Identification and yield of carbonic acid and formaldehyde in irradiated ices

Carbonic acid ((OH)2CO) was tentatively identified in the infrared spectrum of a proton irradiated CO2 + H2O ice mixture [Moore and Khanna, 1991]. In this report we present additional evidence for a more definitive identification of (OH)2CO with (1) the infrared spectrum of a residue obtained by proton irradiation of CO2 + D2O ice mixture, and (2) the infrared spectra of solid phases of formaldehyde (H2CO), acetone [(CH3)2CO], and dimethyl carbonate [(OCH3)2CO], which are structurally similar to (OH)2CO. 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 H2CO and (OH)2CO by proton irradiation of ice mixtures. Both H2CO and (OH)2CO are possible irradiation products of cometary and planetary ices.