Energy Migration and Isotopic Effects in Irradiated Solids at Low Temperature

Energy migration to impurity molecules of hydrogen and methane trapped in solid matrices of argon and krypton at low temperature is shown to occur. The solid containing the impurity molecules in minute quantities was irradiated at 4.2°K with $\gamma$ rays from ${\mathrm{Co}}^{60}$, and the free radicals produced by dissociation of impurity molecules were measured from the intensity of their electron spin resonance signals. For concentrations of 0.001 mole fraction of C${\mathrm{D}}_4$ in A or Kr, over 500 times as many molecules of C${\mathrm{D}}_4$ were dissociated as would be expected from the same quantity of pure C${\mathrm{D}}_4$ given the same exposure dose. It is concluded that the dissociation of the dilute impurity is produced almost wholly by the energy absorbed by the matrix and transferred to the impurity molecules. In the mixed isotopic species—HC${\mathrm{D}}_3$, ${\mathrm{H}}_2$C${\mathrm{D}}_2$, and ${\mathrm{H}}_3$CD—it is shown from the relative strength of the ESR signals of H and D atoms that the C-H bond has about 5 times greater probability of being broken by the migrating energy than has a given C-D in the same molecule. This large isotopic effect is attributed to the more rapid escape of H over D from the parent molecule within the lattice.