Flux and fluence effects on the vacuum-UV photodesorption and photoprocessing of CO2 ices

CO2 is a major component of the icy mantles surrounding dust grains in planet and star formation regions. Understanding its photodesorption is crucial for explaining gas phase abundances in the coldest environments of the interstellar medium irradiated by vacuum-UV (VUV) photons. Photodesorption yields determined experimentally from CO2 samples grown at low temperatures (T = 15 K) have been found to be very sensitive to experimental methods and conditions. Several mechanisms have been suggested for explaining the desorption of CO2, O-2 and CO from CO2 ices. In the present study, the cross-sections characterizing the dynamics of photodesorption as a function of photon fluence (determined from released molecules in the gas phase) and of ice composition modification (determined in situ in the solid phase) are compared for the first time for different photon flux conditions (from 7.3 x 10(12) photon per s cm(-2) to 2.2x 10(14) photon per s cm(-2)) using monochromatic synchrotron radiation in the VUV range (on the DESIRS beamline at SOLEIL). This approach reveals that CO and O-2 desorptions are decorrelated from that of CO2. CO and O-2 photodesorption yields depend on photon flux conditions and can be linked to surface chemistry. In contrast, the photodesorption yield of CO2 is independent of the photon flux conditions and can be linked to bulk ice chemical modification, consistently with indirect desorption induced by an electronic transition (DIET) process.