Time-resolving the ultrafast H₂ roaming chemistry and H₃⁺ formation using extreme-ultraviolet pulses

Trihydrogen cations (H₃⁺) are abundant in the universe and play a key role in forming complex molecules in the interstellar medium (ISM). Their formation and destruction mechanisms are actively researched. While typically formed through H₂⁺ + H₂ collisions, H₃⁺ is a frequent product of organic molecule ionization from various methods including electron impact, fast ion bombardment, multi-photon strong-field laser ionization, and single EUV photon ionization. However, the underlying mechanisms for H₃⁺ formation and the related H₂ product remain poorly understood. Early strong-field laser ionization experiments suggested a long ( >1.4 ps) lifetime for the methanol dication before H₃⁺ formation, implying significant structural rearrangement. Theoretical work proposed a mechanism involving a roaming neutral H₂, lasting hundreds of femtoseconds to thousands of picoseconds, culminating in proton abstraction and H₃⁺ formation. Strong-field laser ionization experiments, however, present challenges due to the coexistence of direct and indirect ionization mechanisms, leading to conflicting results regarding H₃⁺ formation timescales (e.g., ~100 fs vs. a ~38 fs beating related to intermediate states). Furthermore, strong-field laser ionization is computationally challenging to model accurately. Previous ab initio molecular dynamics (AIMD) simulations only captured a small fraction of the experimentally observed H₃⁺ formation probability. This study aims to develop a combined experimental and theoretical approach using single-photon double-ionization via an ultrafast EUV pump pulse followed by a time-delayed near-IR (nIR) probe. This method bypasses the uncertainties inherent in strong-field laser experiments and is well-suited for ab initio theoretical modeling. The study uses XMS-CASPT2 calculations and nonadiabatic AIMD simulations to time-resolve the roaming H₂ chemistry responsible for H₃⁺ formation.

The literature extensively covers H₃⁺'s role in the interstellar medium, its formation via H₂⁺ + H₂ collisions, and its production through various ionization methods applied to organic molecules. Prior experiments using strong-field laser ionization, particularly on methanol, yielded conflicting results regarding the timescale of H₃⁺ formation, with some studies suggesting ultrafast formation while others indicated much longer timescales due to rotational depolarization and intermediate state dynamics. Theoretical studies have proposed mechanisms involving roaming neutral H₂ dynamics, but these studies often had limitations in capturing the full complexity of the process or predicting experimental branching ratios. Earlier computational efforts using ground-state dynamics simulations were unable to reproduce observed branching ratios and lacked the ability to account for other observed products. The limitations of previous strong-field ionization studies highlighted the need for a more controlled experimental approach suitable for detailed ab initio modeling. This gap motivated the current study to combine single photon ionization using ultrafast EUV pulses with time-resolved near-IR probing, improving on previous attempts to resolve the time evolution of H₃⁺ formation.

The experimental setup involved a single-photon Coulomb explosion (CE) imaging system using ultrafast EUV pulses generated via high-harmonic generation (HHG) from a near-IR laser. A time-delayed near-IR probe pulse was used to investigate the dynamics following EUV double ionization of methanol. The cationic products were detected using a 3D coincidence imaging spectrometer, allowing for the measurement of branching ratios and kinetic energy release (KER) distributions. The experimental data were analyzed to determine the time-dependent changes in branching ratios. On the theoretical side, non-adiabatic ab initio molecular dynamics (AIMD) simulations were performed using the XMS-CASPT2 method with an aug-cc-pVDZ basis set. The BAGEL electronic structure package interfaced with a modified Newton-X program was used to calculate potential surfaces and nonadiabatic coupling terms, enabling the simulation of surface-hopping dynamics. The simulations modeled the methanol dication dynamics, including the roaming H₂ dynamics and the competing proton and electron transfer pathways. The basis set convergence was verified by comparison with calculations using aug-cc-pVTZ. The simulated branching ratios, dissociation times, and KER distributions were compared with the experimental data. The instrumental response time was characterized using the Ne²⁺ signal obtained from neon gas ionization. Data analysis included fitting of the experimental transient signals with an exponential decay function convolved with the instrumental response function to extract lifetimes.

The combined experimental and theoretical results unambiguously demonstrate that H₃⁺ formation in the double ionization of methanol occurs on an ultrafast timescale, below 100 fs. The experimental pump-probe measurements revealed a ~70 fs lifetime for the transient suppression of the H⁺ + COH⁺ branching ratio, correlated with an enhancement of the three-body fragmentation ratio. The non-adiabatic AIMD simulations successfully reproduced this ultrafast timescale for H₃⁺ formation. These simulations revealed that the roaming neutral H₂ dynamics are characterized by a competition between proton abstraction (leading to H⁺) and long-range electron transfer (resulting in H₂⁺ formation). The "inverse harpooning" mechanism, where the initially separated H₂ is drawn back to abstract a proton, was observed in the simulations. The simulations showed that over 1/3 of the ground-state trajectories produced H₃⁺, a much higher probability than found in previous ground-state-only simulations. The probability of H₃⁺ formation decreased for higher-lying excited states and was completely quenched when C-O bond cleavage became possible. Analysis of the AIMD trajectories showed that the competing proton and electron transfer processes occurred on comparable ultrafast timescales (~100 fs), aligning with the experimental observations. The KER distributions were not significantly affected by the near-IR probe pulse. The study's findings clarify previous conflicting results from strong-field laser experiments by demonstrating the ultrafast nature of H₃⁺ formation via roaming H₂ chemistry.

The findings address the long-standing question of H₃⁺ formation timescales in organic molecule ionization, settling previous discrepancies in strong-field laser experiments. The ultrafast (<100 fs) formation timescale is explained by the competing proton and electron transfer processes during the roaming H₂ dynamics. The combination of single-photon EUV ionization with time-resolved near-IR probing proved crucial for avoiding ambiguities inherent in strong-field experiments and providing a system amenable to detailed theoretical analysis. The agreement between experimental results and non-adiabatic AIMD simulations using accurate potential energy surfaces validates the proposed mechanism and computational approach. The ultrafast roaming H₂ chemistry and the competition between proton and electron transfer dynamics identified in this study are likely to occur in other ionized systems, impacting fields such as radiation chemistry in planetary and interstellar environments and single-molecule crystallography. Future work using deuterated methanol and other organic systems could further elucidate different pathways for H⁺ formation.

This study successfully resolved the ultrafast H₃⁺ formation in methanol double ionization, revealing a sub-100 fs timescale governed by competing proton and electron transfer processes during roaming H₂ dynamics. The combined experimental and theoretical approach clarified previous conflicting results from strong-field laser experiments. The findings highlight the importance of considering nonadiabatic effects and accurate potential energy surfaces for understanding ultrafast molecular dynamics in ionized systems. Future work should investigate these processes in other organic molecules and isotopic variants to gain a broader understanding of H₃⁺ formation in diverse chemical environments.

While the study successfully determined the ultrafast timescale of H₃⁺ formation, it focused primarily on the methanol system. Extending the methodology to other organic molecules could provide a more generalizable picture. The study mainly focused on the major fragmentation channels. Investigating minor channels with improved statistical precision could provide a more complete picture of the dynamics. The computational cost limited the number of trajectories simulated; increasing the computational power would enable more extensive simulations. Although the basis set convergence was tested, systematic errors associated with the theoretical method cannot be fully ruled out.