Isotope Effects in the 77°K γ Irradiation of Ice

The radiation yields of H2, D2, and HD obtained by the 77°K γ irradiation of solute-free mixed H2O and D2O ice mixtures have been determined by use of mass spectroscopy. The total gas yields have been determined, too, both by use of a calibrated mass spectrometer and by manometric methods. Significant isotope effects were observed over a wide range of the isotopic compositions studied (i.e., the deuterium content varied from ∼0.016% to 99.8%). The separation factor αG=(H/D)gas(H/D)icevaried from about 70 for the high D content ice to about 1.0 for natural isotopic abundance ice. At (H/D)ice=1.0, αG is 1.7. Values of αG were obtained for 99.80% D2O over a temperature range of 4.2° to 196°K. The separation factor was found to be inversely proportional to temperature. The results of this study are compared with those in a previously published EPR study of stabilized H and D atoms after irradiation at 4.2°K. The observed effects are discussed briefly in terms of several models of selective energy absorption.