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Abstract Roaming is an unconventional type of molecular reaction where fragments, instead of immediately dissociating, remain weakly bound due to long-range Coulombic interactions. Due to its prevalence and ability to form new molecular compounds, roaming is fundamental to photochemical reactions in small molecules. However, the neutral character of the roaming fragment and its indeterminate trajectory make it difficult to identify experimentally. Here, we introduce an approach to image roaming, utilizing intense, femtosecond IR radiation combined with Coulomb explosion imaging to directly reconstruct the momentum vector of the neutral roaming H₂, a precursor to$${{{{{{{{\rm{H}}}}}}}}_{3}}^{+}$$ ${H_3}^{+}$formation, in acetonitrile, CH₃CN. This technique provides a kinematically complete picture of the underlying molecular dynamics and yields an unambiguous experimental signature of roaming. We corroborate these findings with quantum chemistry calculations, resolving this unique dissociative process. 

Understanding the dynamics of various photo-induced physical and chemical processes in molecules is fundamental to physics, chemistry, and biology. These dynamics, which occur on pico- and femtosecond scales, are observable using ultrafast tools like ultrashort laser pulses that can be generated using tabletop laser and free-electron lasers. This thesis systematically studies some of these processes using different light sources and probing techniques, revealing new physics and offering insights into potential future control of these phenomena.