The quantum yield (QY) of diacetylene in the 193.3-nm photolysis of acetylene has been measured as a function of pressure, decomposition, and added gases. The QY of diacetylene is near unity (0.9 f 0.1) when the decomposition is 1% or less. The QY of CZHZ + hv -CZH + H is 0.3 f 0.1, which is determined by the C2HD yield from the photolysis of C2H2 + C2D6 (or Dz) mixtures. The yield is in good agreement with the QY of H atoms photodissociated from C2Hz; C2H reacts with C2H2 to produce an equivalent amount of diacetylene. The remaining 60-70% diacetylene arises from the metastable acetylene (C2H2**) reacting with ground-state acetylene at 193.3 nm and above 0.1 Torr of acetylene. The quenching of diacetylene formed via C2H2** by various foreign gases at various wavelengths is compared. The quenching order at 193.3 nm is N2 C D2 C H2 C CzD6 C n-C4Hlo. It is postulated that an adduct from CzH2** + C2H2 is initially formed before it dissociates into C~H Z + Hz or C2H + C2H3. Yields of C4H2, CZHD, and C2H2 were measured by FT-IR calibrated with pure samples. The C4H2 yield at 193 nmdoes not change with the addition of N2 up to 600 Torr. The absorption cross section of C2H2 has been measured in the 190-230-nm region. The production of C4H2 was examined at wavelengths below and above thedissociation threshold. The metastable acetylene reactions may be important in haze formation in Titan's atmosphere.
The photochemistry of acetylene at 1849 Å has been studied over the pressure range from 13 to 2660 N m−2 (0.1–20 Torr). The quantum yield of diacetylene formation has been measured as a function of C2H2 pressure and the partial pressures of He and N2. The quantum yield of H2 formation has also been obtained. It was concluded that the quantum yield for C2H2→hνC2H+H is 0.06 and that for C2H2→hνC2+H2 is 0.1. The remaining process is the formation of a metastable acetylene which reacts either with C2H2 to form C2H radicals or is deactivated by collisions with the walls or inert gases. The C2H radicals react 0.240±0.015 times as fast with C2H6 and 0.469±0.041 times as fast with C3H8 as with C2H2. The role of acetylene photolysis in the Titan atmosphere is briefly discussed.
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