In this work, the novel reaction mechanism initiated by the fast CH( 2 Π) + C 2 H 2 f C 3 H 2 + H reaction (r13a) and followed by C 3 H 2 + O f C 2 H + HCO (or H + CO) (r25a) was established as the dominant C 2 H formation pathway in low-pressure acetylene/atomic oxygen flames at 600 K. The C 2 H 2 /O/H flames were investigated in an isothermal discharge-flow reactor at a pressure of 2 Torr, with He as bath gas. Concentration vs reactiontime data were obtained by molecular beam sampling and threshold ionization mass spectrometry. The crucial role of CH( 2 Π) was evidenced by CH 4 -addition experiments on room-temperature C 2 H 2 /O/H systems, where CH( 2 Π) and CH 2 ( 1 A 1 ) are the sole intermediates that react rapidly with CH 4 . The observed strong reduction of [C 3 H 2 ], [C 2 H], and [C 4 H 2 ] upon CH 4 addition could be correlated quantitatively with the known removal of CH( 2 Π) by CH 4 . The reaction channels r13a and r25a as sources of C 3 H 2 and C 2 H, respectively, were each established by quasi-steady-state analyses of the pertaining radicals in C 2 H 2 /O/H mixtures at 600 K. By a similar method, the observed C 3 H radicals could be attributed to a minor channel of the CH( 2 Π) + C 2 H 2 reaction (r13) parallel to that producing C 3 H 2 . At 600 K and 2 Torr, the following approximate product yields of reaction r13 were derived: 85 -19 +9 % C 3 H 2 plus H, and 15 -9 +19 % C 3 H plus H 2 . Concomitantly with the identification of reaction r25a as dominant C 2 H source, the rate constant of the reaction C 2 H + O (r19) was determined relative to the well-known kinetic coefficients of C 2 H + C 2 H 2 and C 2 H + O 2 : k 19 ) (9 ( 4) × 10 -11 cm 3 molecule -1 s -1 at 600 K. It is suggested that a sizeable fraction of the ethynyl radicals formed in fuel-rich hydrocarbon flames is produced likewise by oxidation of C 3 H x radicals (x ) 1-3) that arise in the fast reactions of CH(X 2 Π) and CH 2 (a 1 A 1 ) with C 2 H 2 .

acetone molecular ions retain large amounts of internal energy for more than 30 /us after formation by electron impact. (2) These highly excited ions are the dominant CID reactants at such low collision energies. (3) These low-energy (0.45-and 0.65-eV) collisions trigger the release (E -* T) of about 2.9 eV of stored energy into the recoil of the acetone ion and the collider He atom. With reference to the initial ion beam direction, the acetone ions are backward scattered and He atoms are forward scattered in the CM coordinate frame. (4) The recoiling acetone molecular ion rapidly dissociates into acetyl ion and methyl radical with low kinetic energy of separation. We infer from these observations that the collision process triggers the A -X conversion of the acetone molecular ion with the release of most (perhaps all) of the stored electronic energy as translational energy. The vibrationally excited ion then dissociates from the ground-state hypersurface. Our continuing research on polyatomic ion CID dynamics will address the question whether electronically excited states are also important intermediates at higher collision energies.

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