The Photochemistry of Propane at High Photon Energies (8.4-21.2 eV)

Neon and helium resonance lamps, which deliver photons of 16.7-16.8 eV and 21.2 eV energy, respectively, have been used to photolyze C₃H₈, C₃D₈, C₃H₈-C₃D₈ (1:1) mixtures, and the results obtained at the two energies are compared. In particular, it is noted that although the quantum yield of ionization in propane is unity at 16.7-16.8 eV, when the energy is raised still further to 21.2 eV, the probability of ionization apparently diminishes to 0.93, an observation which suggests that at 21.2 eV, superexcited states may be reached whose dissociation into neutral fragments competes with ionization. The quantum yields of the lower hydrocarbon products formed in the presence of a radical scavenger in C₃H₈ and C₃D₈ are reported, and are compared with quantum yields of products formed in the vacuum ultraviolet photolysis at lower energies. (Quantum yields of products formed at 8.4 eV and 10.0 eV are reported here for the first time.) Acetylene is formed as a product in the decomposition of the neutral excited propane molecule, and its yield increases in importance with increasing energy; at 16.7-16.8 eV, where all product formation can be traced to ionic processes, acetylene is formed in negligible yields. It is concluded that ionic processes in propane do not lead to the formation of acetylene, and the observation of this product in radiolytic systems may be a reliable indicator of the relative importance of neutral excited molecule decomposition processes. From the results obtained with the C₃H₈-C₃D₈ (1:1) mixture, and with CD₃CH₂CD₃, details of the ion-molecule reaction mechanisms and the unimolecular decomposition of the propane ion are derived.