Effect of initial vibrational excitation on the methane cation sub-femtosecond photodynamics

Abstract

An ab initio ab quantum dynamics study is attempted to understand the effect of initial vibrational state excitation on the sub-femtosecond photodynamics of the methane cation. A time-dependent wave packet (WP) propagation method is utilised where the WPs are prepared for each initial vibrational state of the neutral species of the title cation and its deuteriated isomer. The ratio of the squared of the nuclear autocorrelation functions (ACFs) of CD and CH on the lowest potential sheet of the state are calculated in full dimensionality, including nonadiabatic coupling of the three electronic sheets. The results predict the above ratio to start at unity and then increase with time before passing through a maximum, irrespective of the initial vibrational state. At the first maximum, such a ratio is found to decrease with the number of nodes of the vibrational wave function while the magnitudes of the ACFs of CD and CH diminish gradually with vibrational excitation. This is enhanced by inclusion of further excited vibrational modes. The present results indicate that nuclear motion on the lowest adiabatic sheet of the states of title cation can be slowed down by initial vibrational state excitation.