Geometrical Reorganization of a Methane Cation upon a Sudden Ionization: An Isotope Effect in Electronic Non Equilibrium Quantum Dynamics

An ultrafast structural, Jahn-Teller (JT) driven, electronic coherence mediated quantum dynamics in the CH₄⁺ and CD₄⁺ cations that follows a sudden ionization by an XUV attopulse, exhibits a strong isotope effect. The JT effect makes the methane cation unstable in the T_d geometry of the neutral. Upon the sudden ionization the cation is produced in a coherent superposition of three electronic states that are strongly coupled. On the ground state of the cation the few femtosecond structural rearrangement leads first to a geometrically less distorted D_2d minimum followed by a 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⁺ and CH₄⁺ autocorrelation functions, is in agreement with experimental measurements of the high harmonic spectra. The high value of the ratio is attributed to the faster electronic coherence dynamics in CH₄⁺.