Probing Structural Change via Time-Resolved Dissociative Ionization

Vladimir S. Petrovic

(PULSE Institute for Ultrafast Energy Science, Stanford University)

On the molecular level the conversion of energy from one form to another occurs via non-born-Oppenheimer interactions. Often this energy flow between different degrees of freedom occurs on a sub-picosecond timescale, which makes time-resolved pump-probe techniques suitable for its study. Time-resolved dissociative ionization relies on the characterization of the dissociation-formed fragments, their correlations, or their coincidences. I will discuss complementary techniques for initiating dissociative ionization via weak-field x-ray absorption or strong-field IR absorption, and their application for study of unimolecular isomerizations. In the first part of the talk I will discuss UV-pump /IR-probe experiments that investigated ring opening in 1,3-cyclohexadiene. In a model-free manner we identify the number of species that participate in this process and we separate their temporal evolution. Via time-resolved characterization of the fragment kinetic energy we observe that the cross-section for multiple ionization increases when the wavepacket is in the vicinity of the S1/S0 conical intersection. In the second part of the talk I will discuss experiments performed at Linac Coherent Light Source that probed the non-Born-Oppenheimer interactions in relaxation of core--‐ionized acetylene. An x-ray pump/x- ray probe experiment, coupled  with momentum- resolved detection, permitted visualization of evolving acetylene dication as it isomerizes to form vinylidene. We find that the timescale for this process is more than an order of magnitude faster than expected based on the transition-state theory. The rapid proton migration has important implications for x-ray initiated radiation damage in biomolecules and for single-particle diffraction-before-destruction experiments.