Photoionization and dissociative photoionization of the allyl radical, C3H5

The photoionization and dissociative photoionization of the allyl radical in the VUV range was investigated using synchrotron radiation. Allyl radicals were generated by flash pyrolysis from allyl iodide and 1,5-hexadiene. Mass spectra show the appearance of C3H3+ as a fragment in the dissociative photoionization of allyl above 10 eV. Between 10.4 and 10.5 eV the C3H3+ signal reaches 50% of its maximum value. Cyclopropenyl cation, c-C3H3+, is the thermochemically most stable product, but accompanying ab initio computations indicate that formation of linear l-C3H3+ becomes competitive at small excess energies. The dissociative photoionization of the precursor molecules is discussed as well. An ionization energy of 8.13 ± 0.01 eV was deduced from threshold photoelectron spectra.