State selected charge transfer reactions in the BC++Ar⇄BC+Ar+ systems: Comparison among BC=H2, O2, and NO

Abstract

The relative state selected cross sections for the low-energy charge transfer (CT) reactions in the systems H+2(X2 Σ+g,v)+Ar, Ar+(2PJ)+H2, O+2(a 4Πu,v) +Ar, Ar+(2PJ)+O2, NO+(a 3Σ+,v)+Ar, and Ar+(2PJ)+NO have been determined using the threshold electron-secondary ion coincidence (TESICO) technique. In the (H2+Ar)+ system, the cross sections for both the forward and backward reactions were found to show a characteristic internal-state dependence which can be consistently interpreted in terms of a single model based on energy defects (ΔE) and Franck–Condon (FC) factors. In contrast, in the (O2+Ar)+ and (NO+Ar)+ systems, the strong dependence of the cross sections on the selected internal states were observed only for the forward (starting from the diatomic ions) reaction, and not for the backward (starting from the Ar+ ion) reaction. The results for the forward reactions were again interpreted, at least partially, by the energy defects and FC factors between the reactant and product states. These features of the internal-state dependence of cross sections have been discussed in conjunction with the characteristics of the relevant potential energy surfaces of each system. The discrepancy between the (H2+Ar)+ system and the (O2+Ar)+ and (NO+Ar)+ systems in the behavior of the forward and backward cross sections was ascribed to the difference in the number of potential surfaces involved; in the former system only two surfaces are involved in both forward and backward reactions allowing exactly the same mechanism for both reactions, whereas in the latter two systems, the occurrence of more than two surfaces causes different mechanisms for the forward and backward reactions. In the (O2+Ar)+ system, the doublet and quartet surfaces participate in the backward reaction, leading to two different states (X 2Πg and a 4Πu) of the O+2 product ion, while only quartet surfaces are involved in the forward reaction. In the (NO+Ar)+ system, different reaction paths arise between the [NO+(a 3Σ+)+Ar] and (Ar++NO) states due to the anisotropy in these interactions. This allows the different behavior of the forward and backward reactions, in spite of the fact that the product state of the backward reaction is predominantly NO+(a 3Σ+)+Ar. An ab initio calculation of partial potential energy surfaces for the triplet states of the (NO+Ar)+ system supported this view. From these considerations, low-energy charge transfer reactions in the (BC+Ar)+ systems have been classified into two groups according to the possible types of nonadiabatic transitions. These groups are considered to correspond respectively to the phenomenological ‘‘direct’’ and ‘‘intimate’’ reaction mechanisms.