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.
© 1992 Optical Society of AmericaFull Article | PDF Article
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