X-ray photodesorption from methanol ice

The abundances of molecules and ions depend on the mechanisms of their formation and destruction that can occur both in the gas phase and in the condensed phase on grain surfaces. Photodesorption of grain surface species may explain the relative high abundances of gaseous neutral or ionic species detected in cold environments. X-ray photons from young stars are able to penetrate cold and dense regions inside protoplanetary discs, leading to molecular dissociation and desorption of photo-products from icy molecules on grain mantles. This paper aims to experimentally investigate the contribution of ion desorption from methanol ice stimulated by soft X-rays for producing chemically active ions in protoplanetary discs. The measurements were carried out at the Brazilian synchrotron light source (LNLS), using X-ray photons at the methanol O1s resonance energy (537 eV). Some possible pathways for the H- and O- formation from singly charged desorbed ions are suggested. The photodesorption yields for positive and negative ions were determined and compared with previous results obtained using different ionization agents, such as electrons, heavy ions and photons at different energies. We also correlate our results to the ion production in protoplanetary discs.