Theoretical analysis of vibronic structure in absorption and fluorescence spectra of polyatomic molecules beyond the Condon approximation: Application to 11Ag ↔ 21Ag and 11Ag ↔ 11Bu electronic transitions in all-trans-1,3,5,7-octatetraene

► Derivation of non-Condon time domain correlation functions at finite temperature. ► Employment of multidimensional FC integrals to calculate the non-Condon spectra. ► Application to 11Ag ↔ 11Bu and 11Ag ↔ 21Ag electronic transitions of octatetraene. ► Observation of a nonradiative decay channel in the vibronic levels of the 21Ag state.