Vibrational analysis of the lowest 1B+u state of trans,trans 1,3,5,7-octatetraenea)

The vibronic structure of the 1 1A−g to 1 1B+u transition of trans,trans-1,3,5,7-octatetraene is measured in alkane matrices at 4.2 K and the results compared with Franck–Condon factors obtained from quantum mechanical calculations of the ground and excited state potential energy surfaces. Excellent agreement between theory and experiment is found for the vibrational intensity distribution in the spectrum, which is characterized by the activity of two carbon–carbon stretching modes with weaker involvement of two low frequency bends. The agreement between theory and experiment suggests that the calculated 1 1B+u excited state equilibrium geometry is quantitatively correct. The excited state normal modes are examined in detail, with particular attention to the degree of Duschinsky mixing upon electronic excitation.