On Extracting Subfemtosecond Data from Femtosecond Quantum Dynamics Calculations: The Methane Cation.

Abstract

Motivated by recent measurements of the subfemtosecond structural rearrangement in the methane cation using high-harmonic generation technology, here, we attempt an ab initio quantum dynamics study to explore such an event by modeling the experiments. This is done by calculating the ratio of the squared autocorrelation functions of wave packets that evolve on the X T2 electronic manifold of the isotopes CD4 and CH4 , in full dimensionality including nonadiabatic coupling. We have found good agreement with the experimental data, and additionally predict an unexpected maximum in the above ratio at ∼1.85 fs, thence outside the experimentally covered time-delay window. We further predict the above structural rearrangements to occur in ∼1.85 fs, and we propose a mechanism for this via a five-dimensional hypersurface developed from the JT active bending vibrations of e and t2 symmetry. Recurrences of such a maximum are also found with an interval of time delays, and the average of such intervals suggested corroboration of the oscillation time between the Td and the C2v geometries of CH4 . Furthermore, we perform a dynamics study up to 200 fs, and we unravel the underlying details of the first photoelectron band of CD4.