Radiative and fragmentation decay of the cations of t r a n s- and c i s-1,3,5-hexatriene and of all t r a n s-1,3,5-heptatriene in the A(π−1) states, studied by emission and photoelectron–photoion coincidence spectroscopy

Gas phase emission spectra of the cations of trans- and cis-1,3,5-hexatriene and of all trans-1,3,5-heptatriene have been obtained using ?25 eV electron impact as excitation. The band systems have been assigned as the ?(π−1)→?(π−1) electronic transitions according to the interpretation of their photoelectron spectra. An analysis of the emission bands yields several vibrational frequencies of the totally symmetric fundamentals for these cations in their ground states. These are compared to the frequencies obtained from the matrix absorption and photodissociation spectra of these cations for the ? states, and from the photoelectron spectra. The title cations in the ? states decay by radiative and fragmentation pathways, which are both detected. The fragmentation decays have been studied by photoelectron–photoion coincidence technique using He(Iα) photon radiation. In the case of the 1,3,5-hexatriene cations, C6H+8 and C6H+7 ions are detected on formation of the ? states, whereas for the all trans-1,3,5-heptatriene cation, in addition to the molecular ion, C7H+9, C6H+7, and C6H+6 fragment ions are also observed. Quantum yields of emission from the lowest vibrational level of the ? state of trans- and cis-1,3,5-hexatriene cations have been estimated as 7% and 0.3%, respectively, from the lifetime and relative emission intensity measurements, and the integrated extinction coefficients of the matrix absorption spectra. In order to explain the much larger branching ratios for the parent ions, and the ?20% deviation of the sum curves from unity obtained in the coincidence measurements, it is postulated that there are two nonradiative pathways for the lowest levels of the ? states. One leads to cyclic C6H+7 fragment ions via a cyclic C6H+8 precursor, whereas the other decay is to a linear vibrationally excited C6H+8 ion which cyclizes only slowly, at a rate <104 s−1. In the case of all trans-1,3,5-heptatriene cation the parent ion branching ratio (2.8%) is probably not much greater than the emission quantum yield.