The incidence of high-energy radiation onto icy surfaces constitutes an important route for leading new neutral or ionized molecular species back to the gas phase in interstellar and circumstellar environments, especially where thermal desorption is negligible. In order to simulate such processes, an acetonitrile ice (CH3CN) frozen at 120 K is bombarded by high energy electrons, and the desorbing positive ions are analyzed by time-of-flight mass spectrometry (TOF-MS). Several fragment and cluster ions were identified, including the Hn=1-3(+), CHn=0-3(+)/NHn=0-1(+); C2Hn=0-3(+)/CHn=0-3N(+), C2Hn=0-6N(+) ion series and the ion clusters (CH3CN)n=1-2(+) and (CH3CN)n=1-2H(+). The energy dependence on the positive ion desorption yield indicates that ion desorption is initiated by Coulomb explosion following Auger electronic decay. The results presented here suggest that non-thermal desorption processes, such as desorption induced by electronic transitions (DIET) may be responsible for delivering neutral and ionic fragments from simple nitrile-bearing ices to the gas-phase, contributing to the production of more complex molecules. The derived desorption yields per electron impact may contribute to chemical evolution models in different cold astrophysical objects, especially where the abundance of CH3CN is expected to be high.

Acetone, one of the most important molecules in organic chemistry, also a precursor of prebiotic species, was found in the interstellar medium associated with star-forming environments. The mechanisms proposed to explain the gas phase abundance of interstellar acetone are based on grain mantle chemistry. High energy photons coming from the stellar radiation field of the nearby stars interact with the ice mantles on dust grains leading to photoionization, photodissociation, and photodesorption processes. In this work we investigate the photodesorption and the photostability of pure acetone ices due to soft X-ray impact. Absolute desorption yields per photon impact for the main positive ionic fragments were determined at the O 1s resonance energy (531.4 eV). The photostability of acetone ice was studied by exposure to different irradiation doses with a white beam of synchrotron radiation. The degradation of the ice was monitored by NEXAFS around the O 1s threshold. From this study we determine the photodissociation cross-section to be about 1.5 × 10 −17 cm 2 which allowed us to estimate the half-life for acetone ice in astrophysical environments where soft X-rays play an important role in chemical processes.

Methanol and ethanol, precursors of prebiotic molecules, are found in interstellar and circumstellar environments. At low temperatures, electrons may interact with these frozen molecules on dust grain surfaces stimulating desorption of atomic and molecular ions and charged clusters. These heavy fragments released from the icy mantles could contribute to the abundance in the gas phase of organic molecules in such environments. In this work, we investigate the ionic fragments desorbed from methanol and ethanol pure ices due to high energy electron impact. Absolute desorption yields (ions/impact) for each fragment desorbed from the ice surface were determined. Several clusters and heavier molecular ions were observed at higher electron energies. Two mechanisms seem to be involved in the desorption process, namely, the Auger stimulated ion desorption and that of secondary electrons. These data may provide support to establish more accurate astrochemical models and contribute to explain the influence of solar wind on condensed alcohols.

The organic compounds HCN and C 2 H 2 , present in protoplanetary disks, may react to form precursor molecules of the nucleobases, such as the pyrimidine molecule, C 4 H 4 N 2 . Depending on the temperature in a given region of the disk, molecules are in the gas phase or condensed onto grain surfaces. The action of X-ray photons and MeV protons, emitted by the young central star, may lead to several physical and chemical processes in such prestellar environments. In this work we have experimentally investigated the ionization, dissociation and desorption processes of pyrimidine in the condensed and the gas phase stimulated by soft X-rays and protons, respectively. Pyrimidine was frozen at temperatures below 130 K and irradiated with X-rays at energies from 394 to 427 eV. In the gas phase experiment, a pyrimidine effusive jet at room temperature was bombarded with protons of 2.5 MeV. In both experiments, the time-of-flight mass-spectrometry technique was employed. Partial photodesorption ion yields as a function of the X-ray photon energy for ions such as C 3 H 2 + , HC 3 NH + and C 4 H + were determined. The experimental results were applied to conditions of the protoplanetary disk of TW Hydra star. Assuming three density profiles of molecular hydrogen, 1 × 10 6 , 1 × 10 7 and 1 × 10 8 cm −3 , we determined HC 3 NH + ion-production rates of the order of 10 −31 up to 10 −8 ions cm −3 s −1 . Integrating over 1 × 10 6 yr, HC 3 NH + column density values, ranging from 3.47 × 10 9 to 1.29 × 10 13 cm −2 , were obtained as a function of the distance from central star. The optical depth is the main variable that affects ions production. In addition, computational simulations were used to determine the kinetic energies of ions desorbed from pyrimidine ice distributed between ∼ 7 and 15 eV.