Indirect x-ray photodesorption of N215 and CO13 from mixed and layered ices

Abstract

X-ray photodesorption yields of [Formula: see text] and [Formula: see text] are derived as a function of the incident photon energy near the N (∼400 eV) and O K-edge (∼500 eV) for pure [Formula: see text] ice and mixed [Formula: see text]:[Formula: see text] ices. The photodesorption spectra from the mixed ices reveal an indirect desorption mechanism for which the desorption of [Formula: see text] and [Formula: see text] is triggered by the photoabsorption of [Formula: see text] and [Formula: see text], respectively. This mechanism is confirmed by the x-ray photodesorption of [Formula: see text] from a layered [Formula: see text]/[Formula: see text] ice irradiated at 401 eV on the N 1s → π* transition of [Formula: see text]. This latter experiment enables us to quantify the relevant depth involved in the indirect desorption process, which is found to be 30–40 monolayers in that case. This value is further related to the energy transport of Auger electrons emitted from the photoabsorbing [Formula: see text] molecules that scatter toward the ice surface, inducing the desorption of [Formula: see text]. The photodesorption yields corrected from the energy that can participate in the desorption process (expressed in molecules desorbed by eV deposited) do not depend on the photon energy; hence, they depend neither on the photoabsorbing molecule nor on its state after Auger decay. This demonstrates that x-ray induced electron stimulated desorption, mediated by Auger scattering, is the dominant process explaining the desorption of [Formula: see text] and [Formula: see text] from the ices studied in this work.