Pure beams of free radicals for studies of radical-surface chemistry

自由基是一种具有一个或多个未成对电子的分子，具有高度的化学活性。这种高反应性意味着自由基对它们形成的任何环境的化学性质产生重大影响，即使它们通常不是最丰富的物种。例如，自由基化学控制着重要的大气现象，如臭氧空洞和光化学烟雾的形成。自由基与表面的相互作用被认为在控制由放电形成薄膜的化学过程中至关重要，即所谓的等离子体辅助沉积。这种薄膜在集成电路和太阳能电池等领域具有巨大的技术重要性。然而，在这些膜形成过程中的自由基与表面的相互作用的细节还没有很好地理解。研究自由基表面化学的主要障碍是，目前还没有通用技术可以仅用感兴趣的自由基（例如CH）给表面注入。当前的自由基源从前体气体（例如来自C2 H2的CH）中产生自由基，并且前体气体分子的数量总是超过自由基。因此，如果一个表面是从一个传统的自由基源剂量，前体分子将是表面上的主要物种，使它几乎不可能研究的自由基与表面的相互作用，使用标准的表面科学techniques.In此应用程序中，我们提出了一个新的自由基源的发展，这将产生“干净”的光束的自由基物种，不受污染的前体分子。源将通过产生负离子（例如CH-）来工作，负离子可以被质量选择以形成干净的束。然后用激光束将负离子中的电子打下来，产生自由基。负离子的光分离将产生一束干净的感兴趣的自由基。该方案中给出的计算结果表明，通过这种方法可以产生一个实用的自由基通量。然后，可以使用干净的自由基束向表面注入自由基物质，并可以使用表面科学的标准技术来研究表面，以揭示自由基粘附和表面化学的细节。我们建议开发的源，然后用它来研究的自由基表面的相互作用，在三个技术上重要的薄膜沉积过程。我们的研究揭示的化学将大大提高我们对这些工业相关表面反应中发生的事情的理解，并使我们能够根据正在发生的化学反应优化和改进这些沉积过程。

Free radicals, molecules with one or more unpaired electrons, are highly chemically reactive. This high reactivity means that free radicals exert a major influence on the chemistry of any environment in which they are formed, even though they are often not the most abundant species. For example, it is free radical chemistry that controls important atmospheric phenomena such as the ozone hole and the formation of photochemical smog.The interactions of free radicals with surfaces are thought to be vitally important in controlling the chemistry of formation of thin-films from electrical discharges, so called plasma-assisted deposition. Such thin-films have enormous technological importance in fields such as integrated circuits and solar cells. However, the details of the interactions of the radicals with the surfaces in these film formation processes are not well understood. The major hurdle to investigating this surface chemistry of free radicals is that, currently, there is no general technique available to dose a surface with only the free radical of interest, for example CH. Current radical sources generate the radicals from a precursor gas (e.g. CH from C2H2) and the precursor gas molecules always outnumber the radicals. Thus if a surface was dosed from a conventional radical source, the precursor molecules would be the dominant species on the surface, making it almost impossible to study the interactions of the radical with the surface using standard surface science techniques.In this application, we propose the development of a new source of free radicals which will generate "clean" beams of the radical species, uncontaminated by the precursor molecule. The source will work by generating negative ions (e.g. CH-), which can be mass selected to form a clean beam. The radicals are then generated from the negative ions by using a laser beam to knock off the electron. This photo-detachment of negative ions will yield a clean beam of the radical species of interest. Calculations given in the proposal show that a practical flux of free radicals can be generated by this methodology.The clean beams of free radicals can then be used to dose the surface with the radical species, and the surface can be studied using the standard techniques of surface science to reveal the details of the radicals sticking and surface chemistry. We propose to develop the source and then use it to study the radical-surface interactions involved in three technologically important film deposition processes. The chemistry revealed by our investigations will dramatically improve our understanding of what is going on in these industrially relevant surface reactions and allow us to optimize and refine these deposition processes in the light of the chemistry that is occurring.