Signatures of the phase structure in multiphoton ionization of aligned N₂ molecules

We experimentally measure the photoelectron momentum distributions of multiphoton ionization of aligned N-2 molecules at 400 nm. The phase structure of the ionized electron wave packet is probed with a weak parallelly polarized field at 800 nm. By inspecting the two-color phase-resolved photoelectron interference fringes, we observe a relative phase shift between the yields of above-threshold ionization and sideband peaks that depends on the molecular alignment angle. With the support of molecular strong-field approximation, we show the measured alignment-dependent relative phase shift is associated with the initial phase of electron wave packet dictated by the molecular orbital at the ionization exit, which is mapped into the intracycle interference. We find that the initial phase which depends on the molecular alignment can be modulated by the two-color synthesized fields and decreases with increasing photoelectron energy. Finally, we demonstrate the alignment-dependent phase shift in momentum space allows the retrieval of the relative spatial displacement for electrons at the ionization exit in position space between different alignments.