Electron interactions with small molecular clusters

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Free electrons are present in many environments (the atmosphere, the outer space, irradiated cells, plasmas). Also present in these environments are atoms and molecules. The electrons can interact with the atoms and molecules and change direction or give up some of their energy; scientists call this process a collision. At the same time, the exchange of energy can modify the state of the molecules (excite them), or even break them into smaller molecules or atoms. Understanding these processes is very important for the understanding of the aforementioned environments. Knowledge of collisional information is required for many practical applications. For example, free electrons are present in the cells of living organisms if they receive radiation. It is now known that these electrons can break-up the DNA, damaging our bodies. But, fortunately, we can also use this interaction in a positive way. Cancer treatments based on radiotherapy make use of these processes to destroy cancerous cells. By understanding how electrons interact with the DNA, we can improve these treatments. Electron-molecule collisions also take place in astrophysical environments. It is though that the precursors of life (organic and biomolecules) may have been created in outer space due to the interaction of electrons with atoms and molecules. There are also technological applications for which the knowledge of the collisional process is important: the microchip industry makes use of electron-molecule collisions to generate radicals (very reactive molecular fragments) that react with a silicon oxide substrate thus etching a circuit on it. Scientists have studied collisions for many years, both by means of experiments and using theoretical models and computers. Many methods have been proposed, but due to the complexity of the processes, not many electron-molecule collisions have been studied properly from a theoretical point of view. To describe a collision correctly one has to be able to describe the molecule (the target) accurately and also its interaction with the projectile (the free electron). The bigger the target is (the more nuclei and electrons it has) the more difficult it is to describe its states properly: one needs to use good, accurate methods but also big, powerful computers in an efficient way. There is an additional complication when one studies collisions in the fields for which information on electron-molecule interactions is important: the molecules are not isolated. The electrons collide with molecules that are surrounded by other molecules or atoms (for example, in a cell there is plenty of water molecules). These surrounding particles can significantly affect the outcome of the collisions in many ways. Hence, it is important to think of models to incorporate these effects in the calculations. Clusters (groups of a few molecules) are somehow in between isolated molecules and solids and liquids. Because they involve fewer particles than the condensed phase, studying them is easier. These studies are a good starting point to develop methods to treat electron collisions with condensed molecules.Hopefully, this will help us to achieve a new level of understanding of electron-molecule process and will also provide us with the tools to investigate from the theoretical point of view a lot of processes that are very relevant in our lives nowadays.