Ultrafast geometrical reorganization of a methane cation upon sudden ionization: an isotope effect on electronic non-equilibrium quantum dynamics

Abstract

The ultrafast structural, Jahn–Teller (JT) driven, electronic coherence mediated quantum dynamics in the CH₄⁺ and CD₄⁺ cations that follows sudden ionization using an XUV attopulse exhibits a strong isotope effect. The JT effect makes the methane cation unstable in the T_d geometry of the neutral molecule. Upon the sudden ionization the cation is produced in a coherent superposition of three electronic states that are strongly coupled and neither is in equilibrium with the nuclei. In the ground state of the cation the few femtosecond structural rearrangement leads first to a geometrically less distorted D_2d minimum followed by a geometrical reorganization to a shallow C_2v minimum. The dynamics is computed for an ensemble of 8000 ions randomly oriented with respect to the polarization of the XUV pulse. The ratio, about 3, of the CD₄⁺ to CH₄⁺ autocorrelation functions, is in agreement with experimental measurements of high harmonic spectra. The high value of the ratio is attributed to the faster electronic coherence dynamics in CH₄⁺.