On the kinetic aspects of exciplex formation under molecular supersonic beam conditions

The existence of various orientational isomers in the ground and excited states of electron-donor-acceptor molecular systems under molecular supersonic beam conditions has been confirmed in a number of experiments. The analysis of the potential energy surfaces of such systems suggests that the photokinetic scheme of formation of an exciplex (charge-transfer excited state) should be modified (beyond the classical two-state model) and should also include, among the others, the relaxation pathways of excited Franck-Condon structures corresponding to various ground-state orientational isomers of a Van der Waals complex. A modified kinetic scheme is proposed, solved and applied to analysis of available experimental data. Computer simulations of the time profiles of exciplex emission have shown that the proposed photokinetic scheme is capable of handling qualitatively different features of the rise and decay curves observed for different electron-donor-acceptor systems under molecular supersonic beams conditions. (C) 1997 Elsevier Science B.V.