  Coherent superposition of electronic states, created by ionizing a molecule,
can initiate ultrafast dynamics of the electron density. Correlation between
nuclear and electron motions, however, typically dissipates the electronic
coherence in only a few femtoseconds, especially in larger and more flexible
molecules. We, therefore, use ab initio semiclassical dynamics to study
decoherence in a sequence of organic molecules of increasing size and find,
surprisingly, that extending the carbon skeleton in propynal analogs slows down
decoherence and extends the duration of charge migration. To elucidate this
observation, we decompose the overall decoherence into contributions from
individual vibrational modes and show that: (1) The initial decay of electronic
coherence is caused by high- and intermediate-frequency vibrations via momentum
separation of nuclear wavepackets evolving on different electronic surfaces.
(2) At later times, the coherence disappears completely due to the increasing
position separation in the low-frequency modes. (3) In agreement with another
study, we observe that only normal modes preserving the molecule's symmetry
induce decoherence. All together, we justify the enhanced charge migration by a
combination of increased hole-mixing and the disappearance of decoherence
contributions from specific vibrational modes CO stretching in butynal and
various H rockings in pentynal.
