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Consensus has developed that the mechanism of nonsequential multiple
ionization, at least for near-infrared laser frequencies, is related to
recollision of a tunnel-ionized electron with its parent ion. S-matrix
theory allows for comparatively straightforward computation, once
the diagram responsible has been identified. A crucial element of such a
description is the electron-electron interaction by which the recolliding
electron kicks out the second electron (or more). I consider different
choices for this interaction and their consequences for the ion and
electron momentum distributions that have been recorded in experiments.
I discuss various methods of how to compute the S-matrix element,
including saddle-point methods that lead to the concept of quantum
orbits and a certain limit that is classical except for the initial
tunneling
of the first electron. If the electron-electron interaction is of contact
type, the latter model becomes a statistical model, which only depends
on the
tunneling rate, the rescattering kinematics, and the volume of phase
space for
given final momenta. This statistical model can also be applied for an
elliptically polarized laser field. For ellipticities exceeding
approximately 0.3, interesting effects begin to develop in the momentum
distributions. An additional parameter that reflects the joint action
of the electron-electron, electron-ion, and electron-field dynamics
can be introduced by assuming a delay between the time of recollision
and the later time when a subset of electrons has thermalized with the
returning electron and leaves the immediate vicinity of the ion.
The existence of such a delay is supported by classical-trajectory
calculations. Comparing model calculations with reality one can infer a
value for this delay time.
In collaboration with P.B. Corkum, C.F.M. Faria, S.P. Goreslavski, P.J. Ho, X. Liu, S.V. Popruzhenko, H. Schomerus, N. Shvetsov-Shilovski Supported in part by Deutsche Forschungsgemeinschaft |
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