Carbonic acid by ion implantation in water/carbon dioxide ice mixtures

We present the results of experiments performed by keV ion (He and H) bombardment of frozen mixtures of H2O:CO2 and of pure CO2 ice. Using keV He ions we confirm the already reported measurement of carbonic acid (H2CO3) production in an ice mixture of frozen water and carbon dioxide (1:1) after irradiation by 0.7 MeV H ions (Moore, M. H., and R. K. Khanna 1991. Spectrochim. Acta 47, 255-262; Moore, M. H., R. K. Khanna, and B. Donn 1991. J. Geophys. Res. E 96(2), 17,541-17,545.). Contrary to a previous report (Pirronello, V., W. L. Brown, L. J. Lanzerotti, K. J. Marcantonio, and E. H. Simmons 1982. Astrophys. J. 262, 636-640.), formaldehyde (H2CO), if any, is not a major product. Implantation with hydrogen ions demonstrates that carbonic acid is formed even if the irradiated target is pure CO2; i.e., the implanted ion is incorporated into the target and forms new bonds. Some possible astrophysical applications on Solar System objects or (pre-solar) interstellar grains are discussed. (C) 1997 Academic Press