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Semiheavy water (HOD) is one of the simplest molecules in which the bonds are labeled by isotope. We demonstrate that a pair of intense few-femtosecond infrared laser pulses can be used to selectively tunnel ionize along one of the two bonds. The first pulse doubly ionizes HOD, inducing rapid bond stretching and unbending. Femtoseconds later, the second pulse arrives and further ionization is selectively enhanced along the OH bond. These conclusions arise from 3D time-resolved measurements of ${\mathrm{H}}^{+}, {\mathrm{D}}^{+}$, and ${\mathrm{O}}^{+}$momenta following triple ionization. Published by the American Physical Society2025 

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 

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. 

Site-selective probing of iodine 4d orbitals at 13.1 nm was used to characterize the photolysis of CH2I2 and CH2BrI initiated at 202.5 nm. Time-dependent fragment ion momenta were recorded using Coulomb explosion imaging mass spectrometry and used to determine the structural dynamics of the dissociating molecules. Correlations between these fragment momenta, as well as the onset times of electron transfer reactions between them, indicate that each molecule can undergo neutral three-body photolysis. For CH2I2, the structural evolution of the neutral molecule was simultaneously characterized along the C–I and I–C–I coordinates, demonstrating the sensitivity of these measurements to nuclear motion along multiple degrees of freedom.