M. Unverzagt

A 4π recoil-ion electron momentum analyzer: a high-resolution “microscope” for the investigation of the dynamics of atomic, molecular and nuclear reactions✩

Abstract A high-resolution recoil-ion momentum spectrometer based on a precooled localized supersonic jet target (COLTRIMS) has been combined with a novel low-energy electron analyzer with 4π solid angle for electrons with energies E e ≤ 30 eV including E e = 0 eV. Thus, three recoil-ion momentum components, the recoil-ion charge state and three momentum components of one electron emitted in any collision-induced ionization reaction are measured simultaneously with a coincidence efficiency of 28%. In order to accept large recoil-ion longitudinal momenta (along the beam) of p R ‖ ≤ 160 a.u. and simultaneously guarantee a superior resolution in this direction (Δ P R ‖ ≤ ± 0.08 a.u.), recoil ions are extracted in the longitudinal direction different from all former concepts. Test measurements, details on the present design and results of a kinematically complete experiment for single ionization are presented and possible further improvements are discussed. The future potential of such spectrometers for the investigation of collision-induced atomic many-particle reactions, the “Coulomb-explosion” of molecules and the spectroscopy of electronic states in heavy few-electron systems is illustrated. Similar techniques might be used to measure angular correlations and even the neutrino mass in β-decay experiments.

Kinematically complete experiments using cold target recoil ion momentum spectroscopy

Abstract Cold Target Recoil Ion Momentum Spectroscopy allows the detection of the three-dimensional momentum vector of the recoiling product ion from ion, electron or photon atom collisions with 4π solid angle and high resolution. It can be combined with large area position-sensitive detectors for electron detection or measurement of the projectile charge-state and scattering angle. Such ‘reaction microscopes’ cover the full correlated momentum space of all fragments of an atomic reaction yielding kinematically complete information for each reaction event. For the first time in atomic collision physics fully differential data became available in the sense that not only the momenta of all fragments, but also the complete momentum space is observed in one experiment. Recent results achieved with this new technique for slow p-He collisions and threshold photo ionization of He will be discussed.

A design study for an internal gas-jet target for the heavy-ion storage ring CRYRING

This paper presents the design of the internal gas-jet target, CRYJET, which is being constructed for investigations of, e.g., fast ion--atom collisions in the heavy-ion storage and cooler ring CRYRING at the Manne Siegbahn Laboratory, Stockholm University. The goal for the design work was to create an ultra-cold He target (< = 10 mK in the longitudinal direction and 0.5 mK transverse temperature) with a density of ∼ 1012 atoms/cm3. Care was taken in order to minimize the influence from the jet on the very low background pressure in the storage ring (∼ 10-11 mbar). The low temperature is essential for the resolution in the experiments. The high density will enable us to get sufficient luminosities for investigations of processes with cross sections down to the 10-27 cm2 range. The gas-jet target will be equipped with two recoil-ion-momentum spectrometers in order to extract detailed information about the collision dynamics.

A high resolution 4π multi-electron spectrometer for soft electrons

Electrons emerging form ionizing collisions of any projectiles (ions, photons, electrons) with the particles in a supersonic jet target (atoms, clusters, molecules) are projected onto three large-area, multi-hit capable position sensitive detectors applying well controlled electric and solenoidal magnetic fields. Up to three electrons with an energy bandwidth for each between 0 eV ≤ Ee ≤ 1 keV are detected simultaneously with a solid angle close to 4π. Each momentum vector (three spatial components pei) is calculated from the measured hitting positions and time-of-flights with a resolution of Δ pei < 2 × 10−2 a.u.. Thus, the complete final many-electron momentum space (up to 9 momentum components) is visualized. The theoretical limits in resolution achievable with such a method is in the neV-regime. In addition, the recoil-ion momentum vector is determined simultaneously applying “conventional” recoil-ion momentum spectroscopy.

