Comparison between IR spectra of interstellar and laboratory ices. The top three curves are infrared spectra of young stellar sources obtained by the Infrared Space Observatory (ISO). Lower traces indicate different laboratory spectra of irradiated H₂O:CO₂ ices: a) and c) (this work); b) (Hudson & Moore 2001); d) (Gerakines et al. 2000); e) (Strazzulla et al. 2005b); f) (UV photons, Gerakines et al. 2000). Vertical dashed lines indicate the location of vibrational modes of frozen H₂CO₃.