Excited ions endoergic reactions

Figure 3.1 Translational energy dependence of the reaction cross-section, ctrIEt) for the Hj"(v = 0) -h He HeH -f- H reaction [adapted from T. Turner, O. Dutuit, and Y. T. Lee, J. Chem Phys. 81, 3475 (1984)]. For this ion-molecule reaction the observed threshold energy is equal to the minimal possible value, the endoergicity of the reaction. Exoergic ion-molecule reactions often have no threshold. By exciting the vibrations of the Hj reactant the cross-section for the reaction above can be considerably enhanced.

Many dissociative charge-transfer reactions6 that are endoergic for ground-state ions also become energetically allowed for some electronically excited state of the reactant ion. For example, the reaction of excited Kr + ions with methane has been observed18,19... 

C0+ are nonreactive. These processes have rather high endoergicities (5.5 to 5.6 eV), based on calculations for the ground-state reactants. The energies required to form the excited reactant ions in these instances ( 20 to 23 eV) are not readily accessible with ordinary photoionization sources 85 therefore, these reactions have been studied only with electron-impact ionization. The identity of the excited state responsible for the analogous reaction... 

The enthalpy of formation of the azide radical is 467 SkJmoR. The spin-allowed dissociation to N( D) and N2(X 1 +) is endoergic by 225kJmol, the dissociation enthalpy to N( S) - -N2(X i +) is 0.5 IkJmol. The azide radical is only stable because this spin-forbidden decomposition pathway has an appreciable energy barrier. In aqueous solution, it primarily exists as a monomer, in contrast to other halide or pseudohaUde radicals that exist as the less reactive dimers (e. g. Brs (SCN)2 ). Reaction ofthe azide radical with halogen atoms or other small molecules hke O2, NO, CO, and CO2 produces molecules in electronically excited states because of propensity rules, which can be used for chemically pumped lasers. The azide ion is also formed during high-pressure photolysis of sodium azide. 

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