TEMPERATURE DEPENDENCE OF THE FORMATION OF HYDROGEN, OXYGEN, AND HYDROGEN PEROXIDE IN ELECTRON-IRRADIATED CRYSTALLINE WATER ICE

摘要

We conducted a systematic study of the irradiation of crystalline water ice in an ultrahigh vacuum chamber at pressures of about 10~(-10) torrr. Crystalline water ices of 115 ± 30 nm thickness were irradiated with energetic electrons at 12, 40, 60, and 90 K to simulate energetic particle interaction with solar system and interstellar water ices. The production rates of molecular hydrogen (H_2), molecular oxygen (O_2), and hydrogen peroxide (H_2O_2) decrease as the temperature rises from 12 to 90 K. These findings strongly indicate a thermal, possibly diffusion-controlled component of the reaction mechanism, which could facilitate the back-reaction of the primarily formed irradiation products such as the recombination of atomic hydrogen (H) with the hydroxyl radical (OH) to "recycle" water. This study underlines the necessity to provide temperature-dependent rate constants on the formation of molecules in planetary and interstellar ices and to include nonequilibrium as well as thermal chemistry in reaction models simulating the chemical processing of extraterrestrial ices. We estimate the concentrations of newly formed H_2O_2 molecules in the irradiated water ice at 90 K to be in the same order of magnitude as the concentration of H_2O_2 observed on the Galilean satellites.