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A pulsed Laval nozzle, low Mach number supersonic expansion kinetics apparatus has been constructed to study neutral–neutral kinetics by a rather general laser photolysis initiation and laser photoionization detection of the product species. This new apparatus permits laboratory studies of low temperature rate coefficients (e.g., 70–170 K) on condensable gases that have insufficient vapor pressures at low temperatures for conventional methods of kinetic measurements. The design considerations, the uniformity of the reaction zone over 10–20 cm, and the skimmer sampling of the pulsed Laval expansion are examined. The direct measurement of a rate coefficient at 90 K is also demonstrated using this new apparatus.

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Low-temperature rate coefficients for the reactions C 2 H + C 3 H 4 f products [C 3 H 4 ) methylacetylene (propyne) CH 3 CCH and allene (propadiene) CH 2 CCH 2 ] are measured over the temperature range of 155-298 K. Absolute rate constants are determined using laser photolysis and transient infrared absorption spectroscopy. Both reactions are fast, approaching the kinetic limit and have either no temperature dependence or a slight negative temperature dependence. Over the experimental temperature range, the rate constants are fit to the following Arrhenius expressions: k CH 3 CCH ) (1.6 ( 0.4) × 10 -10 exp[(71 ( 50)/T] cm 3 molecule -1 s -1 and k CH 2 CCH 2 ) (1.3 ( 0.6) × 10 -10 exp[(103 ( 136)/T] cm 3 molecule -1 s -1 . The corresponding rate constants at 298 K are k CH 3 CCH ) (1.9 ( 0.3) × 10 -10 cm 3 molecule -1 s -1 and k CH 2 CCH 2 ) (1.7 ( 0.3) × 10 -10 cm 3 molecule -1 s -1 . These measurements provide new reaction rate coefficients for the ethynyl radical, C 2 H, with C 3 H 4 unsaturated hydrocarbons. These are necessary to extend current photochemical models of Jupiter, Saturn, and its satellite Titan.