Ultraviolet photochemistry of diacetylene: Reactions with benzene and toluene

The reactions of metastable diacetylene with benzene and toluene are explor ed using a molecular beam pump-probe time-of-flight mass spectrometer. Diac etylene is laser-excited to the 2(0)(1)6(0)(1) band of the (1)Delta (u) <-- X-1<Sigma>(+)(g) transition, whereupon rapid intersystem crossing occurs t o the lowest triplet states. The triplet state diacetylene then reacts with either benzene or toluene as the gas mixture traverses a short reaction tu be (similar to 20 mus). The reactions are quenched as the gas mixture expan ds into the ion source region of a time-of-night mass spectrometer where th e primary photoproducts are detected using vacuum ultraviolet (VUV) photoio nization or resonant two-photon ionization (R2PI). The major products from the reaction of diacetylene and benzene have molecular formulas C8H6 and C1 0H6. and are identified as phenylacetylene and phenyldiacetylene using R2PI spectroscopy. The major products from metastable diacetylene's reaction wi th toluene are C9H8 and C11H8. The C9H8 product is confirmed as a mixture o f o-, m-, and p-ethynyltoluene, with the ortho product dominating. Mechanis ms for the formation of the above products are proposed based on deuterium substitution studies of the reactions. The potential importance of these re actions is discussed as they relate to hydrocarbon growth in sooting flames .