Infrared study of ion-irradiated N2-dominated ices relevant to Triton and Pluto:: formation of HCN and HNC

Infrared spectra and radiation chemical behavior of N-2-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated N-2-rich ices containing CH, and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (CH2N2) form in the solid phase, along with several radicals. NH3 is also identified in irradiated N-2 + CH4 and N-2 + CH4 + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial N-2/CH4 ratios from 100 to 4, and in three-component mixtures have an initial N-2/(CH4 + CO) ratio of 50. HCN and HNC are not detected in N-2-dominated ices when CH4 is replaced with C2H6, C2H2, or CH3OH. The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated N-2 + CH4 and N-2 + CH4 + CO ices. In addition, the HNC/HCN ratio is calculated to be similar to 1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in N2-rich ices containing CH4. We examine the evolution of spectral features due to acid-base reactions (acids such as HCN, HNC, and HNCO and a base, NH3) triggered by warming irradiated ices from 12 K to 30-35 K. We identify anions (OCN-, CN-, and N-3(-)) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, NH3, NH4OCN, and NH4CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides. (C) 2003 Elsevier Science (USA). All rights reserved.