Photoinduced dynamics of ethene in the N, V, and Z valence states: a six-dimensional nonadiabatic quantum dynamics investigation.

The photoinduced dynamics of ethene following pi-->pi(*) excitation is investigated by quantum wave-packet dynamics on three coupled six-dimensional diabatic potential-energy surfaces representing the N, V, and Z valence states, which have been developed previously. The C-C stretching and torsion, as well as the pyramidalization and scissoring of both CH(2) groups are included in this description. The wave-packet calculations have been performed using the multiconfigurational time-dependent Hartree method for a time period up to 100 fs. While a small amount of population transfer to the electronic ground state is found within this period, the overall population decay time of the V state is found to exceed the 100 fs range significantly. The autocorrelation function of the wave packet and the stationary absorption spectrum of the V state also have been calculated. It is found that both the torsional mode as well as the C-C stretching mode contribute to the very extended vibrational structure of the absorption spectrum, and that both modes are strongly coupled. At least on the present ab initio surface of limited dimensionality, the speed of pyramidalization of 90 degrees twisted ethene appears as the bottleneck for the ultrafast radiationless decay of the V state.