THE DYNAMICS OF TARGET IONIZATION BY FAST HIGHLY CHARGED PROJECTILES

Abstract We report on the first kinematically complete investigation of single target ionization by fast heavy ions, on the measurement of all low energy electrons down to zero emission velocities and on the determination of the projectile energy loss on the level of ΔE p /E p ≈ 10 −7 . This has been achieved by combining a high-resolution recoil-ion momentum spectrometer with a novel 4π electron analyser. The complete momentum balance between electron, recoil-ion and projectile for single ionization of helium by 3.6 MeV/u Ni 24+ was explored. Low energy electrons are found to be ejected mainly into the forward direction with a most likely longitudinal energy of only 2 eV. The electron momentum is not balanced, as might be expected, by the projectile momentum but is nearly completely compensated by the recoil ion. Surprisingly, the momenta of the helium-atom “fragments”, the electron and the He 1+ recoil ion, are considerably larger than the total momentum loss of the projectile: the target atom seems to dissociate in the strong, long ranging projectile potential. The collision has to be considered as a real three-body interaction.

He2+ on He: State-selective, scattering-angle-dependent capture cross sections measured by cold target recoil ion momentum spectroscopy (COLTRIMS)

Abstract We present the first state-selective, scattering-angle-dependent cross sections for the reaction 0.25–1 MeV He 2+ + He → He 1+ + He 1+ . The experiment was performed, using a new generation of recoil ion momentum spectroscopy, which enabled us to determine the three momentum components and the charge state of the recoil ion simultaneously. An absolute momentum resolution of ±0.13 a.u. for the longitudinal and ±0.02 a.u. for the transverse momentum component has been achieved, which is equivalent to a resolution in projectile energy loss of ±5.6 eV and an uncertainty of the projectile scattering angle of better than ±1 μrad.

Ionization collision dynamics in 3.6 MeV/u Ni24+ on He encounters

Abstract The momentum balance between all emerging particles (electron, recoil ion and projectile) was explored for single ionization of helium by 3.6 MeV/u Ni24+ impact in a kinematically complete experiment. Technically this was achieved by integrating a novel 4π low-energy electron analyzer into a high-resolution cold-target recoil-ion momentum spectrometer. More than 90% of the “soft electrons” (0 eV ≤ Ee ≲ 50 eV) are ejected in forward direction with a most probable longitudinal energy (along the ion-beam) of E e⊥ ≈ 3 eV . Not the projectile, but the backwards ejected recoil-ion ( E R⊥ ≈ 0.4 meV ) compensates the electron longitudinal momentum except of a small contribution from the inelasticity of the reaction. Energy losses of the 0.2 GeV projectiles as small as ΔE p E p = 3.4 × 10 −7 , transverse momentum balances, as well as electron energy and angular distributions for defined final recoil-ion charge state become accessible with this technique.

Cold Target Recoil Ion Momentum Spectroscopy

The experimental technique of Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) is described. It allows a three dimensional imaging of momentum space of the recoiling ion for all ionizing atomic reaction with 4π solid angle for momentum measurement. The resolution presently achieved is ±0.035 a.u.. Depending on the collision system this corresponds to a resolution in projectile energy loss of down to ΔE/E=10−9 and a scattering angle resolution of down to 10−9 rad for fast heavy ion collisions. We discuss the experimental technique and some recent results on dynamics of recoil ion production for electron capture, target ionization and projectile electron loss.

Capture, ionisation and loss in swift He1,2+He collisions investigated by cold target recoil ion momentum spectroscopy

Abstract For 0.25 – 2MeV He 2+ He and He + He collisions we have measured the recoil ion momentum distribution in three dimensions for single capture, target ionisation and projectile electron loss. From these double differential cross sections we obtain state selective and scattering angle dependent cross sections for the single capture process and are able to distinguish between electron-electron and nucleus-electron interactions for the projectile electron loss.

Single and double ionization of helium for fast, highly-charged ion-impact☆

Abstract Using bare Ne, Ni and Kr projectiles, the ratio R of helium double-to-single ionization cross sections was measured at projectile energies of 80 MeV/u ≤ Ep ≤ 1.5 MeV/u (0.39 ≤ νp/ c ≤ 0.93). For Ne10+ impact R was observed to become independent of νp for νp / c ≳ 0.73, indicating that the high-velocity limit for one-step double ionization (“shake-off” limit) has been reached for the first time for highly charged ions. Exploiting recoil-ion momentum spectroscopy (RIMS), first results for R, differential in the recoil-ion transverse momentum pR⊥, are presented for 10 MeV C5+ impact. Detailed investigation of the ionization collision dynamics indicates that pR⊥ is reasonably related to the internuclear impact parameter b. This enables comparison of the experimental data with results of b-dependent ab initio calculations.