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Atoms and molecules submitted to a strong laser field can emit electrons of high energies in the above-threshold ionization process. This process finds a highly intuitive and also quantitative explanation in terms of Feynman's path integral and the concept of quantum orbits. In theoretical description of this process one usually starts from the S-matrix theory and applies the so-called strong-field approximation (SFA). The so obtained transition matrix element is in the form of a multi-dimensional integral. This integral can be evaluated numerically or using the saddle-point method, which results in a sum over relevant parts, called the quantum orbits in strong-field physics. However, this result has not been derived starting directly from the Feynman path-integral formalism and their connection is explained by intuition and analogy.
We present an explicit derivation of the SFA starting from the phase space path-integral formulation of above-threshold ionization. The approximations made allow us to see how it is possible to go beyond the SFA, which is important of one wants to include systematically the influence of the atomic (molecular) binding potential. |
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