Mechanisms of Radiation-Induced Degradation of CFCl3 and CF2Cl2 in Noble-Gas Matrixes: An Evidence for "Hot" Ionic Channels in the Solid Phase.

Abstract

The X-ray-induced transformations of simple chlorofluorocarbons (CFCl3 and CF2Cl2) in solid noble-gas matrixes (Ne, Ar, Kr, and Xe) at 7 K were studied in order to elucidate basic mechanisms of the radiation-chemical degradation with possible implications for stratospheric and extraterrestrial ice chemistry. The decomposition of parent molecules and formation of products were monitored by FTIR spectroscopy, and the identification was supported by ab initio calculations at the CCSD(T) level. It was shown that the ionic reaction channels were predominating in most cases (except for CF2Cl2/Xe system). The primary radical cations (CFCl3+• and CF2Cl2+•) are either stabilized in matrixes or undergo fragmentation to yield the corresponding secondary cations (CFCl2+, CCl3+, CF2Cl+) and halogen atoms. The probability of fragmentation through different channels demonstrates a remarkable matrix dependence, which was explained by the effect of excess energy resulting from the exothermic positive hole transfer from matrix atoms to freon molecules. A qualitative correlation between "hot" ionic fragmentation at low temperatures and gas-phase ion energetics was found. However, dissociative electron attachment leads to formation of neutral radicals (CFCl2• or CF2Cl•) and chloride anions. One more possible way of dissociative electron attachment in the case of CF2Cl2 is formation of CF2•• and Cl2-•. A general scheme of the radiation-induced processes is proposed.