Plyler Prize (2022): DC Slice Velocity Map Imaging and My Dalliances with Ozone and Formaldehyde

There are two molecules that hold a special place in my heart: ozone and formaldehyde. Ozone, despite its apparent simplicity, has rich and complex electronic structure and excited state decay pathways. The fascination of formaldehyde lies in accidents of nature that make it a marvelous laboratory for the study of chemical dynamics on its electronic ground state. These molecules have in common the fact that velocity map imaging studies have revealed deep insights into their dynamical properties that also carry general implications. In ozone, these excited state dynamics have been revealed through imaging studies probing both atomic and molecular products in a range of electronic states. One revealing aspect of this has been studies of angular momentum polarization in the O(¹D₂) product that carry rich information about the nature of the electronic excitation and decay pathways. We will show that coherent orbital orientation in the O(¹D₂) product must arise from Herzberg-Teller interactions in the excitation step. Furthermore, we show that a novel manifestation of the geometric phase can account for a range of polarization phenomena that have hitherto eluded explanation. In formaldehyde, state-correlated imaging studies reveal deep insight into the behavior and dissociation dynamics involving three distinct continua. We will tell the story of roaming in formaldehyde, in which near-dissociation to radical products leads to an intramolecular reaction with unanticipated consequences.  The roaming mechanism is now considered a general aspect of molecular behavior, but has almost always been treated classically.  We will present very recent work that shows quantum resonances at the onset of roaming strongly modulate the roaming yield and profoundly impact the molecular and radical channels as well. All this work has been enabled by DC slice velocity map imaging, and we take the opportunity to describe the virtues of this variant of the powerful VMI technique.