Control of radiationless transitions

Radiationless transitions provide a mechanism for ultrafast conversion of electronic energy into nuclear motion of a molecule. This effect results from strong vibronic mixing at geometries where two or more electronic states are degenerate (or nearly degenerate in regions of avoiding crossing).We review in this chapter different approaches to controlling such transitions. General control strategies include (1) tuning the frequency of the actinic light to enhance the yield of a specific product, (2) tailoring the properties (spectral phase and amplitude) of the excitation source to guide the generated vibrational wave packet to a specific region of the conical seam between the potential energy surfaces, and (3) modifying the shape and location of the conical intersection either by altering the chemical environment of the molecule or by introducing a strong electromagnetic field. The goals of this chapter are to reveal underlying mechanistic similarities among these general methods and to outline areas for future research.