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

自由基是具有一个或多个未配对电子的分子，具有高度的化学反应。这种高度的反应性意味着自由基对形成它们的任何环境的化学成分都有很大的影响，尽管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，如臭氧层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电子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，在这些成膜过程中，自由基与表面相互作用的细节还没有被很好地理解。研究这种自由基表面化学的主要障碍是，目前还没有通用的技术来给只含有感兴趣的自由基的表面进行剂量，例如CH。电流自由基源从前体气体(例如来自C2H2的CH)产生自由基，并且前体气体分子的数量总是多于自由基。因此，如果一个表面是从传统的自由基源剂量，前体分子将是表面上的主要物种，使得几乎不可能用标准的表面科学技术来研究自由基与表面的相互作用。在这一应用中，我们建议开发一种新的自由基源，它将产生不受前体分子污染的自由基物种的“干净”光束。光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光束。然后，通过使用激光束击穿电子，从负离子中产生自由基。这种负离子的光分离将产生感兴趣的自由基物种的干净光束。建议中给出的计算表明，这种方法可以产生实际的自由基通量。然后，可以使用干净的自由基束向表面添加自由基物种，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。我们建议开发源，然后使用它来研究三个具有重要技术意义的薄膜沉积过程中涉及的自由基-表面相互作用。我们的研究揭示的化学成分将极大地提高我们对这些与工业相关的表面反应中正在发生的事情的了解，并使我们能够根据正在发生的化学物质来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.