Resolving Sub-Angstrom Vibrational Dynamics via Strong-Field Rescattering*

Capturing molecular dynamics with sub-angstrom and sub-femtosecond spatial-temporal resolution is an important goal in ultrafast science. Emerging scattering techniques offer promising avenues for achieving this goal, with Laser-induced Electron Diffraction (LIED) standing out as a method to image molecules. In LIED a coherent electron wave packet is created by tunnel ionization and then the molecule is probed by its own tunneled electrons. Unlike LIED which extracts the differential cross section (DCS) from a 2-D photoelectron angular distribution, Fixed-Angle Broadband Laser-driven Electron Scattering (FABLES) connects 1-D photoelectron spectrum to atomic position with sub-picometer resolution by a simple Fourier Transform. FABLES in N₂ has previously been explored using a single color, but here we employ two colors (1700nm+850nm). The 1700 nm light performs FABLES, while the novelty in this research lies in the addition of the 850 nm light, which induces a transition from the X state to the A state of N₂⁺ as the electron travels in the continuum. This transition modifies the electron density upon the electron’s return, resulting in a new diffraction pattern. Changes in bond length indicate that it was possible to image nuclei movement in N₂⁺ due to the X to A transition. Our results demonstrate a change when transitioning from a single color (1700 nm) to two colors (1700 nm + 850 nm). .Additionally, variations in the diffraction pattern were observed with changes in the phase between the two colors.Theoretical calculations provide agreement with experiment and also shows the phase reconstruction between the X and A state is possible This experiment highlights the effect of extremely fast one-photon resonant transitions and reveals how electronic excitations in the ion between ionization and recollision alter FABLES. These findings open the door to new interesting studies in LIED and FABLES using a two-color scheme.