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I will first review various atomic processes in strong laser fields [1].
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 [2].
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.
**

- [1] D. B. Milošević and F. Ehlotzky, “Scattering and reaction
processes in powerful laser fields”, Adv. At. Mol. Opt. Phys.
**49**, 373-532 (2003). - [2] D. B. Milošević, D. Bauer, and W. Becker, “Quantum-orbit
theory of high-order atomic processes in strong fields”, J. Mod. Opt.
**53**, 125-134 (2006). - [3] W. Becker, F. Grasbon, R. Kopold, D. B. Milošević, G. G.
Paulus, and H. Walther, “Above-threshold ionization: from classical
features to quantum effects”, Adv. At. Mol. Opt. Phys.
**48**, 35-98 (2002). - [4] D. B. Milošević, G. G. Paulus, D. Bauer, and W. Becker,
“Above-threshold ionisation by few-cycle pulses”, J. Phys. B
**39**, R203-R262 (2006).

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