Abstract We theoretically study the determination of the time delay between the subsequent absorption of photons in two-photon ionization with the attosecond streaking technique. An analytical model for the absorption time delay based on analysis of the process using perturbation theory is obtained. Model predictions at central frequencies that are resonant and non-resonant with an intermediate bound state agree well with results of numerical streaking simulations, if the intermediate state is well-isolated from other states and the effect of the streaking field on the intermediate state is negligible.
A nonresonant two-photon absorption process can be manipulated by tailoring the ultra-short laser pulse. In this paper, we theoretically demonstrate a highly selective population of two excited states in the nonresonant two-photon absorption process by rationally designing a spectral phase distribution. Our results show that one excited state is maximally populated while the other state population is widely tunable from zero to the maximum value. We believe that the theoretical results may play an important role in the selective population of a more complex nonlinear process comprising nonresonant two-photon absorption, such as resonance-mediated (2+1)-three-photon absorption and (2+1)-resonant multiphoton ionization.
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 $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2$ 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.