Cryogenic Chemistry and Quantitative Non-Thermal Desorption from Pure Methanol Ices: High-Energy Electron versus X-Ray Induced Processes.

ABSTRACT

X-Ray irradiation of interstellar ice analogues has recently been proven to induce desorption of molecules, thus being a potential source for the still-unexplained presence of gaseous organics in the coldest regions of the interstellar medium, especially in protoplanetary disks. The proposed desorption mechanism involves the Auger decay of excited molecules following soft X-ray absorption, known as X-ray induced electron-stimulated desorption (XESD). Aiming to quantify electron induced desorption in XESD, we irradiated pure methanol (CH3 OH) ices at 23 K with 505 eV electrons, to simulate the Auger electrons originating from the O 1s core absorption. Desorption yields of neutral fragments and the effective methanol depletion cross-section were quantitatively determined by mass spectrometry. We derived desorption yields in molecules per incident electron for CO, CO2 , CH3 OH, CH4 /O, H2 O, H2 CO, C2 H6 and other less abundant but more complex organic products. We obtained desorption yields remarkably similar to XESD values.