Presence and absence of electronic mixing in shorter-chain and longer-chain carotenoids: Assignment of the symmetries of and states located just below the state

In spheroidene (having the number of conjugated double bonds n = 10), stimulated emission was observed from the mixed vibronic levels of 1B u + ( 0 ) + 1B u − ( 2 ) and 1B u + ( 1 ) + 1B u − ( 3 ) , whereas in lycopene, anhydrorhodovibrin and spirilloxanthin (n = 11–13), stimulated emission, from the pure vibronic levels of 1B u + ( 0 ) and 1B u + ( 1 ) . Thus, the 1B u + state can mix with the 1B u − state but not with the 3A g − state, both being located just below the 1B u + state. The presence and absence of the mixing of the neighboring diabatic states support the symmetries of the next low-lying 1B u − and 3A g − states.