We explore quantum beats in the photoelectron signal produced when a bound electron wave packet created by an isolated attosecond pulse is ionized by a delayed, few-cycle infrared pulse. Our calculations for helium atoms show that the broad bandwidth of the few-cycle pulse creates spectrally overlapping photoelectron peaks that result from one-, two-, or three-photon ionization processes. The beat signals can, in principle, be interferometrically resolved with high resolution, giving access to the relative phase between different multiphoton ionization pathways. For few-cycle near-infrared fields the relative spectral phases can be extracted over a large energy region, and dynamical information becomes available. We find that multiphoton ionization is temporally shifted with respect to one-photon ionization by several hundred attoseconds. Our results also reveal the impact of depletion and resonant pathways on the phase of the quantum beats.
Publication Source (Journal or Book title)
Physical Review A
Pazourek, R., Reduzzi, M., Carpeggiani, P., Sansone, G., Gaarde, M., & Schafer, K. (2016). Ionization delays in few-cycle-pulse multiphoton quantum-beat spectroscopy in helium. Physical Review A, 93 (2) https://doi.org/10.1103/PhysRevA.93.023420