Photo-induced molecular processes and charge recombination reactions driven by non-adiabatic couplings

Abstract: This thesis is based on a number of theoretical studies of molecular processes in which non-adiabatic effects play a crucial role. The main part is devoted to the photophysics and photochemistry of five-membered aromatic heterocyclic compounds, with particular focus on furan and thiophene. These species absorb light in the UV region of the spectrum, but exhibits neither fluorescence nor phosphorescence. A plausible explanation for this is the presence of very fast radiationless deactivation mechanisms that depopulate the excited states before emission can occur. In the present thesis, several possible such mechanisms are considered based on the results of static electronic structure calculations as well as mixed quantum-classical dynamics simulations. A common feature of the investigated pathways is the presence of easily accessible conical intersections (surface crossings) through which non-adiabatic population transfer to the electronic ground state may take place. Also considered in the present work are the bond-breaking/bond-forming processes that are expected to follow once the ground state has been reached.Another prominent example of non-adiabatic processes is charge recombination reactions involving either two oppositely charged ions or a cation and an electron. Here, the reactants and products belong to different electronic states of the system as a whole and therefore a non-adiabatic transition is necessary for the recombination to occur. Two such processes are considered in the present thesis; mutual neutralization of oppositely charged hydrogen ions and dissociative recombination of the formyl and iso-formyl cations. The cross sections (rate coefficients) and underlying mechanisms of these reactions are investigated at a fully quantum mechanical level.