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来自紫外线、X射线或其他类型的高能辐射的能量时，它很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，可以在固体中移动很远，在此过程中电离其他分子。然而，大多数情况下，电子的能量很低，不能传播很远，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，但它们的影响可能令人惊讶地重要。这项研究项目涉及LEE和它们在不同类型的材料中可以诱导的化学作用。当LEE束在薄的固体样品上发射时，我们经常观察到电子可以被凝聚的分子暂时俘获而形成瞬时负离子(TnI)，这极大地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，有时也不能进行电子激发时，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可能会与邻近的分子发生反应，产生进一步的化学变化。例如，我们的工作表明，像DNA一样大的分子可以被LEE分解，这一结果对如何利用辐射治疗癌症具有重大意义。在这里，我们建议在两个李氏应用领域进行研究。在聚焦电子束诱导沉积中，高能电子束可以通过将易挥发的“前体分子”分解成更小但更粘性的碎片来建立纳米级结构。许多LEE是由高能束产生的，并被认为对沉积过程有贡献，可能是通过形成TnI。我们打算测量LEE与前体分子的相互作用，以帮助我们更好地理解和控制沉积过程。在星际尘埃云中，通过聚集在颗粒上的凝聚气体进行辐射，李也会被释放出来，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.********