ASSOCIATION REACTIONS AT LOW-PRESSURE .3. THE C2H2+/C2H2 SYSTEM

The association reactions, C4H2+ + C 2H2 and C4H3+ + C 2H2 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 X 10-10 cm3 s -1 and 2.0 × 10-10 cm3 s-1 for C4H2+ and C4 H3+, 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 C4H2+ and C4H3 +, respectively, when M = C2 H2. 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 (C6H4 + ) * and ≥ 18 μs for (C6H5 + ) *. At pressures below 1 × 10-6 Torr, ∼ 11% of the (C6H4+ ) * were stabilized by photon emission and the remaining ∼ 89% reverted back to reactants, while ∼ 24% of the (C6H5+ ) * were stabilized by photon emission and the remaining ∼ 76% reverted back to reactants. The ionic products of the C2H2+ + C2H2 reaction, C4H2+ and C4H3+, 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. © 1990 American Institute of Physics.