Ly-alpha photon induced amorphization of Ic water ice at 16 Kelvin - Effects and quantitative comparison with ion irradiation

Water ice is a ubiquitous material in space, on the surface of planets and their moons, in comets and interstellar dust grains. The structure of water ice strongly depends on the deposition temperature. It can be amorphous if deposited at low temperature, i.e. 16 K, or crystalline if the deposition temperature is higher than 140 K. In this paper we report the first experimental study of the structural effects induced by Lyman-alpha UV photons in crystalline water ice carried out by in situ infrared spectroscopy. The effects induced by Lyman-alpha photolysis are compared with those induced by ion irradiation carried out with the same experimental apparatus. We found that, as already observed in the case of fast ions, also UV Lyman-alpha photons are able to fully amorphize the crystalline water ice structure after a dose of few eV per molecule. This study has important astrophysical implications, in particular because photons and fast ions are both present in space, they can induce variations in the profile of the 3 mum band of crystalline water ice. Thus these effects have to be considered when one attempt to deduce information on the physical status (e.g. temperature history) of the responsible dust from the profile of the observed ice band.