Secondary ion emission from CO 2–H 2O ice irradiated by energetic heavy ions : Part I. Measurement of the mass spectra

Abstract

Secondary ion mass spectrometry is used to investigate ion emission from a frozen-gas mixture ( T = 80–90 K) of CO 2 and H 2O bombarded by MeV nitrogen ions and by 252Cf fission fragments (FF). The aim of the experiments is to produce organic molecules in the highly excited material around the nuclear track and to detect them in the flux of sputtered particles. Such sputter processes are known to occur at the icy surfaces of planetary or interstellar objects. Time-of-flight (TOF) mass spectrometry is employed to identify the desorbed ions. Mass spectra of positive and negative ions were taken for several molecular H 2O/CO 2 ratios. In special, positive ions induced by MeV nitrogen beam were analyzed for 9 and 18% H 2O concentrations of the CO 2–H 2O ice and negative ions for ∼5% H 2O. The ion peaks are separated to generate exclusive the spectra of CO 2 specific ions, H 2O specific ions and hybrid molecular ions, the latter ones corresponding to ions that contain mostly H and C atoms. In the mass range from 10 to 320 u, the latter exhibits 35 positive and 58 negative ions. The total yield of the positive ions is 0.35 and 0.57 ions/impact, respectively, and of negative ions 0.066 ions/impact. Unexpected effects of secondary ion sputtering yields on H 2O/CO 2 ratio are attributed to the influence of water molecules concentration on the ionization process.