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A central issue in modem atomic physics is the understanding of the role of static and dynamic electron-electron correlation in multi-electron transitions. In this respect the investigation of double ionization of helium induced in collisions with projectiles is fundamental since it presents the simplest particle impact induced three body break-up and it is the first step in order to understand multiple ionization processes. Experiments aim to obtain fully differential cross sections since they provide the most stringent tests for theory and to give an understanding of the mechanisms involved for a large range of incident energies and of the final state momentum space. For double ionization of helium by electron impact very few experimental results are available and besides the present study only one kinematically complete experiment using conventional electron spectrometry was performed by (1998).

In recent experiments at the velocity filter Separator for Heavy Ion reaction Products (SHIP) (GSI, Darmstadt), an extended and improved set of $\ensuremath{\alpha}$-decay data for more than 20 of the most neutron-deficient isotopes in the region from lead to thorium was obtained. The combined analysis of this newly available $\ensuremath{\alpha}$-decay data, of which the $^{186}\mathrm{Po}$ decay is reported here, allowed us for the first time to clearly show that crossing the $Z=82$ shell to higher proton numbers strongly accelerates the $\ensuremath{\alpha}$ decay. From the experimental data, the $\ensuremath{\alpha}$-particle formation probabilities are deduced following the Universal Decay Law approach. The formation probabilities are discussed in the framework of the pairing force acting among the protons and the neutrons forming the $\ensuremath{\alpha}$ particle. A striking resemblance between the phenomenological pairing gap deduced from experimental binding energies and the formation probabilities is noted. These findings support the conjecture that both the $N=126$ and $Z=82$ shell closures strongly influence the $\ensuremath{\alpha}$-formation probability.

In recent experiments at the velocity filter Separator for Heavy Ion reaction Products (SHIP) (GSI, Darmstadt), an extended and improved set of α-decay data for more than 20 of the most neutron-deficient isotopes in the region from lead to thorium was obtained. The combined analysis of this newly available α-decay data, of which the (186)Po decay is reported here, allowed us for the first time to clearly show that crossing the Z = 82 shell to higher proton numbers strongly accelerates the α decay. From the experimental data, the α-particle formation probabilities are deduced following the Universal Decay Law approach. The formation probabilities are discussed in the framework of the pairing force acting among the protons and the neutrons forming the α particle. A striking resemblance between the phenomenological pairing gap deduced from experimental binding energies and the formation probabilities is noted. These findings support the conjecture that both the N = 126 and Z = 82 shell closures strongly influence the α-formation probability.

For impact of slow multicharged N, Ne, Ar, and I ions on clean polycrystalline gold (impact velocity 1×10 4

Dissociative as well as nondissociative single ionization of H2 by 6 MeV proton impact has been studied in a kinematically complete experiment by measuring the momentum vectors of the electron and the H+ fragment or the H+2 target ion, respectively. For the two ionization pathways, the electron spectra reveal the role of autoionization of the doubly and singly excited states of H2. The latter explicitly involve the coupling between the electronic and the nuclear motion of the molecule. This is a clear manifestation of a breakdown of the Born-Oppenheimer approximation.

Energy-gain spectra and total cross sections have been measured for projectiles of 492-eV Ne/sup +10/ and 443-eV Ne/sup +9/ capturing electrons from rare-gas targets. For each collision system a single, or at most two, principal quantum numbers on the projectile are found to be populated. The effective electron-transfer probabilities are found to be quite dependent upon the collision system.