Oxygen negative ion reactions with carbon dioxide and carbon monoxide. Part 1

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.