tokens: an effective exit channel for molecules to desorb to the surface, in line with the conclusion from Yuan Yates ( 2014 ). In contrast, this study demonstrates that the photodesorption yield of CO 2 is not ice-structure dependent, which is incompatible with the preceding works based on IR spectroscopy for quantification (Öberg et al. 2009 ; Yuan Yates 2014 ). As mentioned in Muñoz Caro et al. ( 2016 ), the photodesorption yield of CO ice is independent of ice structure, and the linearly decreasing photodesorption yield for increasing deposition temperature might be due to orientational disorder. Therefore, whether they are polar CO or non-polar CO 2 , their structures are not directly related to the photodesorption yield. This work has implications for the interpretation of gas-phase observations toward cold interstellar and circumstellar environments where ice mantles were submitted to UV irradiation. The ice analog component studied in this paper, pure CO 2 ice, is believed to form in ice mantles by two processes. One is the CO 2 segregation out of the CO 2 –H 2 O mixture, leading to CO 2 inclusions in water ice. The other is a distillation process, in which CO evaporates from a CO 2 –CO ice mixture, leaving pure CO 2 behind (Pontoppidan et al. 2008 ; Kim et al. 2012 ). In the former process, UV irradiation of CO 2 inclusions in water ice produces CO, O 2 and CO 3 molecules that will be mainly retained in the ice with a negligible desorption. The latter evolutionary phase of the ice is favorable for the photodesorption of CO 2 , because CO 2 ice is exposed to the surface of ice mantles, and our experimental results are more directly applicable to this scenario. Only CO 3 is not expected to photodesorb in this case, according to the experiments. In particular, the photodesorption yields of CO 2 , the CO and O 2 photoproducts, in molecules photon −1 , should be valid to estimate the density of these species in the gas of cold environments. In dense clouds, the secondary UV field originated by excitation of molecular hydrogen is about 10 3 –10 4 photons cm −2 s −1 , and should be dominated by the molecular emission lines with a smaller contribution of the more energetic Ly α photons produced by atomic hydrogen. This UV emission spectrum, along with the total value of the UV flux, dictates the values of the photodesorption yields in our experiments. In Section 3.3 of this paper, we report the linear dependence of the photodesorption yields with the UV lamp emission and ice absorption spectra in the UV. Pure CO 2 ice is expected to be in the crystalline form in ice mantles, since the band around 4.30 μ m (2328 cm −1 ) of amorphous CO 2 ice is absent in the observed spectra toward dense clouds and protostars; this suggests that CO 2 ice was formed on the dust at temperatures above 25 K or was submitted later onto these temperatures (Escribano et al. 2013 ). We confirmed that this ice structure reflects the temperature of ice deposition in the
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