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Fully differential cross-sections in momentum space for multiple ionization processes of atoms and molecules have been investigated by a multi-coincidence imaging technique, called COLTRIMS (cold target recoil ion momentum spectroscopy) (J. Phys. B 30 (1997) 2917; Nucl. Instrum. Methods B 108 (1996) 425; In: Ullrich, J., Shevelko, V.P. (Eds.), Many Particle Quantum Dynamics in Atomic Fragmentation, Series Atomic, Optical, and Plasma Physics, Vol. 35. Springer, Berlin, 2003; Phys. Rep. 330 (2000) 95). This technique is as powerful as the bubble chamber system in high-energy physics. It has opened a new observation window into the hidden world of many-particle dynamics: correlated many-particle dynamics in Coulombic systems can now be experimentally approached with unprecedented completeness and precision.The principle of the method, namely measuring the momentum of the emitted charged particles from an atomic or molecular fragmentation process, is as simple as determining the trajectory of a thrown stone. From knowing the position from where the stone was slung and where it hits the target, as well as measuring its time-of-flight, the trajectory of the stone and thus its initial velocity vector can be determined precisely. Furthermore, in order to achieve good precision we have to know whether the person, who throws the stone, was at rest in the frame of observation or with which relative velocity this person was moving. Thus, to obtain optimal momentum resolution for the exploding fragments one has to bring the fragmenting object to a complete rest in the frame of measurement before the reaction occurs, i.e. if the object is a gas atom or molecule one has to cool it down to sub-milli Kelvin temperatures. [Copyright &y& Elsevier]

Many particle dynamics in atomic and molecular physics has been investigated by using the COLTRIMS method. The method and its power is discussed. The COLTRIMS technique visualizes many-particle fragmentation processes in the eV and sub milli-eV regime and reveals like the bubble chamber the complete momentum pattern in atomic and molecular particles reactions. Complete differential cross sections in momentum space have been measured for the transfer ionization channel in fast four-body (p+He→H°+He2++e) collisions. The correlated kinematical transfer ionization channel has been used to probe the highly correlated contributions to the asymptotic parts (high momenta and large nuclear impact parameters) of the He ground-state momentum wave function. In this reaction, one electron with selected initial momentum (2.5–7.5 a.u.) in the He ground state is kinematically captured by the proton (tunneling through the two-center barrier). The measured three-particle final-state momentum distributions show well-structured patterns, which reflect special features of the three-particle initial-state momentum wave function. [Copyright &y& Elsevier]

We study the decay of a helium/neon dimer after ionization and simultaneous excitation of either the neon or the helium atom using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS). We find that, depending on the decaying state, either direct Interatomic Coulombic Decay (ICD) (i.e. mediated by a virtual photon exchange), exchange ICD (mediated by electron exchange) or radiative charge transfer occurs. The corresponding channels are identified. [ABSTRACT FROM AUTHOR]

We investigate the single-photon double ionization of helium at photon energies of 440 and 800 eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique fingerprint of an elusive quasifree photon double ionization mechanism predicted by Amusia et al. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods. [ABSTRACT FROM AUTHOR]

We investigate the photodouble ionization of H2 molecules with 400 eV photons. We find that the emitted electrons do not show any sign of two-center interference fringes in their angular emission distributions if considered separately. In contrast, the quasiparticle consisting of both electrons (i.e., the "dielectron") does. The work highlights the fact that nonlocal effects are embedded everywhere in nature where many-particle processes are involved. [ABSTRACT FROM AUTHOR]

Even though the study of ion-atom collisions is a mature field of atomic physics, large discrepancies between experiment and theoretical calculations are still common. Here we present experimental results with high momentum resolution on the single ionization of helium induced by 1-MeV protons, and we compare these to theoretical calculations. The overall agreement is strikingly good, and even the first Born approximation yields good agreement between theory and experiment. This has been expected for several decades, but so far has not been accomplished. The influence of projectile coherence effects on the measured data is briefly discussed in terms of an ongoing dispute on the existence of nodal structures in the electron angular emission distributions. [ABSTRACT FROM AUTHOR]