PARAMAGNETIC SPECIES IN IRRADIATED ORGANIC GLASSES .2. FORMATION AND DECAY OF TETRAMETHYLETHYLENE RADICAL CATIONS AND EFFECTS OF SOME ADDITIVES ON FREE-RADICAL YIELDS

Electron spin resonance (ESR) study shows that glassy 3-methylpentane (3-MP) with 0.3% tetramethylethylene (TME) γ irradiated at 77 K yields the monomer radical cations of TME (TME+), the trapped electrons (et-), and the free radicals from 3-MP. Both the TME+ and the et- are unstable at 77 K, and most of them disappear within 1 h. When 0.2% CO2 is added to 0.3% TME-3-MP, TME+ together with the CO2- formed is stabilized. TME+ and et- in 0.3% TME-3-methylheptane (3-MHP) are bleachable by light with λ >800 nm, while TME+ and CO2- in 0.3% TME-0.2% CO2-3-MHP are stable to photoirradiation with λ >800 nm and unstable to light with λ <400 nm. These photo- and thermal decays are concluded to be caused by charge recombination with a mobile electron activated from its trapping site. The radical cations of TME dimer complexes (TME2+) are formed at TME concentrations higher than 1% and disappear by neutralization with electrons as concluded in the case of TME+. Optical absorption study shows that a broad band with λmax ∼860 nm observed in γ-irradiated 1% CO2-3-MP with TME higher than 0.3% is attributed to TME2+ and that TME+ does not give a distinct band at wavelengths longer than 500 nm. The 100-eV yields (G values) of paramagnetic species in 3-MHP glasses with and without additives (CO2 and TME) are obtained by ESR methods. Free-radical yields, G(R·), in CO2-3-MHP are found to decrease with an increase in negative-charge yields, G(et-) + G(CO2-), the result of which indicates the importance of the charge recombination reaction between electrons and positive charges to the production of R· (e- + RH+ → RH* → R· + H·). The yields of TME+ in the TME-3-MHP system with and without 0.5% CO2 are found to be approximately equal to increases in G(CO2-) and G(et-), respectively. It is found that TME+ in TME-R′X-3-MP (R′X = CH3I, (CH3)3CCl, CCl4) decreases with decay of R′· radicals produced by dissociative electron attachment to R′X and that yields of TME+ in TME-HI-3-MP are very small. Consideration is given to the TME+ decay accompanying the R′· decay in relation with the ionization potentials of the R′· radicals. Radiolysis of glassy n-butyl chloride and polycrystalline CCl4 each containing TME yields TME+; however, polycrystalline n-heptane containing TME and CO2 or n-butyl chloride does not give TME+. It is observed that ESR signals from TME+ in the CCl4 matrix are enhanced by photoirradiation with λ >800 nm or by warming. The intensity change of TME+ is reversible, indicating formation of something like a complex between TME+ and CCl4 or a decomposition product from CCl4. © 1979 American Chemical Society.