Identification and yield of carbonic acid and formaldehyde in irradiated ices

Abstract

Carbonic acid ((OH) 2 CO) was tentatively identified in the infrared spectrum of a proton irradiated CO 2 + H 2 O ice mixture [ Moore and Khanna , 1991]. In this report we present additional evidence for a more definitive identification of (OH) 2 CO with (1) the infrared spectrum of a residue obtained by proton irradiation of CO 2 + D 2 O ice mixture, and (2) the infrared spectra of solid phases of formaldehyde (H 2 CO), acetone [(CH 3 ) 2 CO], and dimethyl carbonate [(OCH 3 ) 2 CO], which are structurally similar to (OH) 2 CO. Infrared characteristics (peak frequencies and complex refractive indices of the compounds in point 2) are also reported. In particular, the integrated absorption coefficients for the C=O band for the compounds in point 2 do not vary by more than 20%. Based on these values, we estimate the yields of H 2 CO and (OH) 2 CO by proton irradiation of ice mixtures. Both H 2 CO and (OH) 2 CO are possible irradiation products of cometary and planetary ices.