Nitrogen condensation on water ice

Abstract

We studied, by infrared absorption spectroscopy, icy samples (16 K) of pure water, a mixture N2:H2O=100:1, and a sample made of N2 condensed on water ice and diffused in it after warm up to 30 K. We concentrated our efforts in two spectral regions around 3700 cm–1 where the feature due to the O–H dangling bonds in porous amorphous water falls and around 5000 cm–1 where a broad water band is present. We found that in the N2:H2=100:1 mixture the profile of the broad water feature at about 5000 cm–1 dramatically changed to a very narrow band at about 5300 cm–1. When N2 diffuses in water ice a feature at about 5300 cm–1 appears along with the broad 5000 cm–1 band. We also studied some of the effects of ion irradiation (Ar++, 60 keV ions) on these icy samples. We found that after processing the feature due to the O–H dangling bonds it reduced in intensity and eventually disappeared. Here we present the experimental results, discuss their astrophysical relevance and suggest that a band at about 5300 cm–1 (1.88 µm) should be searched for on icy surfaces in the outer Solar System, namely Pluto, Triton, Edgeworth–Kuiper Belt Objects, and Centaurs. PACS No.: 68.43Pg