Knowledge Bank

trans, trans-1,3,5,7-Octatetrane in n-octane at 4.2K is an ideal polyene system in which the full implications of inversion symmetry are evident in the vibronically induced nature of the $11Ag-21Ag$ electronic transition (1). With the aid of a high resolution two photon excitation spectrum, it is possible to quantitatively analyze the fluorescence and one photon absorption spectra in terms of the Duschinsky effect and adiabatic (Herzberg-Teller) and non-adiabatic (Born-Oppenheimer) vibronic coupling (2) between the $21Ag$ and $11Bu$ electronic states. Although the fluorescence spectrum can be explained by the usual Herzberg-Teller scheme, it is found that the one photon absorption is completely dominated by Born-Oppenheimer coupling. The form of this coupling is such that the relative signs of the normal mode displacements in the $21Ag$ and $11Bu$ states can be determined within a harmonic oscillator model.