Temperature dependence of three-body ion-molecule reactions

The temperature dependence of the ion-molecule association reactions (i) N2+ + N2 + M → N4+ + M (M=N2, He), (ii) O2+ + O2 + M → O4+ + M (M=O2, He) and (iii) He+ + 2He → He2+ + He have been studied over an extended temperature range to temperatures as low as 30K with a recently constructed liquid helium-cooled ion drift tube. Over most of the temperature range the threebody reaction rate coefficients show an inverse temperature dependence proportional to Tsup(-n) with n in the range 0.6 to 2.9. This temperature dependence is quite consistent with current theories of ion molecule association. At low temperatures, however, a deviation from the Tsup(-n) dependence was observed for the association reactions (ii). For reactions (i) different temperature dependences were obtained for N2 and He third bodies indicating an additional temperature dependence of the collisional stabilisation process. (Authors)