Chemical consequences of low energy I+ or I+2 implantation in solid hydrocarbons

Abstract

I+ and I+2 ions, produced by electron bombardment of iodine vapour in a specially designed mass spectrometer, were allowed to strike thick solid hydrocarbon targets (propane, n- and iso-butane, n-pentane and benzene) at 77 K. The organic iodides formed were analysed with a gas chromatograph equipped with an electron capture detector. The total and relative yields were determined as a function of the beam energy from 5 to 3000 eV.The reaction mechanism involves cage effects which favour the recombination of iodine with one of the free radicals formed in the close vicinity of the thermalized halogen, after the slowing down and the neutralization of a beam component. The contribution of charge transfer reactions is important in the low energy region and with alkane targets, increasing steadily from pentane to butane and to propane but playing a minor role with C6H6.Accelerated iodine ions, (n,γ) activated 128I and 125I formed in the electron capture of 125Xe lead to similar results with solid alkanes hence weakening the validity of the autoradiolysis model. This similarity does not appear however with a more radiation-resistant compound such as C6H6: the yields of C6H5I in I+ irradiated targets being one order of magnitude lower than those obtained after nuclear transformations. These discrepancies must be attributed to the radiolytic effects brought about in the formation of nucleogenetic hot atoms, especially by Auger electrons.