High yield of radical pairs in deuterated n-alkane single crystals γ-irradiated at 4.2 K

Abstract

Abstract Following γ-irradiation at 4.2 K ca. 40% of the radicals in an n-decane- d 22 crystal are paired compared to ca 3% following irradiation at 77 K. Two structurally different kinds of radical pairs have been observed. One kind is similar to that formed on irradiation at 77 K and shows a hyperfine splitting of 2.5 G. The second kind shows no hyperfine structure and is thought to consist of two slightly differently oriented alkyl radicals that are trapped in the damaged regions of the crystal lattice. The distance between the radicals in this kind of pair is clustered around two values, namely 10.0±0.5 and 7.5±0.5A˚. The value of 10A˚is believed to be a good measure of the spur radius. The high yield of radical pairs at 4.2 K as compared to 77 K is explained by a mechanism where the excess excitation energy is trapped more efficiently at the original site of deposition at 4.2 K. This will also lead to less energy transfer to the surrounding molecules at 4.2 K.