I will first review various atomic processes in strong laser fields . A particular emphasis will be on the explanation of common features of different high-order atomic processes, which is based on the following sequence of theoretical methods: S-matrix formalism – strong-field approximation (SFA) – quantum-orbit theory .
In the second part of my talk I will consider above-threshold ionization (ATI) by ultrashort laser pulses [3,4]. It has been commonly assumed that S-matrix theory, as an in-out formalism, conveys no information about what happens in between. Applying the SFA and the formalism of quantum orbits to ATI I will show that this is not so. By expanding an S-matrix element in terms of such quantum orbits, whose space-time evolution is known, one can review the options that are available to the electron and follow their paths on the attosecond time scale.
In the third part of my talk I will present simulations of two recently performed experiments: ATI of inert gases (Garching), and above-threshold detachment of halogen negative ions (Freiburg).
Finally, I will consider how the presented theory can be generalized to molecular processes in strong fields. In particular, I will analyze the results for the strong-field ionization of diatomic molecules, obtained applying different gauges.
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