Few-femtosecond electronic and structural rearrangements of CH\(_4^+\) driven by the Jahn-Teller effect

The Jahn-Teller effect (JTE) is central to the understanding of the physical and chemical properties of a broad variety of molecules and materials. Whereas the manifestations of the JTE on stationary properties of matter are relatively well studied, the study of JTE-induced dynamics is still in its infancy, largely owing to its ultrafast and non-adiabatic nature. For example, the time scales reported for the distortion of CH\(_4^+\) from the initial \(T_{\rm d}\) geometry to a nominal \(C_{\rm 2v}\) relaxed structure range from 1.85~fs over 10\(\pm\)2~fs to 20\(\pm\)7~fs. Here, by combining element-specific attosecond transient-absorption spectroscopy and quantum-dynamics simulations, we show that the initial electronic relaxation occurs within 5~fs and that the subsequent nuclear dynamics are dominated by the \(Q_2\) scissoring and \(Q_1\) symmetric stretching modes, which dephase in 41\(\pm\)10~fs and 13\(\pm\)3~fs, respectively. Significant structural relaxation is found to take place only along the e-symmetry \(Q_2\) mode. These results demonstrate that CH\(_4^+\) created by ionization of CH\(_4\) is best thought of as a highly fluxional species that possesses a long-time-averaged vibrational distribution centered around a \(D_{\rm 2d}\) structure. The methods demonstrated in our work provide guidelines for the understanding of Jahn-Teller driven non-adiabatic dynamics in other, more complex systems.