| Résumé | We studied the ultrafast dissociation dynamics of two prototypical molecular systems using Coulomb explosion imaging (CEI). First, we investigated the unimolecular dissociation reactions in the deuterated formaldehyde molecule (D2CO). Asides from the conventional –molecular and radical– dissociation channels, a third channel termed roaming was discovered in 2004, which bypasses the transition state. Identified through spectroscopic footprints in high-resolution spectroscopy and found in a variety of molecules, roaming fragments had never been observed directly and in a time-resolved way due to the signal’s statistical nature. We succeeded in doing so by using the highly sensitive CEI method in combination with high-level first-principles simulations of all critical experimental steps. Distinctive roaming signatures were identified in the experimental data. These were rendered observable by extracting rare stochastic events out of an overwhelming background. We further explored how the overall dissociation depends on the employed UV pump wavelength.
Second, we investigated and controlled post-ionization dynamics in the asymmetric OCS molecule upon excitation with intense phase-locked asymmetric two-color laser fields. The fragment ejection direction of both break-up channels of the dication, OCS²⁺ →O⁺ + CS⁺ and OC⁺ + S⁺, reflects the anisotropy of the tunneling ionization rate, following a 2π-periodicity. Additionally, we see signatures of post-ionization dynamics. These observations, supported by state-of-the-art theoretical simulations, reveal the importance of post-ionization population dynamics in addition to tunneling ionization in the molecular fragmentation processes, even for heavy polar molecules. |
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