Numerical simulation of nonadiabatic electron excitation in the strong field regime. 2. Linear polyene cations.

Time-dependent Hartree-Fock theory has been used to study the electronic optical response of a series of linear polyene cations (+1 and +2) in strong laser fields. The interaction of ethylene, butadiene, and hexatriene, with pulsed and CW fields corresponding to 8.75 x 10(13) W/cm(2) and 760 nm, have been calculated using the 6-31G(d,p) basis set. Nonadiabatic processes including nonlinear response of the dipole moment to the field and non-resonant energy deposition into excited states were more pronounced for the monocations in comparison with dications. For a given charge state and geometry, the nonadiabatic effects in the charge distribution and instantaneous dipole increased with the length of the polyene. For pulsed fields, the instantaneous dipole continued to oscillate after the field returned to zero and corresponded to a non-resonant electronic excitation involving primarily the lowest electronic transition. For a given molecule and fixed charge state, the degree of nonadiabatic coupling and excitation was greater for geometries with lower excitation energies.