Association reactions at low pressure. III. The C2H+2/C2H2 system

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Abstract
The association reactions, C4H+2+C2H2 and C4H+3+C2H2 have been examined at pressures between 8×10−8 and 1×10−4 Torr at 298 K in an ion cyclotron resonance mass spectrometer. Association occurred via two different mechanisms. At pressures below ∼2×10−6 Torr, the association was bimolecular having rate coefficients k2=2.7×10−10 cm3 s−1 and 2.0×10−10 cm3 s−1 for C4H+2 and C4H+3, respectively. At pressures above ∼2×10−6 Torr, termolecular association was observed with rate coefficients, k3=5.7×10−23 cm6 s−1 and 1.3×10−23 cm6 s−1 for C4H+2 and C4H+3, respectively, when M=C2H2. The termolecular rate constants with N2, Ar, Ne, and He as the third body, M, are also reported. We propose that the low pressure bimolecular association process was the result of radiative stabilization of the complex and the termolecular association process was the result of collisional stabilization. Elementary rate coefficients were obtained and the lifetime of the collision complex was ≥57 μs for (C6H+4)* and ≥18 μs for (C6H+5)*. At pressures below 1×10−6 Torr, ∼11% of the (C6H+4)* were stabilized by photon emission and the remaining ∼89% reverted back to reactants, while ∼24% of the (C6H+5)* were stabilized by photon emission and the remaining ∼76% reverted back to reactants. The ionic products of the C2H+2+C2H2 reaction, C4H+2 and C4H+3, were found to be formed with enough internal energy that they did not react by the radiative association channel until relaxed by several nonreactive collisions with the bath gas.

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