threshold.The reaction energy threshold Eo can be no lower than the minimum energy AEo thermochemically required for the reaction,but may be higher or even significantly higher. An important class of reactions,of particular interest in atmospheric chemistry (aeronomy) and,in general,for interstellar chemistry,is that of the exoergic ion-molecule reactions e.g., N++O2 NO+O N+O Such reactions often show no threshold energy*and the reaction cross-sections are found to be a decreasing function of the translational energy,roughly as OR(ET)=AET12 (3.6) as shown in figure 3.2.It is the preference for low collision energies that makes ion-molecule reactions so important for the synthesis of molecules in the interstellar medium. MRD Chapter 3 page 8 ©R D Levine(2003)threshold. The reaction energy threshold E0 can be no lower than the minimum energy ∆E0 thermochemically required for the reaction, but may be higher or even significantly higher. An important class of reactions, of particular interest in atmospheric chemistry (aeronomy) and, in general, for interstellar chemistry, is that of the exoergic ion-molecule reactions e.g., N+ + O2 → NO+ + O N + O2 +      Such reactions often show no threshold energy4 and the reaction cross-sections are found to be a decreasing function of the translational energy, roughly as σ R(ET ) = AET −1/2 (3.6) as shown in figure 3.2. It is the preference for low collision energies that makes ion-molecule reactions so important for the synthesis of molecules in the interstellar medium. MRD Chapter 3 page 8 © R D Levine (2003)