|dc.description.abstract||Astronomical observations show that the interstellar space is not empty. Typically, interstellar regions are characterized by low density and low kinetic temperature, although at some locations extreme densities and temperatures can occur. The interstellar medium is exposed to various radiation sources, such as cosmic rays and starlight (ultraviolet and X-rays), and contains magnetic fields.
In regions over-pressured with respect to the average pressure of the general interstellar medium (called diffuse and dense molecular clouds) are present a number of free floating simple molecules (such H2 and CO), able to reach concentrations high enough to be detected. These molecules owe their existence to tiny solid particles (the so called cosmic dust) ~ extendash~ sparsely populating the interstellar medium ~ extendash~ composed by silicate and carbonaceous materials. The primary role of dust in molecular survival consist in the (partial) shield of clouds from ambient starlight. Therefore, molecules in cloud interiors are protected from photodissociation by UV starlight and have a long lifetime. A complex chemistry is therefore able to build up in cloud interiors. However, hydrogen molecules cannot be formed by gas-phase reactions and are formed in reactions at the surfaces of dust grains, as demonstrated by a number of appropriate laboratory experiments.
In highly shielded regions (dark clouds) where the starlight is largely excluded, dust grains should be coated with icy mantles of condensed gas. This is indeed what is observed in very dense regions, in high- and low-mass cores in star-forming regions, where water, carbon monoxide and dioxide, methanol, formaldehyde, methane, ammonia, and other species are observe to be resident on dust grains. More complex species, such as ethanol, acetic acid, and glycolaldehyde, are detected at relatively high abundances in these regions. These molecules, composed by the most important biogenic elements (hydrogen, carbon, nitrogen, and oxygen) are mainly organic species, and are considered by some astrochemists to be related to the emerging subject of astrobiology. These larger astronomical molecular species are called complex organic molecules, or COMs, for short. The basic idea for the formation of these molecules is that simple hydrogenated molecules like H2CO, CH3OH and NH3 are formed on the cold grain surfaces. Then, the chemical complexity can be dramatically enhanced, through some form of solid-state chemistry.
Stars are formed by the gravitational collapse of molecular clouds. In the very initial phases, through the combined action of accretion and jet flows, the nascent star (called protostar), gradually transforms the collapsing cloud into a flat disk called the protoplanetary disk, because it constitutes the raw material from which planets will form.
In this work, I will perform laboratory experiments, investigating how simple interstellar ices may form COMs under the transforming action of energetic sources characteristic of interstellar and circumstellar regions, electrons and X-rays, and I shall also address the specific problem on how chemical complexity arising in the ices could be ejected into the gas-phase without thermal desorption.
The experiments presented in this dissertation have been performed with the new Interstellar Energetic-Process System, an ultra-high vacuum facility specifically designed for the study of the irradiation of interstellar and circumstellar ices. Electrons are produced trough an electron gun, while X-rays have been collected at the at National Synchrotron Radiation Research Center. Synchrotron light sources are ideal because of their high intensity and wide wavelength coverage. The X-ray spectrum ranges between 250 and 1250 eV, with a shape roughly resembling the X-ray spectrum of a young solar-type star. In the electron radiolysis I use energies from 150 to 1000 eV. Such a range is similar to the primary electron spectrum produced by X-rays emitted by T-Tauri stars or cosmic-rays interacting with the gaseous interstellar medium. In this thesis I study three kinds of samples, pure CO ices, a binary mixtures of H2O:CO, and a ternary mixture of H2O:CO:NH3. In the first cases, irradiation has been performed with both X-rays and electrons, while the ternary mixture is irradiated with just X-rays.
Pure CO ice and H2O:CO ice mixture irradiated by X-rays and electrons produce the same chemical species. X-rays irradiation of ices is the result of photons and particles processing. The absorption of X-rays results in the ionization of an inner shell of atoms composing a molecule, in which a photo-electron (the primary electron) is ejected. As an electron from the higher energy levels fills the core vacancy a second electron, the Auger electron, is ejected into the continuum. These two electrons will deposit and degrade their energies interacting with the ice materials, and creating a cascade of secondary electron that drives the chemistry in the ices. On the other hand, in the electron radiolysis, electrons impinging on the ice can excite and ionize the molecules or if sufficiently energetic can (not the case in our experiments), as X-rays, ionize inner-shell electrons of the atoms in the molecules. As in the previous case involving X-rays, the electrons slow down loosing their energy interacting with the ice, and producing secondary electrons. Therefore, the chemistry promoted in X-ray irradiation or electron radiolysis are mainly promoted by the secondary electrons.
In H2O:CO ice mixture, because of the rich environment in H atoms and OH radicals, hydrogenation of carbon monoxide (e.g., formaldehyde, and methanol) and oxidation (e.g., carbon dioxide) are the main reactions. X-rays irradiated H2O:CO:NH3 produces many organic compounds of prebiotic relevance, such as isocyanic acid, formamide, and the simplest amino acid, glycine.
During X-rays irradiation, several masses have been observed to leave the ice, despite the very low ice temperature, 11 K, well below its sublimation temperature. The desorption of carbon dioxide have been detected in all samples, and for all radiolysis. In electron irradiated CO ice experiments, it is found that almost all of the CO2 formed in the ice desorbes, while only partial CO2 desorption occurred in water mixtures. The desorbing ice fraction is related to the energy of the impinging electrons.
In X-rays irradiated H2O:CO:NH3 ice mixtures, QMS detections are consistent with photo-desorption of COMs such as methyl isocyanate, formic acid, and formamide. If this would be actually the case, the controversy on the chemical origin of such species would be definitely solved in favor of solid state reaction channels.||en_US|