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Abstract Filming atomic motion within molecules is an active pursuit of molecular physics and quantum chemistry. A promising method is laser-induced Coulomb Explosion Imaging (CEI) where a laser pulse rapidly ionizes many electrons from a molecule, causing the remaining ions to undergo Coulomb repulsion. The ion momenta are used to reconstruct the molecular geometry which is tracked over time (i.e., filmed) by ionizing at an adjustable delay with respect to the start of interatomic motion. Results are distorted, however, by ultrafast motion during the ionizing pulse. We studied this effect in water and filmed the rapid “slingshot” motion that enhances ionization and distorts CEI results. Our investigation uncovered both the geometry and mechanism of the enhancement which may inform CEI experiments in many other polyatomic molecules. 

We develop mathematical tools to compute higher order covariances in charged particle detection, and demonstrate fourfold covariance measurements for molecular imaging with intense ultrafast laser pulses. 

It has recently been shown that strong field multiple ionization of water depends on the duration and intensity of the laser pulse. While the polarizability of neutral water is isotropic, the polarizability of the molecular ions can be significant and evolve in time. If the molecular ions spend enough time in the field, dynamic alignment can reorient them and modify the yield of dissociating fragments as a function of angle relative to the polarization of the laser. Unbending motion is one way that the polarizability of the molecular ions increases. Here, we study strong field ionization of water in the long pulse regime where dynamic alignment and unbending are known to contribute at 800 nm, and we tune the laser wavelength to modify coupling between the states of the monocation. A resonance between the X and A states at 660 nm should excite the monocation and initiate unbending motion, but our results cannot be explained without considering the dynamics and structure of the dication and trication. To conduct these measurements, we utilize laser pulses with a duration of 40 fs and central wavelengths of 660 nm, 800 nm, and 1330 nm to multiply-ionize an effusive molecular beam of water. The resulting charged fragments are detected using a velocity map imaging apparatus. Our results provide additional clues about the strong field ionization of water. *M.B., G.A.M., A.J.H., N.P., and P.H.B. were supported by the National Science Foundation. A.J.H. was additionally supported under a Stanford Graduate Fellowship as the 2019 Albion Walter Hewlett Fellow. N.P. was additionally supported by the Hertz Foundation. R.F. was supported by the Department of Energy office of Basic Energy Science, Facilities Division.