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

Dry ice (solid CO2) 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 CO2 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 (g1 = 2.0057, g2 = 2.0126, g3 = 2.0161; activation energy E = 0.10 eV; frequency factor ν0=4×101 s−1) and HO2 (g1 = 2.0040, g2 = 2.0055, g3 = 2.0360), which have E = 0.28 eV and ν0=7×105 s−1). Hence, HO2 is more thermally stable, and use of HO2 is promising for ESR dating.