Radiation effects in dry ice: models for a peak on the Arrhenius curve

摘要

Dry ice (solid CO_(2)) occurs in the polar caps of Mars, on the surface of Triton, and in places in the outer planets of our solar system. Radicals in γ-irradiated solid CO_(2) have been studied by ESR for future applications of ESR dating on outer planets. The annealing curves for CO_(3)~(-) radical (ESR signal at g=2.0126) can be described neither by the first-order nor the second-order decay kinetics. The peak observed in the Arrhenius plot can result from two parallel first-order kinetic processes. Radicals that provide overlapping signals are CO_(3)~(-) (g_(1)=2.0057, g_(2)=2.0126, g_(3)=2.0161; activation energy E=0.10 eV; frequency factor v_(0)=4×10~(1) s~(-1)) and HO_(2) (g_(1)=2.0040, g_(2)=2.0055, g_(3)=2.0360), which have E=0.28 eV and v_(0)=7×10~(5) s~(-1)). Hence, HO_(2) is more thermally stable, and use of HO_(2) is promising for ESR dating.