Heteronuclear associative ionization

Homonuclear associative ionization) (1) (Heteronuclear associative ionization) (2)... 

A more typical example of heteronuclear associative ionization is the... 

Inspection of Tables IV and V, in which are presented known heteronuclear associative ionization reactions, leads to the conclusion that usually the metastable levels of the reactant with the higher ionization potential are involved, a characteristic not encountered in the homonuclear reactions. However, this need not be true, as exemplified by the existence of reaction (28), in which metastable levels are definitely not involved. A particularly interesting case is the formation of HeAr. Munson et reported that the onset potential of this ion is 17.9 eV and that the ionization-efficiency curve of HeAr shows a sharp upward break at about 19.9 eV corresponding to the expected process... 

TABLE V. Heteronuclear Associative Ionization Reactions in Polyatomic Gases... 

Very few measurements of the reaction cross sections or of the specific reaction rates of heteronuclear associative ionization processes have been... 

Several authors have measured abundances of heteronuclear associative ionization products relative to the abundance of the corresponding homonuclear association product and have related this in a simple manner to the ratio of corresponding specific reaction rates. This is probably valid in some cases, but certainly not in all. For example, consider the formation of HeNe and He2 in mixtures of helium and neon. The... 

Nearly all the mass-spectrometric studies of these rearrangement reactions have yielded relative abundances of the ionic products formed in Penning ionization, associative ionization, and rearrangement ioniza-tion. " For those cases in which the Penning ionization cross sections are known or could be estimated, specific reaction rates have been calculated and these are depicted also in Table VI. Comparison of Tables V and VI shows that when rearrangement ionization and heteronuclear associative ionization are in competition, the former occurs about an order of magnitude faster. 

Cermak has also introduced a new method for the study of Penning and heteronuclear associative ionization of long-lived electronically excited atoms and molecules, which he has called Penning-ionization electron spectroscopy. Since Penning ionization usually predominates, the name is quite an appropriate one. Cermak has used the method to measure ionization potentials of some 40 molecules and the values obtained agree very well with those determined by photoelectron spectroscopy. The method has been further shown by Cermak and Herman, Fuchs and Niehaus, and Hotop and Niehaus to be of considerable value in mechanistic considerations of Penning and heteronuclear associa-... 

For those cases in which heteronuclear associative ionization and/or rearrangement ionization compete with Penning ionization, the relative probabilities of reaction are shown in Table VIII. It is seen here that Penning ionization generally predominates, although recent work with rare gases has shown that the probability of associative ionization relative to Penning ionization increases considerably as the temperature is decreased. On the other hand, cross sections for total ionization are not very sensitive to the temperature. ... 

In heteronuclear associative ionization and in Penning ionization, metastable states usually contribute and when this is possible, these long-lived states usually dominate the reaction. In those cases in which only higher-lying states contribute to the reactions, the same considerations as discussed for the homonuclear case apply. 

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