FAR-INFRARED SPECTRAL STUDIES OF PHASE-CHANGES IN WATER ICE INDUCED BY PROTON IRRADIATION

Far-infrared spectra from 20 mum (500 cm-1) to 100 mum (100 cm-1) of water ice have been measured over the temperature range of 13-155 K. Amorphous and crystalline water ice are easily identified in far-infrared spectra since amorphous ice has one broad absorption peak near 45 mum (220 cm-1) and crystalline has absorptions near 44 mum (229 cm-1) and 62 mum (162 cm-1). We have observed radiation-induced phase changes in both amorphous and crystalline ice. Crystalline ice converts to an amorphous phase when irradiated at temperatures between 77 and 13 K. The conversion rate increases as the temperature is decreased and the converted fraction is dose dependent. No radiation-induced changes are detected in amorphous ice between 125 and 36 K. However, far-infrared spectra of proton-irradiated ices near 13 K show interconversion between the amorphous and crystalline ice phases beginning at doses near 2 eV molecule-1 and continuing cyclically with increased dose. A quadrupole mass spectrometer was used to detect H-2, H2O, and 02 releases associated with the cyclic phase conversions. Radiation-induced phase changes in these two forms will be discussed in relation to interstellar ices, and ices in comets.