Mechanism of Indirect Photon-Induced Desorption at the Water Ice Surface

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, O2 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 × 1012 photon per s cm-2 to 2.2× 1014 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 O2 desorptions are decorrelated from that of CO2. CO and O2 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.

Vacuum UV photodesorption of organics in the interstellar medium: an experimental study of formic acid HCOOH and methyl formate HCOOCH3-containing ices

Being a potential process that could explain gas phase abundances of so-called complex organic molecules (COMs) in the cold interstellar medium (ISM), the UV photon-induced desorption from organics-containing molecular ices has been experimentally studied. In this work, we focused on the observation of the photodesorbed products and the measurement of the associated photodesorption yields from pure and mixed molecular ices, each containing organic molecules whose detection has been achieved in the gas phase of the cold ISM, namely formic acid HCOOH and methyl formate HCOOCH3. Each molecule, in pure ice or in ice mixed with CO or water, was irradiated at 15 K with monochromatic vacuum UV photons in the 7-14 eV range using synchrotron radiation from the SOLEIL synchrotron facility, DESIRS beamline. Photodesorption yields of the intact molecules and of the photoproducts were derived as a function of the incident photon energy. Experiments have revealed that the desorbing species match the photodissociation pattern of each isolated molecule, with little influence of the kind of ice (pure or mixed in CO or H2O-rich environment). For both species, the photodesorption of the intact organics is found to be negligible in our experimental conditions, resulting in yields typically below 10-5 ejected molecules per incident photon. The results obtained on HCOOH and HCOOCH3-containing ices are similar to what has already been found for methanol-containing ices, but contrast with the case of another complex molecule, CH3CN, photodesorption of which has been recently studied. Such experimental results may be linked to the observation of COMs in protoplanetary disks, in which CH3CN is commonly observed whereas HCOOH or methanol are detected only in some sources, HCOOCH3 not being detected at all.

Indirect X-ray photodesorption of 15N2 and 13CO from mixed and layered ices

X-ray photodesorption yields of 15N2 and 13CO are derived as a function of the incident photon energy near the N (~ 400 eV) and O K-edge (~ 500 eV) for pure 15N2 ice and mixed 13CO:15N2 ices. The photodesorption spectra from the mixed ices reveal an indirect desorption mechanism for which the desorption of 15N2 and 13CO is triggered by the photo-absorption of respectively 13CO and 15N2. This mechanism is confirmed by the X-ray photodesorption of 13CO from a layered 13CO/15N2 ice irradiated at 401 eV, on the N 1s -> π* transition of 15N2. This latter experiment enables to quantify the relevant depth involved in the indirect desorption process, which is found to be 30 - 40 ML in that case. This value is further related to the energy transport of Auger electrons emitted from the photo-absorbing 15N2molecules that scatter towards the ice surface, inducing the desorption of 13CO. The photodesorption yields corrected from the energy that can participate to the desorption process (expressed in molecules desorbed by eV deposited) do not depend on the photon energy hence neither on the photo-absorbing molecule nor on its state after Auger decay. This demonstrates that X-ray induced electron stimulated desorption (XESD), mediated by Auger scattering, is the dominant process explaining the desorption of 15N2 and 13CO from the ices studied in this work.