Compaction of porous ices rich in water by swift heavy ions

•We irradiated porous ices with heavy ions and monitored compaction by FTIR.•The OH dangling bonds decrease exponentially with projectile fluence.•The compaction cross section is proportional to the electronic stopping power.•These findings may serve as diagnostic tools for the morphology of ices in the Solar System and the ISM. Porous water ice and water ice mixtures H2O:X (X=CO, CO2 and CH4) produced at 15K, with film thicknesses in the 0.5–1μm range, were irradiated by swift ions and monitored by mid-infrared spectroscopy (FTIR). The analysis of the evolution of the pure water ice infrared absorption on ion beam dose reveals a strong correlation among three quantities: (i) the absorbance of the most intense band (3250cm−1), (ii) the wavelength of the maximum absorbance of this band and (iii) the absorbance of the OH-dangling bonds. This correlation is interpreted as indications of the water ice compaction by irradiation: as the beam fluence increases, the ice porosity decreases, the dangling bond peaks collapse and the area and position of the 3250cm−1 band vary exponentially, all of them evolving with the same compaction cross section (σc). The linear dependence σc∝Se (Se being the electronic stopping power) is observed for both pure and mixed water ices, confirming previous results. We suggests that the infrared absorption A-value varies with dose as (1-ζe-D/D0) during the compaction process (D0=0.2eV/molec being the effective energy density to eliminate the OH-db, and ζ is a parameter characterizing the porosity). These findings may be used as a diagnostic tool to probe the morphology of water ices occurring in the outer Solar System and in the ISM.