Photodissociation, a common photochemical process, often proceeds through minimum energy pathways. However, a more complex pathway, known as roaming, involves neutral fragments traversing relatively flat regions of the potential energy surface due to long-range, weakly-bound interactions. Roaming's significance lies in its prevalence and contribution to new molecular compound formation, making it crucial for understanding photochemical reactions. While indirect observation of roaming has been achieved in several small molecules (formaldehyde, acetaldehyde, acetone, nitrate, methyl formate, propane, and 2-propanol), real-time, direct visualization remains elusive due to the neutral character of the roaming fragment and its indeterminate trajectory. This study focuses on the formation of H₃⁺, a ubiquitous molecular ion in the universe, whose dynamics are pivotal in gas-phase astrochemistry. Previous research, using both IR and XUV femtosecond laser pulses combined with Coulomb explosion imaging (CEI), has explored roaming's role in H₃⁺ formation, often initiating from the molecular dicationic state. This work aims to overcome the limitations of previous methods by directly tracking the roaming process, achieving real-time, direct visualization of the roaming neutral H₂ in acetonitrile (CH₃CN), a relatively simple model system with three hydrogen atoms all bonded to the same carbon.

The roaming phenomenon, initially indirectly observed in formaldehyde decomposition, has since been suggested to occur in various small molecules. Studies have employed time-resolved mass spectra to infer the role of roaming neutral H₂ in H₃⁺ formation, but lacked the coincident momentum imaging necessary for unambiguous identification of contributing fragmentation channels. The complexity of the process necessitates high-level simulations to disentangle the signature of roaming from experimental results. Recent works have specifically focused on the formation of D₃⁺ (an isotope of H₃⁺) from bimolecular reactions in D₂, offering potential explanations for its high abundance in interstellar molecular clouds. High-powered, ultrafast lasers have facilitated exploration of H₃⁺ formation dynamics via photodissociation of organic molecules, particularly focusing on roaming H₂ triggered by intense femtosecond IR laser pulses, leading to H₃⁺ formation in monohydric alcohols. Complementary experiments using XUV femtosecond laser pulses and CEI further explored this mechanism. However, despite these efforts, the direct, real-time visualization of the roaming process has remained a challenge.

This study directly tracks the roaming process by experimentally measuring the complex dynamics of neutral H₂, a crucial intermediate in H₃⁺ formation in CH₃CN. The researchers utilized coincident momentum imaging combined with femtosecond 800 nm IR-IR pump-probe spectroscopy to obtain time-resolved 3D momentum information of each detected fragment. Momentum conservation allows for the reconstruction of the momentum vector of the neutral fragment, even in channels involving a neutral fragment. Acetonitrile was chosen for its simple structure, eliminating ambiguity in H₃⁺ formation pathways. The experimental setup involved a femtosecond IR pump pulse to excite CH₃CN to the dicationic state, followed by the dissociation and roaming of neutral H₂ near C₂N⁺. A probe pulse can ionize roaming H₂, leading to a triple-ion coincidence channel. The Cold Target Recoil Ion Momentum Spectrometer (COLTRIMS) technique was used to measure the momenta of all ionic fragments in coincidence, providing a kinematically complete picture. Analogous measurements using deuterated acetonitrile were performed to better resolve particle masses. Quantum mechanical calculations were performed using the Atom-Centered Density Matrix Propagation method, employing density functional theory (B3LYP functional with the 6-31+G(d,p) basis set). The simulations involved a vertical double ionization of acetonitrile, introducing a specific amount of internal energy to match the experimentally measured KER.

The study successfully imaged the dynamics of roaming neutral D₂ (precursor to D₃⁺ formation). The Newton diagram for the D⁺ + D₂ + C₂N⁺ channel, integrated over the first 200 fs of pump-probe delay, shows the momentum vector of C₂N⁺ lies within those of D₂. Analysis of the kinetic energy release (KER) map of the H₃⁺ + C₂N⁺ channel reveals a horizontal KER band around 5 eV, indicative of direct excitation to a diabatic state. Time-resolved measurements showed no significant difference between H₃⁺ and D₃⁺ formation timescales, suggesting that electronic properties, rather than nuclear dynamics, primarily govern roaming reactions in acetonitrile. Fitting the KER projections yields time constants indicating that H₂ roaming and H₃⁺ formation occur on ultrafast timescales (approximately 200 fs). Molecular dynamics simulations support the experimental findings, showing H₃⁺ formation via neutral H₂ roaming within 100-400 fs. The calculated nuclear kinetic energies agree well with the experimental KER. Analysis of competing channels (roaming neutral D₂, D₃⁺ formation, D₂⁺ formation, and three-body ionization) reveals an inverse correlation between the proton transfer (PT) and three-body ionization channels due to direct competition between fragmentation processes. The electron transfer (ET) channel exhibits a different time-dependent behavior, possibly due to suppression by the probe pulse or interatomic separations.

The results demonstrate the successful direct imaging of neutral roaming reactions in small molecular systems. The observation of H₃⁺ formation via neutral H₂ roaming in acetonitrile within a few hundred femtoseconds provides direct evidence for this reaction pathway. The combination of ultrafast IR pump-probe spectroscopy and coincident CEI enables tracking of the invisible neutral precursor to H₃⁺ formation, even from incomplete fragmentation channels. The agreement between experimental and simulation results validates the methodology and strengthens the conclusions about the mechanism of H₃⁺ formation. The study highlights the importance of considering neutral fragments in understanding molecular dynamics, and showcases the technique's potential for investigating neutral reactivity in complex systems.

This study successfully developed a novel technique for directly imaging neutral roaming reactions in small molecules. The real-time tracking of H₃⁺ formation from neutral H₂ roaming in acetonitrile, occurring within a few hundred femtoseconds, was achieved. This method overcomes limitations of previous techniques and opens new avenues for investigating neutral reactivity in complex systems. Future research could explore other molecular systems and expand the technique's capabilities for more detailed analysis of neutral fragmentation processes.

The study focuses on acetonitrile, a relatively simple model system. The generalizability of the findings to other molecular systems remains to be tested. The experimental resolution might limit the detection of subtle differences in some of the time-resolved measurements. The computational model used certain approximations that could affect the accuracy of the simulations.