Watching Ultrafast Chemistry After Ionization

Ionization initiates chemistry in processes ranging from cosmic radiation and radiation damage to electron ionization mass spectrometry (EI-MS). Following electron removal, charge redistribution and nuclear motion trigger competing pathways involving bond breaking, bond formation, and molecular rearrangement. Two fundamental experimental challenges arise: How can we reproduce electron-impact-induced chemistry with femtosecond time resolution, and how can we simultaneously track the many reactions that follow ionization? We address these challenges using strong-field ionization, energy- and time-resolved mass spectrometry, femtosecond disruptive probing, and ultrafast X-ray scattering [1–4]. By reproducing fragmentation patterns characteristic of EI-MS while establishing a precise time zero, we can follow up to 80 mass-resolved product channels simultaneously using disruptive probing [5], revealing when competing pathways emerge and how they evolve. These measurements uncover surprising timescales, including a molecular rearrangement that occurs 120 times faster than electron transfer [6], and unexpected reaction pathways such as H₃⁺ formation following double ionization, driven by hydrogen roaming and large-amplitude nuclear motion [7–9]. I will also discuss studies of phenyl triflate using disruptive probing [10] and ultrafast X-ray scattering [11], which provide complementary temporal and structural views of fragmentation and distinguish rapid nonstatistical bond cleavage from slower statistical dissociation. Together with electronic-structure calculations, molecular dynamics, and quantum wavepacket models, these measurements are moving us beyond identifying reaction products toward molecular movies that reveal how complex chemistry unfolds following ionization.

References:

[1] M. Dantus, Science 385, eadk1833 (2024).

[2] M. Dantus, Acc. Chem. Res. 57, 033003 (2024).

[3] M. Dantus, Phys. Chem. Chem. Phys., 28, 20853 (2026).

[4] J. Stamm, S. Kwon, S. Sandhu, J. Sandhu, B. G. Levine, M. Dantus, Commun. Chem. 7, 103 (2024).

[5] S. Li, B. Jochim, J. E. Jackson, M. Dantus, J. Chem. Phys. 155, 044303 (2021).

[6] J. Stamm, S. Kwon, S. Sandhu, M. Shaik, R. Das, et al., J. Phys. Chem. Lett. 14, 10088 (2023).

[7] N. Ekanayake, M. Nairat, B. Kaderiya, et al., Sci. Rep. 7, 4703 (2017).

[8] N. Ekanayake, T. Severt, M. Nairat, et al., Nat. Commun. 9, 5186 (2018).

[9] J. Stamm, S. S. Priyadarsini, S. Sandhu, et al., Nat. Commun. 16, 410 (2025).

[10] S. Kwon, J. Stamm, M. Dantus, J. Phys. Chem. Lett. 16, 3397 (2025).

[11] J. Stamm, et al. DOI: 10.26434/chemrxiv.15000365