Context. Astrophysical observations show complex organic molecules (COMs) in the gas phase of protoplanetary disks. X-rays emitted from the central young stellar object that irradiate interstellar ices in the disk, followed by the ejection of molecules in the gas phase, are a possible route to explain the abundances observed in the cold regions. This process, known as X-ray photodesorption, needs to be quantified for methanol-containing ices. This Paper I focuses on the case of X-ray photodesorption from pure methanol ices. Aims. We aim at experimentally measuring X-ray photodesorption yields (in molecule desorbed per incident photon, displayed as molecule/photon for more simplicity) of methanol and its photo-products from pure CH3OH ices, and to shed light on the mechanisms responsible for the desorption process. Methods. We irradiated methanol ices at 15 K with X-rays in the 525–570 eV range from the SEXTANTS beam line of the SOLEIL synchrotron facility. The release of species in the gas phase was monitored by quadrupole mass spectrometry, and photodesorption yields were derived. Results. Under our experimental conditions, the CH3OH X-ray photodesorption yield from pure methanol ice is ~10−2 molecule/photon at 564 eV. Photo-products such as CH4, H2CO, H2O, CO2, and CO also desorb at increasing efficiency. X-ray photodesorption of larger COMs, which can be attributed to either ethanol, dimethyl ether, and/or formic acid, is also detected. The physical mechanisms at play are discussed and must likely involve the thermalization of Auger electrons in the ice, thus indicating that its composition plays an important role. Finally, we provide desorption yields applicable to protoplanetary disk environments for astrochemical models. Conclusions. The X-rays are shown to be a potential candidate to explain gas-phase abundances of methanol in disks. However, more relevant desorption yields derived from experiments on mixed ices are mandatory to properly support the role played by X-rays in nonthermal desorption of methanol (see Paper II).

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.

Pure acetonitrile (CH3CN) and mixed CO:CH3CN and H2O:CH3CN ices have been irradiated at 15 K with vacuum ultraviolet (VUV) photons in the 7–13.6 eV range using synchrotron radiation. VUV photodesorption yields of CH3CN and of photoproducts have been derived as a function of the incident photon energy. The coadsorption of CH3CN with CO and H2O molecules, which are expected to be among the main constituents of interstellar ices, is found to have no significant influence on the VUV photodesorption spectra of CH3CN, CHCN•, HCN, CN•, and CH3•. Contrary to what has generally been evidenced for most of the condensed molecules, these findings point toward a desorption process for which the CH3CN molecule that absorbs the VUV photon is the one desorbing. It can be ejected in the gas phase as intact CH3CN or in the form of its photodissociation fragments. Astrophysical VUV photodesorption yields, applicable to different locations, are derived and can be incorporated into astrochemical modeling. They vary from 0.67(± 0.33) × 10−5 to 2.0(± 1.0) × 10−5 molecule photon−1 for CH3CN depending on the region considered, which is high compared to other organic molecules such as methanol. These results could explain the multiple detections of gas-phase CH3CN in different regions of the interstellar medium and are well correlated to astrophysical observations of the Horsehead nebula and of protoplanetary disks (such as TW Hya and HD 163296).

Context. Astrophysical observations show complex organic molecules (COMs) in the gas phase of protoplanetary disks. X-rays emitted from the central young stellar object (YSO) that irradiate interstellar ices in the disk, followed by the ejection of molecules in the gas phase, are a possible route to explain the abundances observed in the cold regions. This process, known as X-ray photodesorption, needs to be quantified for methanol-containing ices. Aims. We aim at experimentally measuring X-ray photodesorption yields (in molecule desorbed per incident photon, displayed as molecule/photon for more simplicity) of methanol and its photo-products from binary mixed ices: 13CO:CH3OH ice and H2O:CH3OH ice. Methods. We irradiated these ices at 15 K with X-rays in the 525–570 eV range from the SEXTANTS beam line of the SOLEIL synchrotron facility. The release of species in the gas phase was monitored by quadrupole mass spectrometry, and photodesorption yields were derived. Results. For 13CO:CH3OH ice, CH3OH X-ray photodesorption yield is estimated to be ∼10−2 molecule/photon at 564 eV. X-ray photodesorption of larger COMs, which can be attributed to either ethanol, dimethyl ether, and/or formic acid, is detected with a yield of ∼10−3 molecule/photon. When methanol is mixed with water, X-ray photodesorption of methanol and of the previous COMs is not detected. X-ray induced chemistry, dominated by low-energy secondary electrons, is found to be the main mechanism that explains these results. We also provide desorption yields that are applicable to protoplanetary disk environments for astrochemical models. Conclusions. The X-ray emission from YSOs should participate in the enrichment of the protoplanetary disk gas phase with COMs such as methanol in the cold and X-ray dominated regions because of X-ray photodesorption from methanol-containing ices.