Showing papers by "Maria Elisabetta Palumbo published in 2004"

Formation of CO and CO2 Molecules by Ion Irradiation of Water Ice-covered Hydrogenated Carbon Grains

Abstract: We present the results of experiments aimed at studying the influence of the type of grain on the chemical composition of the ice mantles during energetic processing under simulated dense medium conditions. Formation of CO and CO2 molecules occurs when hydrogenated carbon grains with a water ice cap are irradiated with 30 keV He+ ions at low temperature. The fraction of carbon in the grains converted to CO and CO2 by ions is at least 0.03 and 0.02, respectively. An estimation of the formation cross section of these molecules by 30 keV He+ ions has been derived from the intensity increase of their infrared stretching bands as a function of the ion fluence. On the basis of the laboratory results, it has been possible to evaluate the contribution of CO and CO2 produced on carbon grain by cosmic rays to the observed column densities of these molecules for dense clouds whose visual extinction is known. The mechanism we have studied does not dominate other CO2 formation processes; however, its contribution is in addition to other processes occurring on ice mantles. The spectral profile and the contribution to the observed column densities make solid CO formed by cosmic-ray irradiation of ice-layered carbon grains a good candidate for the red component of the interstellar CO stretching feature, which is generally attributed to CO mixed in with water ice. As a consequence of the formation of CO and CO2 molecules on carbon grains, a slow chemical erosion of the particles takes place.

A Raman study of ion irradiated icy mixtures

Abstract: In this paper we present a Raman study of pure CH4 ,H 2O:CH4:N2 and CH3OH:N2 frozen films before and after ion irradiation at 12 K, 100 K and 300 K. By means of Raman spectroscopy, we monitor the structural evolution of each film, whose chemical and physical properties are deeply modified by the interaction with the ion beam. For the two methane containing samples, Raman spectra show that the initial ice is partially converted into a refractory residue, which under further irradiation evolves towards an amorphous carbon (AC) with a band near 1560 cm 1 (G line) and a shoulder at about 1360 cm 1 (D line). No evidence of the AC Raman band is seen in the spectra of the methanol-containing mixture. By means of Lorentzian fits, we have determined the specific parameters of the AC band (G and D line peak positions, widths and relative intensities) in our spectra after ion irradiation and we have compared them with the corresponding parameters of the band as observed in the spectra of 11 IDPs (Interplanetary Dust Particles). Here we present the experimental results and discuss their contribution to our knowledge of the origin and evolution of IDPs.

Raman spectroscopy of ion-irradiated interplanetary carbon dust analogues

Abstract: Interplanetary dust particles (IDPs) and meteorites provide a unique opportunity to study extraterrestrial materials in the laboratory. Different Raman studies have shown that most of IDPs exhibit the characteristic amorphous carbon Raman feature. Different degrees of order have been recognized in the amorphous carbon phase of IDPs, testifying either to different origins or to different processing under different physical conditions (temperature, pressure, etc.). This paper presents a comparison between the amorphous carbon Raman features of IDPs and those of carbon dust analogues obtained in the laboratory by ion irradiation of carbon-containing frozen gases and by arc discharge. We propose a possible mechanism able to induce an ‘evolution’ of IDPs. In particular, amorphous carbon with different degrees of order could be indicative of different irradiation doses by solar wind particles and fast solar protons suffered by IDPs in the interplanetary medium before collection in the Earth's atmosphere. Copyright © 2004 John Wiley & Sons, Ltd.