Abstract
Fluorescence quantum yields and cascade-free lifetimes of diacetylene and dideuterodiacetylene cations in selected vibrational levels in the à 2Πu state have been measured. This was accomplished by the detection of coincidences between energy selected electrons and undispersed photons following ionization by He(Iα) photons. The radiative and nonradiative rate constants of such state selected cations under collision-free conditions are therefore directly obtained. The fluorescence quantum yields are high, but not unity, and decrease as do the lifetimes on increasing the internal energy in the à 2Πu state. These first results are used to illustrate the decay behavior of open-shell organic cations for which only the 2Ã→ 2X̃ internal conversion process completes with the 2Ã→ 2X̃ radiative relaxation.

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