Determination of atomic multiphoton-ionization phases by trichromatic multichannel wave-packet interferometry

We present the unambiguous determination of quantum phases in the multiphoton ionization (MPI) of potassium atoms by using a multichannel photoelectron interferometry scheme based on trichromatic pulse shaping. The colors of the laser field are chosen to produce three energetically separated photoelectron interferograms in the continuum. While the red pulse is two-photon resonant with the 3d state resulting in a (2 + 1) resonance-enhanced MPI (REMPI), a (1 + 2) REMPI occurs via the nonresonant intermediate 4p state with an initial green or blue pulse. We show that ionization via a nonresonant intermediate state lifts the degeneracy of photoelectron interferograms from pathways consisting of permutations of the colors. The analysis of the interferograms reveals a phase shift of +/-pi /2 depending on the sign of the detunings in the (1 + 2) REMPI pathways. In addition, we demonstrate that the photoionization time delay in the resonant (2 + 1) REMPI pathway gives rise to a linear spectral phase in the photoelectron spectra. Insights into the underlying MPI processes are gained through an analytic perturbative description and numerical simulations of a trichromatic driven three-level system coupled to the continuum.