Semiclassical electron-radiation-ion dynamics (SERID) and cis-trans photoisomerization of butadiene

Detailed simulations are reported for the dynamics of electrons and nuclei during the cis-trans isomerization of butadiene following a femtosecond-scale laser pulse. Our technique, semiclassical electron-radiation-ion dynamics (SERID), is fully described in the text. The one-electron Hamiltonian and ion-ion interactions employed in the present work are density-functional based. Following excitation of electrons by the laser pulse, all three C-C bonds in the butadiene molecule become longer, as electrons are promoted from the HOMO to both the LUMO and LUMO+1 levels. In the excited electronic state, the molecule rotates about all of its three C-C bonds. There are then non-adiabatic events near avoided crossings, with electronic transitions to the ground state via creation of vibrational excitations. The molecule continues to twist around the central bond and one of the terminal C-C bonds, until the trans-conformation is achieved. Various features in the behaviour of the vibrational modes can also be observed, including initial excitations due to the laser pulse, further excitations at avoided crossings and the redistribution of vibrational energy between modes.