Radiolysis. Dissociative Ionization Processes

Following radiolysis of OF2 at 77 K (3 MeV bremsstrahlung, up to 200 Mrad/h, mass spectro-metric analysis at T 219 K) showed the formation of O2, F2, O2F2, and O3F2 [1]. 

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Neutralization by electrons remains an important process in the radiolysis of alkanes containing chloroalkanes or CO2 as solute. Many of the electrons formed in the ionization process have insufficient energy to escape the Coulomb field of their associated cation and, on returning, neutralize the corresponding radical cations, carbenium ions or carbonium ions (geminate recombination). Part of the electrons formed do not return, however, but react with the chloroalkane solute by dissociative electron attachment. 

At least seven modes of dissociation are theoretically possible below the ionization threshold, although their total yield in radiolysis is small (Platzman, 1967). The dissociation products are H, H2, O, and OH, where the first two are in their ground (electronic) states but the last two may be either in ground or excited states. Only two modes of dissociation, H20 -H + O and H20 H + OH, are possible for all excitation energies UV photolysis indicates that the latter process is by far (90%) the most likely. Accordingly, in radiolysis there is a tendency to lump the decay of all excited states of the water molecule into H and OH. 

Excited states can be formed by a variety of processes, of which the important ones are photolysis (light absorption), impact of electrons or heavy particles (radiolysis), and, especially in the condensed phase, ion neutralization. To these may be added processes such as energy transfer, dissociation from super-excited and ionized states, thermal processes, and chemical reaction. Following Brocklehurst [14], it is instructive to consider some of the direct processes giving excited states and their respective inverses. Thus luminescence is the inverse of light absorption, super-elastic collision is the inverse of charged particle impact excitation, and collisional deactivation is the inverse of the thermal process, etc. 

Photoionization of the hydrocarbon followed by dissociative electron attachment (Reaction 1) should be considered since the ionization potential of a molecule is less in the liquid phase than it is in the gas phase. For hydrocarbons the ionization potential is 1 to 1.5 e.v. less in the liquid phase (24). The photon energy at 1470 A. is about 1.4 e.v. below the gas-phase ionization potentials of cyclohexane and 2,2,4-trimethylpentane (14). Some ionization may therefore occur, but the efficiency of this process is expected to be low. Photoionization is eliminated as a source of N2 for the following reasons. (1) If photoionization occurred and the electron reacted with nitrous oxide, then O" would be formed. It has been shown in the radiolysis of cyclohexane-nitrous oxide solutions that subsequent reactions of O result in the formation of cyclohexene and dicyclohexyl (I, 16, 17) and very little cyclohexanol (16, Table III). In the photolysis nitrous oxide reduces the yield of cyclohexene and does not affect the yield of dicyclohexyl. This indicates that O is not formed in the photolysis, and consequently N2 does not result from electron capture. (2) A further argument against photoionization is that cyclohexane and 2,2,4-trimethylpentane have comparable gas-phase ionization potentials but exhibit quite different behavior with respect to N2 formation. 

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