The cyclopropene radical cation: Rovibrational level structure at low energies from high-resolution photoelectron spectra

The cyclopropene radical cation (c-C3H4+) is an important but poorly characterized three-membered-ring hydrocarbon. We report on a measurement of the high-resolution photoelectron and photoionization spectra of cyclopropene and several deuterated isotopomers, from which we have determined the rovibrational energy level structure of the X+ 2B2 ground electronic state of c-C3H4+ at low energies for the first time. The synthesis of the partially deuterated isotopomers always resulted in mixtures of several isotopomers, differing in their number of D atoms and in the location of these atoms, so that the photoelectron spectra of deuterated samples are superpositions of the spectra of several isotopomers. The rotationally resolved spectra indicate a C2v-symmetric R0 structure for the ground electronic state of c-C3H4+. Two vibrational modes of c-C3H4+ are found to have vibrational wave numbers below 300 cm−1, which is surprising for such a small cyclic hydrocarbon. The analysis of the isotopic shifts of the vi...