用快电子探索分子和生物分子体系电子跃迁的空间特征
Exploring the spatial features of electronic transitions in molecular and biomolecular systems by swift electrons
摘要：
电子能量损失光谱学是一个强大的工具，探索电子（以及振动）激发的物质，包括分子。在扫描透射电子显微镜中进行，这种技术是基于薄样品中快电子的非弹性散射。最近，新的电子光学配置已被引入，开辟了分析出射电子的轨道角动量的单个分量的途径，其传达了关于所研究的激发的空间特征的额外信息：因此建议了用于金属纳米结构的创新的双分散光谱实验。我们建议在这里扩展这种技术来探测分子和超分子系统，设计新的实验：使用最先进的量子化学方法来描述的分子系统中存在的电子束的配置，避免分子损伤，我们表明，散射电子获得诱导分子跃迁势的不同方位角的组件。对尺寸不断增大的系统进行的数值模拟指出，设想的新技术可以开辟探测多极分量甚至分子跃迁手性的可能性，取代通常的光谱学，用于那些有问题的情况，如偶极禁戒跃迁，在非常高的空间分辨率。
英文摘要：
Electron energy loss spectroscopy is consolidating as a powerful tool to explore electronic (as well as vibrational) excitations of matter, including molecules. Performed in a scanning transmission electron microscope, this technique is based on inelastic scattering of fast electrons in a thin specimen. Very recently, new electron optics configuration have been introduced, opening the way to the analysis of the single components of orbital angular momentum of the outcoming electrons, that convey additional information on the spatial features of the investigated excitations: innovative double-dispersed spectroscopic experiments for metallic nanostructures have been therefore suggested. We propose here to extend this technology to probe molecular and supra-molecular systems, devising new kind of experiments: using state of the art quantum chemical methods to describe the molecular system in presence of an electron beam in a configuration that avoid molecular damage, we show that scattered electrons acquire the different azimuthal components of induced molecular transition potentials. Numerical simulations performed for systems of increasing size, point out that the conceived new technique can open up the possibility of probing the multipolar components and even the chirality of molecular transitions, superseding the usual optical spectroscopies for those cases that are problematic, such as dipole-forbidden transitions, at a very high spatial resolution.
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