Reaction microscopes applied to study atomic and molecular fragmentation in intense laser fields:: non-sequential double ionization of helium

Reaction Microscopes enable to detect the momentum vectors of several electrons and ions after the fragmentation of atoms or molecules. Thus, the investigation of pathways to single and multiple electron ejection in fermosecond PW/cm(2) laser fields has become experimentally accessible in unprecedented detail. In this paper, a newly designed machine is described, resolutions achieved for electrons and ions are discussed and examples are given for many-particle fragmentation of atoms and molecules. Moreover, for helium, new results on single as well as first multiple differential data on double ionization are presented. Covering a wide intensity range within the non-sequential (NS) double ionization regime, the importance of different NS double ionization mechanisms is explored as a function of the laser intensity. Contributions due to recollision-excitation plus subsequent field ionization (RESI) are identified and correlated electron emission spectra are discussed in the longitudinal as well as transverse directions. Whereas only weak indications of Coulomb-repulsion between the electrons in the final state are observed the emitted electrons are found to be strongly correlated with the ions. (C) 2004 Elsevier B.V. All rights reserved.