Theoretical description of diabatic mixing and coherent excitation in singlet-excited states of carotenoids

This Letter discusses the following characteristic excited-state properties of closely-overlapped ‘diabatic’ states of carotenoids, including 1 B u + , 1 B u - and 3 A g - , on which we have reported systematic experimental studies. A rigorous general formulation is developed and applied for the adiabatic and diabatic descriptions in which the breakdown of the Born–Oppenheimer approximation is used as a criterion to differentiate them: (i) the observed selection rules concerning electronic mixing and internal conversion between the 1 B u + and 1 B u - or 3 A g - diabatic levels after incoherent excitation, and (ii) a long lifetime of the apparent 1 B u + stimulated emission and the oscillatory change in the fluorescence intensity after coherent excitation of both pairs of diabatic levels.