Reconstructing Molecular-Ion Quantum States with Two-Color Strong-Field Scattering
Strong-field ionization of molecules can prepare coherent superpositions of ionic states whose amplitudes and relative phases govern the ensuing ultrafast dynamics, but accessing this information experimentally remains a challenge. Here we introduce two-color strong-field scattering, in which a strong mid-infrared field ionizes the molecule and launches an electron, while a weak second harmonic field resonantly couples vibronic states of the parent ion during the electron's continuum excursion, thereby introducing structural changes on the sub-laser-cycle timescale. Scanning the relative phase between the two colors controls the ionic evolution and maps it onto phase-dependent sub-angstrom bond-length changes retrieved from the rescattering signal. Using the example of N$_2$, we apply the two-color setup to probe the coherence between different ionization channels and to reconstruct the initial ionic superposition. Our results establish two-color strong-field scattering as a powerful structural approach to characterize the complex amplitudes of ion quantum wavepackets.