Electron interactions with small molecular clusters

自由电子存在于许多环境中（大气、外层空间、受辐照的细胞、等离子体）。原子和分子也存在于这些环境中。电子可以与原子和分子相互作用，改变方向或放弃一些能量；科学家称这个过程为碰撞。同时，能量的交换可以改变分子的状态（激发它们），甚至将它们分解成更小的分子或原子。理解这些过程对于理解上述环境非常重要。许多实际应用都需要了解碰撞信息。例如，如果生物体的细胞受到辐射，它们就会产生自由电子。现在已经知道，这些电子可以分解DNA，损害我们的身体。但幸运的是，我们也可以以积极的方式利用这种互动。基于放射治疗的癌症治疗利用这些过程来摧毁癌细胞。通过了解电子如何与DNA相互作用，我们可以改进这些治疗方法。电子-分子碰撞也发生在天体物理环境中。不过，由于电子与原子和分子的相互作用，生命的前体（有机分子和生物分子）可能是在外层空间产生的。在一些技术应用中，对碰撞过程的了解也很重要：微芯片工业利用电子-分子碰撞产生自由基（非常活跃的分子碎片），这些自由基与氧化硅衬底发生反应，从而在其上蚀刻电路。多年来，科学家们通过实验、理论模型和计算机研究碰撞。人们提出了许多方法，但由于过程的复杂性，从理论的角度对电子-分子碰撞进行了适当的研究。为了正确地描述一次碰撞，人们必须能够准确地描述分子（目标）以及它与抛射体（自由电子）的相互作用。目标越大（原子核和电子越多），就越难以正确地描述它的状态：人们需要使用好的、精确的方法，但也需要高效的、强大的计算机。当研究电子-分子相互作用信息很重要的场中的碰撞时，还有一个额外的复杂问题：分子不是孤立的。电子与被其他分子或原子包围的分子碰撞（例如，在一个细胞中有大量的水分子）。这些周围的粒子可以在许多方面显著地影响碰撞的结果。因此，考虑在计算中纳入这些影响的模型是很重要的。簇（由少数分子组成的群体）介于孤立的分子、固体和液体之间。因为它们比凝聚相包含更少的粒子，所以研究它们更容易。这些研究是发展处理电子与凝聚分子碰撞的方法的一个很好的起点。希望这将有助于我们对电子-分子过程的理解达到一个新的水平，也将为我们从理论的角度研究许多与我们生活中非常相关的过程提供工具。

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.