Flux and fluence effects on the Vacuum-UV photodesorption and photoprocessing of CO$_2$ ices

TL;DR

This study investigates how photon fluence and ice composition affect the vacuum-UV photodesorption of CO2 ices, revealing different mechanisms for CO, O2, and CO2 desorption relevant to interstellar environments.

Contribution

It compares the photodesorption cross sections as functions of photon fluence and ice composition under various flux conditions using synchrotron radiation, highlighting distinct desorption mechanisms.

Findings

CO and O2 desorption depend on photon flux and surface chemistry.

CO2 desorption is flux-independent and linked to bulk ice modifications.

Photodesorption mechanisms differ for CO, O2, and CO2.

Abstract

CO$_2$ 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 CO$_2$ 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 CO$_2$, O$_2$ and CO from CO$_2$ 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$\times 10^{12}$~photon/s/cm$^2$ to 2.2$\times 10^{14}$~photon/s/cm$^2$) using monochromatic synchrotron radiation in the VUV range (on the DESIRS beamline at SOLEIL). This approach reveals that CO and O$_2$ desorption are decorrelated from that of CO$_2$. CO and O$_2$ photodesorption yields depend on photon flux conditions and can be linked to surface chemistry. By contrast, the phodesorption yield of CO$_2$ is independent of the photon flux conditions and can be linked to bulk ice chemical modification, consistently with an indirect desorption induced by electronic transition (DIET) process.