Low energy electron induced processes of relevance to focused electron beam induced processing, astrobiology and related fields

当固体中的一个分子吸收紫外线、X射线或其他类型的高能辐射的能量时，它可能会被电离并射出一个电子。偶尔，这个电子会有相当大的能量，可以在固体中传播很远，电离沿途的其他分子沿着。然而，大多数情况下，电子的能量很低，不能走得很远，也不能电离更多的分子。在大多数辐射损伤模型中，这种低能电子（LEE）及其影响通常被忽略，但它们的影响可能非常重要。该研究项目涉及LEE及其在不同类型材料中诱导的化学反应。当在薄固体样品上发射LEE束时，我们经常观察到电子可以被凝聚分子暂时捕获以形成瞬态负离子（TNI），这大大增强了向分子的能量转移。当入射电子的能量太小而不能电离，有时也不能激发电子时，TNI的形成也会导致分子的碎裂。以这种方式产生的分子碎片然后可以与邻近的分子反应，产生进一步的化学变化。例如，我们的工作表明，像DNA这样大的分子可以被LEE破坏，这一结果对如何使用辐射治疗癌症具有重要意义。在这里，我们提出了两个LEE应用领域的研究。在聚焦电子束诱导沉积中，高能电子束可用于通过将挥发性“前体分子”破碎成更小但更粘的碎片来构建纳米级结构。许多LEE是由高能束产生的，并且被认为可能通过形成TNI而有助于沉积过程。我们打算测量LEE如何与前体分子相互作用，以帮助提高我们对沉积过程的理解和控制。在星际尘埃云中，LEE也被通过聚集在颗粒上的冷凝气体的通道辐射释放出来，因此可能在简单有机分子的合成中发挥作用，这些分子是已经在陨石上检测到的生命的基石。在我们的实验中，我们在实验室中模拟了这样的过程。

When a molecule in a solid adsorbs energy from ultra-violet light, x-rays or another type of high energy radiation, it will likely be ionised and eject an electron. Occasionally this electron will have considerable energy and can travel far in the solid, ionizing other molecules along the way. Mostly however, the electron will be of low energy and unable to travel very far or to ionise further molecules. In most models of radiation damage, such low energy electrons (LEEs) and their effects are usually ignored, but their effect can be surprisingly important. This research program concerns LEEs and the chemistry they can induce in different types of materials. When firing beams of LEEs on thin solid samples, we have often observed that an electron can be temporarily captured by a condensed molecule to form a transient negative ion (TNI) and this greatly enhances energy transfer to the molecule. Formation of a TNI can also cause the fragmentation of the molecule, when the incident electron's energy is too small for ionization and sometimes for electronic excitation. Molecular fragments created in this way may then react with neighbouring molecules producing further chemical change. For example, our work has shown that molecules as large as DNA can be broken by LEEs, a result with significant implications for how radiation is used to treat cancer. Here we propose research in two LEE application areas. In Focused Electron Beam Induced Deposition, beams of high energy electrons can be used to build nanometer scale structures by breaking volatile "precursor molecules" onto smaller, but stickier fragments. Many LEEs are generated by the high energy beams and are thought to contribute to the deposition process possibly by the formation of TNI. We intend to measure how LEEs interact with precursor molecules to help improve our understanding and control of the deposition process. In interstellar dust clouds , LEEs are also liberated by the passage radiation through the condensed gases that accumulate on grains and may thus play a role in the synthesis of the simple organic molecules, building blocks for life that have already been detected on meteorites. In our experiments, we simulate such processes in the laboratory.