Attosecond time delay during resonance-enhanced multiphoton ionization of Ar in strong laser fields

Phase-dependent photoelectron spectra in resonance-enhanced multiphoton ionization (REMPI) of Ar are obtained by scanning the relative phase of two-color femtosecond lasers. Five REMPI channels are resolved and the relative time delays in the attosecond-timescale are extracted. The channel-resolved time delays can be assigned as the contribution of the resonant phase shift of electrons trapped in the specific Rydberg states and also influenced by the phase shift during the continuum-continuum transition. Our observations highlight the important influences of Freeman resonances on photoelectron emission dynamics in strong laser fields and will enable a subfemtosecond photoelectron wavepacket reconstruction in quantum mechanics.