Fundamental physical properties and structure of carbon dioxide and its derivatives

Ab initio investigation of several low-lying states of all-trans octatetraene

“…In the gas phase under free-jet expansion conditions the lines of the 'Bu spectrum have been shown to have widths consistent with rather short excited-state lifetimes.6•10 This suggests the possibility of geometric distortions leading to radiationless decays to lower lying states In solution and solids, the lowest singlet transition (0-0) has been identified as occurring to a state of the same symmetry as the ground state ('Ag).3 •4•5•11•12 This conclusion is based on the large gap separating the absorption and emission bands, the asymmetry between the bands, and direct demonstration of the two-photon-allowed, one-photon-forbidden nature of the lowest state. 5 However, it is interesting to note that gas-phase spectra give a decidedly different picture of the level ordering of octatetraene. The gas-phase fluorescence spectrum shows no Stokes shift relative to the absorption spectrum,3 and no evidence for a low-lying 2*Ag state was obtained in electron-energy-loss spectra in the gas phase.…”