Effect of Internal Energy and Temperature on IM Processes

The effects of energy on ion-molecule rate processes have been investigated by a variety of methods. The influence of reactant translational energy, as studied by SIFDT (selected ion-flow drift-tube) techniques in swarm experiments, by ICR, and by beam and other single-collision techniques, is reviewed in other parts of this chapter or of this book. In this section, we will concentrate specifically on the influence of reactant internal energy on ion-molecule reactions. There are basically two sources of data that address this problem  

Pioneering studies using variable-temperature flowing afterglow tubes (Lindinger et al, 1974) provided data in the temperature range 80 to 900K present 

An interesting case of internal energy effects in the charge-transfer reaction Ar+ + N2 and in the reverse process, (y) + Ar, will be discussed separately. 

Many examples of differences in the reaction rates for ground-state and electronically excited (metastable) states of reactant ions come from SIFT and SIFDT studies. In reactions of simple ions, reactions of ground and excited 0+ ions are of interest While the reaction of the groimd-state ion. 

Metastable oxygen ions 02(a ITu) react fast with atoms or molecules (Ar, N2, CO, H2) in reactions that are endothermic with ground-state oxygen ions the respective rate constants do not depend much on translational energy. An interesting case is the charge-transfer reaction of electronically excited NO+(a S ) with Ar The reaction is endothermic by 0.09 eV, and its rate constant increases with translational energy from the thermal value of 3 x 10 cm s to a value of about 9 x 10 cm s at 3 eV in a way typical of 

---