Unimolecular dissociations of excited C3H6O+: A comparative photoelectron—photoion coincidence study of 1,2-epoxypropane and acetone

The unimolecular fragmentation of internal energy selected 1,2-epoxypropane cations has been studied by fixed-wavelength photoelectron—photoion coincidence spectroscopy. Branching ratios for the prominent fragment ions are reported up to an ionization energy of I = 14 eV. It is shown that 1,2-epoxypropane cations initially formed with none or only little vibrational excitation in the electronic ground state do not dissociate, though their excess energy with respect to the lowest energetic fragmentation pathway is 1.25 eV. As the internal energy is increased, slow fragmentation into several dissociation channels is observed. This is used to explain a comparably slow dissociation process observed in the case of acetone molecular ions initially excited to their electronic à state. CH₂C(OH)CH₃⁺ and/or CH₃CHCHOH⁺ are proposed as precursors for these low-rate unimolecular reactions.