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Title: Molecular anions in circumstellar envelopes, interstellar clouds and planetary atmospheres: quantum dynamics of formation and evolution
Authors: Carelli, Fabio
Keywords: Astrochemistry
negative ions in the interstellar medium
Issue Date: 21-Dec-2011
Abstract: Nowadays, it is a well known fact that most of the matter in our Solar System, in our Galaxy and, probably, within the whole Universe, exists in the form of ionized particles. For decades astronomers and astrophysicists believed that only positively charged ions were worthy of relevance in drawing the networks for possible chemical reactions in the interstellar medium, as well as in modeling the physical conditions in most of astrophysical environments. Thus, negative ions (and especially molecular negative ions) received minor attention until their possible existence was observationally confirmed (discovery of the first interstellar anion, C6H-), about thirty years after the first physically reasonable proposal on their actual detection was theoretically surmised by E.Herbst (1981). From a purely theoretical point of view, negatively charged ions play a peculiar role as they can be formed in large quantities in the gas phase by attachment of low (1.5 eV < E < 10 eV) and very low-energy (E < 1.5 eV) free electrons, while sometimes such a formation process can occur even close to zero eV. In an astrophysical context, roughly speaking, their role should be then found in their involvement in the charge balance as well as in the chemical evolution of the considered environment: depending on their amount and on the global gas density, in fact, the possible evolutive scenario could be susceptible of marked variations on the estimated time needed for reaching the steady state, their presence having thus also important repercussions on the final chemical composition of a given environment. In contrast, the formation of positive ions requires an energy amount equal to or greater than the first ionization potential of the neutral molecular species considered, i.e. an energy around 8-10 eV for most of organic molecules, a value which is considerably high for several astronomical contexts, like cold dense interstellar molecular clouds and, with less importance, within diffuse molecular gaseous regions. Low-energy electrons, usually considered in the 0-10 eV range, can be very reactive in the sense that they are effectively captured by many molecules, which can then undergo either rapid stabilization toward the undissociated species or fast unimolecular decomposition toward specific fragments, where, in the latter case, the extra electron initially captured by the neutral molecular target, can in turn remain stuck to one of the fragments as well as be re-emitted again in the surroundings with a different (usually lower) kinetic energy. Consequently, it can play a role in heating the gaseous medium at a molecular level. Furthermore, low-energy electrons can also be ’simply’ deflected and then causing vibro-rotational excitation of the target molecules, therefore, playing once more a role in the heating processes at a molecular level. On a much larger scale, they could even produce conductivity inhomogeneity, which in turn disturbs the radiowave propagation in such a medium, like on the edge or on the tail of an interstellar shock, or in principle also moving across a dense cloud: from this point of view, electrons can be also seen as an important and versatile source for ’decoupling’, at least in the low frequency range, the energy transfer from the interstellar radiative field to molecules and nanoparticles. Hence, in a qualitative sense, all of these nanoscopic mechanisms could play a crucial role in understanding more deeply the local interactions within different ’phases’ of the ISM, ranging from the cold dense molecular gas up to the diffuse molecular component. In fact, if we look at the stars, with high energy thermonuclear burning reactions in their interiors, as the primary sources of almost all the energy that is released in the seemingly empty interstellar space, then we can look at the free electrons as a flexible ’means’ which, in competition with photons, is able to participate into the complex processes responsible for the coupling between the ionized, the dust and the neutral components of the ISM. Generally speaking, the main reasons that originally motivated us to undertake the present work, were at least two. First of all, we intended to demonstrate the importance of resonances in forming molecular anions in different astrophysical environments. Secondly, we were attracted by the possibility of investigating with a reliable and suitably approximate approach, the occurrence of radiationless paths like intramolecular vibrational redistributions (IVRs) to account for the dissipation of the extra energy initially carried by the impinging electron. The former aim, which is of course a common feature to each of the molecules investigated here, can be easily justified on the basis of the most important attempts recently dedicated to the understanding of anion formation processes in several astrophysical environments, where indeed only s-wave electron attachment processes were expressly considered. In the framework of the electron-molecule collisions, once the physical conditions like the mean free electron density and the mean kinetic electron temperature are defined according to the specific astronomical context under investigation, taking in consideration only s-wave electron attachment processes to account for the formation of molecular anions provides, in fact, a reliable but narrow point of view. In a qualitative sense, resonant electron attachment can be viewed as the doorway for the ensuing possible formation of the stable molecular anion, provided that the neutral molecule has a positive electron affinity. It therefore follows that the resonant contributions associated to partial waves referred to non-spherical angular momentum states, can account for the formation of molecular anions also for non-vanishing collision energies which can be still relevant in the given environment. In this framework, once one has determined which are the astrophysically relevant resonances for a given molecule, it becomes of interest to also investigate which is the possible evolution of those resonant species: the radiative stabilization for the extra energy content that characterizes the resonant species, in fact, is usually a slow process, that can even exceeds the resonance’s lifetime, so that in such a case the autodetachment becomes strongly competitive. On the other hand, the radiationless IVR constitutes in general another reliable evolutive path that, conversely to the spontaneous emission of a 'high-energy' photon, can efficiently lead to the stabilization of the metastable anion; at the same time, according to the non-ergodic vibrational redistribution, which for a pletora of molecules is an actual possibility, such a process can also account for specific fragmentation channels, as will be shown in details in the last chapter, where all the findings shall be analised and discussed. The present PhD thesis is focusing on electron-molecule interactions with astrophysically relevant molecules, and therefore represents a theoretical/computational work which deals with an area placed at the boundary between (molecular) astrophysics, quantum collision thery, and of course theoretical chemistry. The three molecular species that will be computationally investigated for their behaviour under low-energy electron collisions are the ortho-benzyne (o-C6H4), the coronene (C24H12), and the carbon nitride (NC2N), respectively. Due to their sizes, their peculiar structures, their chemical reactivity and their physical and chemical properties as well as according to astronomical observations, when available, the above three molecules provide representative examples for different astrophysical contexts. However, each of them is linked to a specific astrophysical aspect that currently constitutes an intriguing scientific challenge, hence the present study.
Research interests: Theoretical Chemistry, Quantum Collision Theory, Molecular Astrophysics

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