Benzene, absorption spectrum vibronic coupling

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absorption spectrum [109,173,174,195], and recently to study the photoisomerization of retinal [196],... 

The 30 normal vibrations available in benzene include two a g, one a2g, two bjg, one eig, four ejg, one aju, two bi , two b2 , three ei , and two e2 modes. For a vibronically induced El transition from Sq to S, the latter state must be coupled to either an Ei state (which transforms as x, y) or an Aju state (which transforms as z). In the latter case, the Sq absorption spectrum would exhibit parallel bands polarized along the z axis. For states having Bi and B2u symmetry, respectively, b2g and b g modes would be required to effect coupling to an A2 state. Benzene has two b2g modes, but no big modes. Since no parallel bands have ever turned up in the Si Sq spectrum, the Si state is a B2 rather than a Bi state. 

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