Ionization: electron-exchange, 156 partial

Another important aspect is the question of the reaction mechanisms. Since the sonochemical activation of Diels-Alder systems seems to have no direct link with the volatility of the substrates, the reactions should not take place in the "microreactor" bubble, and a purely thermal interpretation based on the hot-spot theory then seems inadequate. Alternative explanations could be based on a redox reaction between the two partners, or an ionization of one reactant. The existence of intermediate charge transfer complexes in Diels-Alder chemistry has been postulated for a long time, but most authors consider that no complete electron exchange is actually involved. l With the strong electron-accepting tetracyano-ethylene, a partial transfer occurs, but not at all with acrylic compounds. 

Here c is the collision velocity, the asymptotic coefficient is expressed through the atom ionization potential I, and in atoinic units it is etjual to n = / (see also formula (23)). This formula is valid for transfer of an. s—electron or in the case when transitions for states with given quantum numbers may be separated. In particular, the partial cross sections of resonant charge exchange in the chlorine case are given in Table 4. 

In most heterocomplexes the two constituents differ in their ionization potentials and electron affinities. This can promote a partial charge transfer upon contact of the two molecules causing a considerable electric dipole to form across the interface. The importance of the charge-transfer interactions can therefore become dominant over the excitation exchange interactions. Indeed, most exciplexes appear to be stabilized mainly by charge-transfer interactions [24],... 

The first RE is of limited interest, as many molecules cannot absorb an energy of the order of 54 eV in an ionization process. This is probably the case for all hydrocarbons, as they are built up of carbon atomic orbitals with orbital energies of the order of 300, 24, and 13 eV, and hydrogen atomic orbitals at 13 eV. As the third RE is too low for ionization of most molecules, the electron will enter the 2s or 2p orbital of He if partial charge exchange takes place when He meets a molecule. The effective RE will then be somewhat lower than 13.6 eV. 

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