Dissociative Ionization of Molecules by Rare-Gas Ion Impact

Abstract

Reactions of rare-gas ions with N2O, C2H2, C2H4, and D2 are studied, and cross sections are given for primary-ion energies below 120 eV. These cross sections are compared for qualitative differences. Most of the reactions have rather sharp onsets, and a semiempirical formula is fitted to the data near the apparent threshold energy. The results of this investigation show that useful information about bond energies can be obtained from ion-molecule reactions. The data are consistent with D(H2C=CH2)=8.11 +0.10−0.18 eVand thus with D(CH–H)=3.99 +0.05−0.09 eV,although smaller values of D(H2C=CH2), and thus larger values of D(CH–H), cannot be entirely excluded. The results indicate that ion-impact methods may possess inherent advantages over electron-impact methods in certain cases; for example, the present data show that the electron-impact appearance potential of C2H3+ from C2H4 is certainly 0.3 eV and probably 0.6 eV larger than the true threshold energy. The threshold behavior of an endothermic charge-transfer process Kr++D2→D2++Kr is studied, and the cross section for this reaction is comparable in size to many of the other endothermic reactions observed, despite the fact that kinetic energy of the colliding particles must be converted into electronic energy for the reaction to proceed.