Double ionisation of helium in fast ion collisions: the role of momentum transfer

Double ionisation of helium in the perturbative regime has been explored in a kinematically complete collision experiment using 100 MeV/u C"6"+ ions. Different ionisation mechanisms are identified by inspecting the angular distribution of the electrons as a function of the momentum transfer q to the target by the projectile. For q<1.2 a.u., both electrons are distributed uniformly in the plane perpendicular to the projectile axis, and distinct similarities with photoionisation are identified. For q>1.2 a.u., the faster electron resulting from a binary encounter with the projectile is emitted along the direction of momentum transfer, while the other electron is distributed uniformly. Experimental data are compared with various model calculations based on the Bethe-Born approximation with shake-off. Surprisingly, the effect of the final state interaction is found to depend decisively on the choice of the initial state wave function. (orig.)Available from TIB Hannover: RO 801(99-02) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Atoms in extreme virtual photon fields of fast, highly charged ions

Highly-charged ions at velocities close to the speed of light generate strong (I = 10"1"7 - 10"2"0 W/cm"2), ultra-short (t = 10"-"1"9 - 10"-"1"7 s) electromagnetic pulses when passing target atoms or molecules. Single and multiple ionization occurs and can be interpreted in terms of photoionization: at small field strength ionization is due to the absorption or the scattering of a single virtual photon whereas the incoherent, ''simultaneous'' absorption of ''many'' field quanta gives rise to multiple ionization in strong fields. In this paper analogies to photoionization are investigated by comparison of kinematically complete experimental data with results of various theoretical calculations for collisions of 100 MeV/u C"6"+, 1000 MeV/u U"9"2"+ and 3.6 MeV/u Au"5"3"+ on He, Ne and Ar targets. Dynamical multiple ionization mechanisms are identified as a function of the momentum transfer. Two-electron final states are found to sensitively depend on the correlated initial state, and the many-particle Coulomb continuum is investigated for triple ionization of neon in super-strong fields. (orig.)29 refs.SIGLEAvailable from TIB Hannover: RO 801(99-29) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman