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

TL;DR

This study uses attosecond transient absorption spectroscopy to observe ultrafast symmetry breaking and vibrational relaxation in methane cations, revealing dynamics within tens of femtoseconds.

Contribution

It provides the first detailed real-time observation of Jahn-Teller distortion and vibrational coherence in methane cations at the carbon K-edge.

Findings

Symmetry breaking occurs within 10 femtoseconds after ionization.

Coherent oscillations in the scissoring mode are detected and damped within 58 femtoseconds.

Vibrational energy redistributes into lower-frequency modes during relaxation.

Abstract

Understanding the relaxation pathways of photoexcited molecules is essential to gain atomistic level insight into photochemistry. Herein, we perform 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 reveals that the distortion occurs within $10\pm 2$ femtoseconds after few-femtosecond strong-field ionization of methane. The distortion activates coherent oscillations 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 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.