The Astrophysical Journal, 727:27 (15pp), 2011 January 20 doi:10.1088/0004-637X/727/1/27 C 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A. LABORATORY STUDIES ON THE FORMATION OF FORMIC ACID (HCOOH) IN INTERSTELLAR AND COMETARY ICES Chris J. Bennett1,2, Tetsuya Hama3,5, Yong Seol Kim1, Masahiro Kawasaki3, and Ralf I. Kaiser1,2,4 1 Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA;
[email protected] 2 NASA Astrobiology Institute, University of Hawaii, Honolulu, HI 96822, USA 3 Department of Molecular Engineering, Kyoto University, Kyoto 615-8510, Japan Received 2010 July 9; accepted 2010 October 5; published 2010 December 29 ABSTRACT Mixtures of water (H2O) and carbon monoxide (CO) ices were irradiated at 10 K with energetic electrons to simulate the energy transfer processes that occur in the track of galactic cosmic-ray particles penetrating interstellar ices. We identified formic acid (HCOOH) through new absorption bands in the infrared spectra at 1690 and 1224 cm−1 (5.92 and 8.17 μm, respectively). During the subsequent warm-up of the irradiated samples, formic acid is evident from the mass spectrometer signal at the mass-to-charge ratio, m/z = 46 (HCOOH+) as the ice sublimates. The detection of formic acid was confirmed using isotopically labeled water-d2 with carbon monoxide, leading to formic acid-d2 (DCOOD). The temporal fits of the reactants, reaction intermediates, and products elucidate two reaction pathways to formic acid in carbon monoxide–water ices. The reaction is induced by unimolecular decomposition of water forming atomic hydrogen (H) and the hydroxyl radical (OH).