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# Femtosecond Symmetry Breaking and Coherent Relaxation of Methane Cations at the Carbon K-Edge

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Dec 2022

Understanding the relaxation pathways of photoexcited molecules is essentialto gain atomistic level insight into photochemistry. Herein, we perform atime-resolved study of ultrafast molecular symmetry breaking via geometricrelaxation (Jahn-Teller distortion) on the methane cation. Attosecond transientabsorption spectroscopy with soft X-rays at the carbon K-edge reveals that thedistortion occurs within $10\pm 2$ femtoseconds after few-femtosecondstrong-field ionization of methane. The distortion activates coherentoscillations in the scissoring vibrational mode of the symmetry broken cation,which are detected in the X-ray signal. These oscillations are damped within$58\pm13$ femtoseconds, as vibrational coherence is lost with the energyredistributing into lower-frequency vibrational modes. This study completelyreconstructs the molecular relaxation dynamics of this prototypical example andopens new avenues for exploring complex systems.

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