Molecular structure can be encoded both in the direct photoelectrons resulting from tunneling in a strong laser field, as well as in the elastically rescattered electrons, which are driven back to the molecular ion within one half-cycle of the laser field. The latter process, Laser-Induced Electron Diffraction (LIED), is often described as self-imaging of a molecule by its own electron. Due to the high degree of spatial and temporal localization of the continuum electron wavepacket, high currents of about 1011 Amperes/cm2 are reached by the driven motion of a single electron, comparable to conventional diffraction experiments with electron beams. Importantly, in contrast to the latter, LIED inherently is a time-resolved technique and due to the small impact parameters and high-angle scattering can reach Ångström spatial resolution when mid-IR laser fields are applied, in which the continuum electrons gain enough energy to sufficiently reduce the deBroglie wavelength. Moreover, since different electron trajectories can be separated by electron momentum detection, attosecond resolution can be reached.
Here we aim to advance laser-driven electron recollision techniques to image molecular structure and dynamics in simple molecules and in molecular complexes. On the one hand, an exquisite control over the molecular size, conformer, quantum state and alignment distribution is achieved by combining an electric field Stark deflector and tailored laser pulses. Such initial control offers the unique opportunity to perform experiments in a sample of “almost” perfectly aligned gas-phase molecules and therefore to directly unravel a three-dimensional molecular structure by means of laser-induced electron diffraction. On the other hand, we combine laser-induced field free alignment and electron-ion coincidence spectroscopy in a reaction microscope, and use control over the ellipticity of the strong laser field to decipher the role of different molecular orbitals in the strong-field ionization and rescattering process . Performing coincidence rescattering experiments with two parallel ionization channels on the same molecule allows to test and refine some of the fundamental assumptions underlying LIED. Our studies pave the way to applying laser-driven rescattering to image complex molecular structures and dynamics. On the theory side, strong-field ionization and recollision is modeled by time-resolved resolution in ionic state (TD-RIS) theory. Experiment and theory benefit a lot from this in-house collaboration.