Abstract In this review, we summarize recent progress in experimental approaches to the investigation of the unoccupied electronic structures of organic ultrathin films, based on a combination of spectroscopic and microscopic techniques. At the molecule/substrate interface, electronic structures are greatly affected by the geometrical structures of adsorbed molecules. In addition, a delicate balance between substrate-molecule and intermolecular interactions plays an important role in the formation of complex polymorphism. In this context, we have clarified the correlation between geometric and electronic structures using a combination of two-photon photoemission (2PPE) spectroscopy, low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Organic ultrathin films of metal phthalocyanines and polycyclic aromatic hydrocarbons (naphthalene, rubrene and perylene) on graphite substrates were examined as model systems. Depending on the substrate temperature and coverage, unique morphologies, including well-ordered films, a metastable phase and a two-dimensional gas-like phase, were determined at the molecular level. The data show that variations in molecular orientation have a significant impact on the occupied/unoccupied electronic structures. In addition to static information regarding electronic states, ultrafast electron excitation and relaxation dynamics can be tracked in real time on the femtosecond scale by time-resolved 2PPE spectroscopy. The excited electron dynamics of rubrene films are discussed herein, taking into account structural information, in the presence and absence of an overlap of the wave function with the substrate. Spatial resolution at the molecular level is also obtainable via STM-based local spectroscopy and mapping, which have been utilized to elucidate the spatial extent of unoccupied orbitals in real space. Visible photon emissions from the unoccupied states of perylene monolayer films were observed using 2PPE, representing a characteristic deexcitation process from electronically excited states, depending on the surface structure. These spectroscopic and molecular level microscopic investigations provide fundamental insights into the electronic properties of organic/substrate interfaces.

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有机超薄膜的光谱和微观研究：几何结构与未占据电子状态之间的相关性

摘要在这篇综述中，我们总结了基于光谱和显微技术相结合的有机超薄膜未占据电子结构研究的实验方法的最新进展。在分子/底物界面，电子结构受吸附分子几何结构的影响很大。此外，底物-分子和分子间相互作用之间的微妙平衡在复杂多态性的形成中起着重要作用。在这种情况下，我们使用双光子光电发射 (2PPE) 光谱、低能电子衍射 (LEED) 和扫描隧道显微镜 (STM) 的组合阐明了几何结构和电子结构之间的相关性。研究了石墨基材上金属酞菁和多环芳烃（萘、红荧烯和苝）的有机超薄膜作为模型系统。根据基板温度和覆盖范围，在分子水平上确定了独特的形态，包括有序的薄膜、亚稳态相和二维气体状相。数据表明，分子取向的变化对占据/未占据的电子结构有显着影响。除了有关电子状态的静态信息外，还可以通过时间分辨 2PPE 光谱在飞秒级实时跟踪超快电子激发和弛豫动力学。考虑到结构信息，本文讨论了红荧烯薄膜的激发电子动力学，在存在和不存在波函数与基底重叠的情况下。分子水平的空间分辨率也可以通过基于 STM 的局部光谱和映射获得，这些已经被用来阐明真实空间中未占据轨道的空间范围。使用 2PPE 观察到苝单层膜的未占据状态的可见光子发射，这代表了电子激发态的特征去激发过程，具体取决于表面结构。这些光谱和分子水平的显微研究为有机/基材界面的电子特性提供了基本的见解。已被用来阐明实际空间中未占据轨道的空间范围。使用 2PPE 观察到苝单层膜的未占据状态的可见光子发射，这代表了电子激发态的特征去激发过程，具体取决于表面结构。这些光谱和分子水平的显微研究为有机/基材界面的电子特性提供了基本的见解。已被用来阐明实际空间中未占据轨道的空间范围。使用 2PPE 观察到苝单层膜的未占据状态的可见光子发射，这代表了电子激发态的特征去激发过程，具体取决于表面结构。这些光谱和分子水平的显微研究为有机/基材界面的电子特性提供了基本的见解。