Photodissociation of gaseous olefinic cations

An ion cyclotron resonance spectrometer was used to study the wavelength dependence of the photodissociation of cations in propene and several isomeric butenes. Photodissociation onsets were determined and compared with electron impact and thermodynamic values. In all cases the photodissociation onsets were found to be above both the thermodynamic onset and the first excited state of the cation as determined by photoelectron spectroscopy. The onset results were compatible with a photodissociation mechanism that proceeds by initial excitation into the first excited state, followed by dissociation. A large increase in the photodissociation cross section for all the cations studied was observed at [inverted lazy s] 3.7 eV. This increase correlated with the second excited states of all the cations except 1‐butene. Internal excitation of the parent cations was investigated by the addition of inert gas and by changing the electron beam energy. Photodissociation onsets were also found to be valuable in choosing between alternative structures for gas‐phase cations. At least two noninterconverting isomers of ${\mathrm{C}}_4{\mathrm{H}}_8^{+}$ were shown to exist.