nuclei have so far been determined with resolving powers exceeding one million and an accuracy of typically 10−7.

In-flight capture of ions into a penning trap☆

Abstract A bunched beam of alkali ions with a pulse length of about 10 μs and an energy of 1 KeV has been retarded electrostatically and captured in flight into a Penning trap. A trapping efficiency of up to 70% has been determined. Subsequently the cyclotron resonance was induced. In the case of K a line width of the resonance of 4 Hz was measured at a resonance frequency of 2.3 MHz. This enables mass determinations of unstable nuclei produced at on-line mass separators with an accuracy in the sub-ppm region.

Excitation modes for Fourier transform-ion cyclotron resonance mass spectrometry

Various geometric configurations for the excitation of coherent ion motion in Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR/MS) are analyzed (in some cases for the first time) with unified notation. The instantaneous power absorption, F v, in which v is ion velocity and F the force produced by the applied excitation electric field (harmonic, single frequency, on-resonance, in-phase), is time averaged and then set equal to the time rate of change of ion total (cyclotron + magnetron + trapping) energy, to yield a differential equation that is readily solved for the (time-dependent) amplitude of each of the various ion motions. The standard FT-ICR excitation (namely, radial dipolar) is reviewed. The effects of quadrature and radial quadrupolar excitation on ion radial (cyclotron and magnetron) motions are also reviewed. Frictional damping is shown to decrease the ion cyclotron orbital radius and trapping amplitude but increase the magnetron radius. Feedback excitation (i.e., excitation at the simultaneously detected ion cyclotron orbital frequency of the same ion packet) is introduced and analyzed as a means for exciting ions whose cyclotron frequency changes during excitation (as for relativistically shifted low-mass ions). In contrast to conventional radial dipolar excitation, axial dipolar excitation of the trapping motion leads to a mass-dependent ion motional amplitude. Parametric (i.e., axial quadrupolar) excitation is shown to produce an exponential increase in the ion motional amplitudes (hyperbolic sine and hyperbolic cosine amplitude for cyclotron and magnetron radii, respectively). More detailed consideration of parametric excitation leads to an optimal ion initial radial position in parametric-mode FT-ICRjMS.

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.

Multiple-collision induced dissociation of trapped silver clusters Agn+ (2⩽n⩽25)

The dissociation energies of singly charged silver cluster cations, Agn+ (2⩽n⩽25), are determined by multiple-collision induced dissociation (MCID) in a Penning trap. The fragment yield is analyzed in terms of a linearized impulsive collision theory for the energy transfer in the multicollisional process and the delayed decay as predicted by the Rice–Ramsperger–Kassel (RRK) model. Previous photofragmentation experiments performed in the size range (9⩽n⩽21) are found to be in good agreement with the present results. Theoretical predictions agree for most clusters sizes.

The magnetic trapping mode of an electron beam ion trap: New opportunities for highly charged ion research

Using x‐ray spectroscopic techniques, we have investigated the properties of an electron beam ion trap (EBIT) after the electron beam is switched off. In the absence of the electron beam, bare, and hydrogenlike Kr35+ and Kr36+ ions remain trapped due to externally applied magnetic and electric fields for at least 5 s; xenon ions with an open L shell, i.e., Xe45+–Xe52+, remain trapped at least as long as 20 s. The ion storage time in this ‘‘magnetic trapping mode’’ depends on the pressure of background atoms as well as on the value of the externally applied trapping potential, and even longer ion storage times appear possible. The magnetic trapping mode enables a variety of new opportunities for atomic physics research involving highly charged ions, which include the study of charge transfer reactions, Doppler‐shift‐free measurements of the Lamb shift, measurements of radiative lifetimes of long‐lived metastable levels, or ion‐ion collision studies, by x‐ray or laser spectroscopy, and mass spectrometry. Be...

Mass measurements of very neutron-deficient Mo and Tc isotopes and their impact on rp process nucleosynthesis.

The masses of ten proton-rich nuclides, including the N=Z+1 nuclides ⁸⁵Mo and ⁸⁷Tc, were measured with the Penning trap mass spectrometer SHIPTRAP. Compared to the Atomic Mass Evaluation 2003 a systematic shift of the mass surface by up to 1.6 MeV is observed causing significant abundance changes of the ashes of astrophysical x-ray bursts. Surprisingly low α separation energies for neutron-deficient Mo and Tc are found, making the formation of a ZrNb cycle in the rp process possible. Such a cycle would impose an upper temperature limit for the synthesis of elements beyond Nb in the rp process.

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.

Multiply Charged Metal Cluster Anions

Formation and stability patterns of silver dianionic and gold trianionic clusters are investigated with Penning-trap experiments and a shell-correction method including shape deformations. The theoretical predictions pertaining to the appearance sizes and electronic shell effects are in remarkable agreement with the experiments. Decay of the multiply anionic clusters occurs predominantly by electron tunneling through a Coulomb barrier rather than via fission, leading to appearance sizes unrelated to those of multiply cationic clusters.