Reactions of Ne²⁺, Ar²⁺, Kr²⁺ and Xe²⁺ with the rare gases at low energies

The rate coefficients and product ions for the reactions Ne²⁺, Ar²⁺, Kr²⁺ and Xe²⁺ ions with He, Ne, Ar, Kr and Xe have been determined in the Birmingham selected-ion flow tube (SIFT) at 300K, and the energy variation of several of these reactions has been studied in the Innsbruck drift tube (IDT). In the case of Xe²⁺, the reactions of the ground state (³P) and the metastable electronic states (¹D₂ and ¹S₀), were studied whereas for Ar²⁺ a study of only the ³P and ¹S₀ state reactions was possible. In the case of Ne²⁺ no state selection was achieved and for Kr²⁺ state selection was only achieved in the reaction with Ne. Single charge transfer was the only observed mechanism in all of the reactions except the Ne²⁺ reactions with Kr and Xe in which double charge transfer channels were also evident. The widely varying rate coefficients are interpreted in terms of a model involving the internuclear separation, R_x, at which the ion-induced dipole attractive and the Coulombic repulsive potential curves cross, and which indicates that a 'reaction window' exists for R_x between 2 and 6 AA. The drift tube data are totally consistent with the model and the increasing rate coefficients with increasing energy for the Kr²⁺(¹D₂)+Ne and Xe²⁺(³P)+Ar reactions clearly indicate an increasing efficiency for the production of the ²P_1/2 states of Kr⁺ and Xe⁺ rather than the ²P_3/2 states which are the favoured products at thermal energies.