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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.

 

Covariance mapping is widely used to study correlations of different variables in the dataset. The power of the method has been demonstrated in multi-particle imaging, including two- and three-body covariance on molecules of biological relevance and Coulomb explosion imaging (CEI) of molecular dissociation dynamics. While covariance for two particles is rather straightforward, for four-body correlations, one needs to extend covariance mapping to cumulant mapping, which has been tested in recent measurements of strong field ionization of formaldehyde. Here, I will discuss the details of how to compute cumulant mapping for the momentum sum of all four fragments of the formaldehyde molecule, and how one can perform the calculation with a faster and better algorithm. 

The electrons and atoms inside molecules can rearrange rapidly during photoexcitation or collisions, moving angstroms in a few femtoseconds or less. This non-classical many-body quantum evolution is far too small and too fast to be resolved in any imaging microscope, but if we could film it, what should we expect to see? New tools based on ultrafast lasers, electron accelerators, and x-ray free-electron lasers have now begun to record this motion with increasing detail, and for a growing array of atomic and molecular systems. Here I will attempt to answer the question, "So what?" What have we learned, and how are molecular movies guiding us toward future discoveries in AMO physics? *Much of this work is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Chemical Sciences, Geosciences, and Biosciences Division (CSGB). Other work described here has been supported by the National Science Foundation 

We present results from an experimental ion imaging study into the fragmentation dynamics of 1-iodopropane and 2-iodopropane following interaction with extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Using covariance imaging analysis, a range of observed fragmentation pathways of the resulting polycations can be isolated and interrogated in detail at relatively high ion count rates (∼12 ions shot −1 ). By incorporating the recently developed native frames analysis approach into the three-dimensional covariance imaging procedure, contributions from three-body concerted and sequential fragmentation mechanisms can be isolated. The angular distribution of the fragment ions is much more complex than in previously reported studies for triatomic polycations, and differs substantially between the two isomeric species. With support of simple simulations of the dissociation channels of interest, detailed physical insights into the fragmentation dynamics are obtained, including how the initial dissociation step in a sequential mechanism influences rovibrational dynamics in the metastable intermediate ion and how signatures of this nuclear motion manifest in the measured signals. 

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. 

We demonstrate an enhancement in the formation of D2O3+ as a consequence of field-free molecular dynamics following the strong-field multiple ionization of deuterated water via 6-fs 800-nm pulse pairs. 

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.