Abstract
The initial steps involved in the thermal decomposition of acetylene and diacetylene in the temperature range 1850 to 3000 K have been investigated by monitoring the hydrogen-atom production behind reflected shock waves in highly diluted mixtures of argon (5–400 ppm, total densities 6 × 10 −6 to 2.7 × 10 −5 mol/cm 3). The technique of atomic resonance absorption spectrophotometry (ARAS) was used to measure the H-atom concentration. In the low pressure region for unimolecular decomposition the initial step of the reaction can be characterized as: C 2 H 2+M → k 1 C 2 H+ H+M for which the rate coefficient k 1=(3.6±0.6)10 16 exp ( −E 1 RT ) cm 3 mol −1 s −1 E 1 R = 53600±400 K T:2100−3000 K M:8×10 −6−2.6×10 −5 mol cm −3 was obtained. The measurements also favor the recent value of 127 kcal/mol for the formation enthalpy of C 2H published by Okabe and Dibeler. In addition, it was possible to derive for the reaction C 4 H 2 → k 3 C 4 H+ H a rate coefficient k 3=(2.2±0.6)10 14 exp ( −E 3 RT ) s −1 E 3 R = 58700±700 K T:1850−2300 K M:1×10 −5−2.4×10 −5 mol cm −3 which is not far from the high pressure limit. It was also possible to derive rate coefficients in certain regimes of the investigated temperature interval at total densities around 10 −5 mol cm −3 for the following reactions: C 2 H 2+ C 2 H → k 2 C 4 H 2+ H k 2=3−4·10 13 cm 3 mol −1 s −1 T:2300−2700 K C 4 H → k 4 C 4+ H k 4=0.5−2.2·10 14 exp ( E 3 RT ) s −1 T:2100−2300 K C 2 H 2+ C 2 H 2 → k 6 C 4 H 2+2 H k 6=(3±1)·10 8 cm 3 mol −1 s −1 T:1850−2000 K C 4 H 2+ C 4 H → k 7 C 8 H 2+ H k 7=1.5−5.5·10 13 cm 3 mol −1 s −1 T:1850−2300 K C 4 H 2+ C 2 H → k 9 C 6 H 2+ H k 9=1−4·10 13 cm 3 mol −1 s −1 T:2400−2700 K
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