Understanding the relaxation pathways of photoexcited molecules is essential to gain atomistic level insight into photochemistry. Herein, a team around our former colleague Enrico Ridente of the Department of Chemistry at the University of California, performed a time-resolved study of ultrafast molecular symmetry breaking via geometric relaxation (Jahn-Teller distortion) on the methane cation. Attosecond transient absorption spectroscopy with soft X-rays at the carbon K-edge revealed that the distortion occurred within 10 ± 2 femtoseconds after few-femtosecond strong-field ionization of methane. The distortion activated coherent oscillations in the asymmetric scissoring vibrational mode of the symmetry broken cation, which were detected in the X-ray signal. These oscillations were damped within 58 ± 13 femtoseconds, as vibrational coherence was lost with the energy redistributing into lower-frequency vibrational modes. This study completely reconstructs the molecular relaxation dynamics of this prototypical example and opens new avenues for exploring complex systems.

Enrico Ridente, Diptarka Hait. Eric A. Haugen, Andrew D. Ross, Daniel M. Neumark, Martin Head-Gordon, Stephen R. Leone
Femtosecond symmetry breaking and coherent relaxation of methane cations via X-ray spectroscopy
Science 380, 713 (2023)