The Magnus expansion for interaction of atoms with attosecond laser pulses

Abstract

The Magnus expansion of the time propagator for an atom in the classical field of a short, strong laser pulse is employed increasingly as attosecond pulses are developed. Here the lowest order terms are given explicitly and shown to be gauge invariant up to a given power of the laser-pulse length. The Kramers–Henneberger frame transformation is shown to lead to the same result and to be particularly suited for use with the Magnus expansion. A recently proposed modification of the first-order Magnus term is shown to be equivalent to the second order for a full-cycle pulse.