Observations of the Thermal Behavior of Radicals in Gamma-Irradiated Ice

Ice was subjected to γ radiation at 77°K and the EPR spectra of the radicals produced were examined as a function of temperature. The spectrum consisted essentially of a doublet centered at g=2.008±0.002 and split by 40 gauss; the corresponding spectrum of irradiated D₂O consisted of a triplet with approximately 6 gauss between adjacent lines. The doublet decayed rapidly above 100°K leaving a residual broad line which was thermally stable until approximately 145°K. The main spectrum has been attributed to the OH radical and the residual line assigned to the HO₂ radical. The decay process of the OH radicals was shown to follow the rate law d(OH)/dt=—k(OH)^{$\frac{3}{2}$} where k=8.5×10¹² exp—6000/RT (liter/mole)^½ sec^—1. The magnitude of the rate constant was shown to be a function of the total irradiation damage experienced by the ice sample. Stabilization of OH radicals at 77°K was studied as a function of total irradiation dosage; saturation is reached at an OH concentration of 1.8×10^—3M or 0.003 mole %. The EPR spectrum of a frozen aqueous NO solution was also observed; it consisted of a broad line centered at approximately g=2.08.