Diagrammatic many-body theory for atoms in high-intensity laser fields Part II

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Abstract
Starting from first principles, I derive a time-dependent diagrammatic many-body formalism that can be used for strong harmonic fields. My aim has been to treat many-electron effects in an atom interacting with an intense laser field. The formalism derived describes the atom in terms of a damped many-body Floquet state. The diagrammatic expansion is similar to the ordinary time-independent atomic many-body theory, but some important changes have been made: The energy denominators contain time-dependent self-energies and the exact ground-state shift; the intermediate levels are dressed atomic states, for which each harmonic of a given atomic state is counted as a separate level; and the ground-state energy shift is given by a time-dependent Brillouin-Wigner-type energy equation, which in general must be solved numerically. The formalism is not linked but contains no divergences in the adiabatic limit of the electron-electron interaction. For weak fields the formalism reduces to the ordinary linked expansion. For a two-level system all diagrams can be summed, and the result is equivalent to the usual continued-fraction solution. I also derive the exact diagrammatic solution for a single level interacting with a continuum.

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