Ionization, associative Penning

Figure 25. Associative Penning ionization total ionization ratio as function of collision energy for Ne(3P2 0)-Kr. Results obtained by three different experimental methods are compared.

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. 

It is assumed here that the probabilities of associative ionization, rearrangement ionization, and Penning ionization are the same for He(2 S) and He(2... 

Fig. 7. Potential energy curve depiction of associative ionization and Penning ionization. (Reproduced with permission of Collections of Czechoslovak Chemical Communications.)...

Non-ionizing electron-neutral interactions create electronically excited neutrals. The ionization reactions occurring when electronically excited neutrals, e.g., noble gas atoms A, collide with ground state species, e.g., some molecule M, can be divided into two classes. [21] The first process is Penning ionization (Eq. 2.6), [22] the second is associative ionization which is also known as the Hombeck-Molnar process (Eq. 2.7). [23]... 

Figure 18. Calculated cross sections and experimental data for He(2J5)-H otouu, total ionization o(Pgl), partial Penning ionization a(AI), partial associative ionization QI, partial formation of quasibound heH + (-o-), quantal calculation 51 (- -), classical calculation.51 Experimental data value obtained with thermal velocity distribution corresponding to average energy of 45 meV 51 value obtained at average energy of 3 + 0 meV I, renormalized data due to Howard et al. 54 x, quantal calculation, thermal average for conditions under which point was obtained.51...

Fig. 11.26 Rb-Rb potential curves showing the origin of the differing rates for Penning and associative ionization. In associative ionization the initial state Rb n( + Rb 5s only is above the lower 2g ionic state at small R where the g - 2U exchange splitting is large. Only at small R does autoionization to the ionic molecular state occur. In contrast, in Penning ionization the initial state Rb n + Rb 5p always lies above the ionic final state, and autoionization can occur at any R.

The potential curves for the 3 S+, and 5 2+ states arising from two triplet metastable He atoms are important in the study of Penning and associative ionization. These have recently been studied by Garrison et a/.,99 using a large Cl expansion, and ionization cross-sections have been calculated. 

Penning and Associative Ionization of Atoms.—Several workers have described chemi-ionization reactions of He with an atom, M, in terms of autoionizing transitions between adiabatic potential curves, V and V, correlating with He + M and He + M respectively. Figure 2 shows schematic curves and transitions, representing the processes ... 

Intermediate to Penning ionization, in which the unexcited reaction partner is ionized by the excited one [reactions (4) and (5)], and associative ionization, in which only one molecular entity is formed, is the process (3), which has been called by Berry rearrangement ionization. ... 

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-... 

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