Abstract:
A drift tube and mass filter have been used to measure the rates of some O– negative ion molecule reactions, thought to be important in the radiolysis of carbon dioxide. Measurements of the clustering reaction O–+ CO2+ M → CO–3+ M in carbon dioxide give a third-order rate constant which falls with increasing pressure. This suggests an intermediate CO–3 ion with a lifetime of approximately 10–8 s. The limiting, low-pressure, rate constant is (1.1 ± 0.1)× 10–27 cm6 molecule–2 s–1 and, in the high-pressure limit, it is (2.7 ± 0.3)× 10–10 cm3 molecule–1 s–1. The rate also falls slowly with increasing reduced field. In O2 the rate constant is a factor of 3.5 lower, but it is difficult to measure the pressure dependence as accurately because CO–3 is also produced by the reaction: O–3+ CO2→ CO–3+ O2k=(5.5 ± 0.5)× 10–10 cm3 molecule–1 s–1. The rate constant measured in O2 for the associative detachment reaction O–+ CO → CO2+ e is (7.3 ± 0.7)× 10–10 cm3 molecule–1 s–1. Similar experiments in carbon dioxide are complicated by changes in the electron energy distribution as CO is added, but an upper limit of significantly less than 10–13 cm3 molecule–1 s–1 is suggested for the competing reaction: CO–3+ CO → 2CO2+ e. The reaction of O–3 with CO is very slow.
