The lateral distribution of optical excitation occurring at the edge part of rubrene (5,6,11,12-tetraphenyltetracene) molecular islands formed on highly oriented pyrolytic graphite (HOPG) substrate has been visualized with microspot two-photon photoemission (2PPE) spectroscopy of 0.4 µm resolution. As we reported previously [T. Ueba, et al., J. Phys. Chem. C 117 (2013) 20098–20103], an unoccupied peak labeled Ln is strongly enhanced in 2PPE spectroscopy by a resonant optical excitation between a diffuse unoccupied Ln molecular orbital and the HOMO. The strong enhancement is due to the hybridization of Ln with the image potential state (IPS) on the HOPG substrate. In this study, using the spatially-resolved microspot 2PPE method, we obtained the real space image of the area where the optical excitation occurs. One-photon photoelectron emission microscope (1PPE-PEEM) with 50 nm resolution is used to compare with the microspot 2PPE. The 1PPE-PEEM images are homogeneous for films below and above 1 monolayer (ML) coverage, indicating that the molecular density of rubrene is homogeneous within the lateral resolution. On the other hand, microspot 2PPE images for sub-ML films show inhomogeneous areas of several µm sizes. The images depend on the detected unoccupied levels of interest. The inhomogeneity is very significant when the image is recorded at the energy of Ln peak. The different appearance between 1PPE-PEEM and microspot 2PPE images is explained on the basis that the Ln excitation occurs at the edges of islands smaller than the resolution of the PEEM. Though the microspot 2PPE achieves only moderate lateral resolution, it is sensitive to the resonant excitation occurring at nm-scale boundaries of the molecular islands. The distribution of the nm-scale islands is the origin of the µm-scale lateral inhomogeneity. The microspot 2PPE images become homogeneous for films thicker than 1 ML, reflecting the quenching of the resonant excitation. These results demonstrate a possibility that spatially- and energy-resolved microspot 2PPE can be a powerful tool to clarify nanoscale geometric and electronic structures at the molecule/substrate interfaces.

光学激发的横向分布发生在高取向热解石墨（HOPG）衬底上形成的红荧烯（5,6,11,12-四苯基并四苯）分子岛的边缘部分，已通过微点双光子光发射（2PPE）光谱进行了可视化。分辨率为0.4 µm。正如我们先前报道的[T. Ueba等，J.Phys。化学 C 117（2013）20098–20103]中，标记为Ln的未占据峰在2PPE光谱中通过弥漫的未占据Ln分子轨道与HOMO之间的共振光激发而得到了显着增强。强烈的增强归因于Ln与HOPG基板上的图像电势状态（IPS）的杂交。在这项研究中，使用空间分辨微点2PPE方法，我们获得了发生光激发的区域的真实空间图像。使用分辨率为50 nm的单光子光电子发射显微镜（1PPE-PEEM）与微点2PPE进行比较。1PPE-PEEM图像对于1个单层（ML）覆盖率以下的膜是均质的，这表明红荧烯的分子密度在横向分辨率内是均质的。另一方面，亚ML胶片的微点2PPE图像显示出几微米大小的不均匀区域。图像取决于检测到的未占用兴趣水平。当以Ln峰的能量记录图像时，不均匀性非常显着。基于Ln激发发生在小于PEEM分辨率的岛的边缘，说明了1PPE-PEEM和微点2PPE图像之间的不同外观。尽管微点2PPE仅获得中等水平分辨率，它对在分子岛的纳米级边界上发生的共振激发敏感。纳米级岛的分布是微米级横向不均匀性的起源。对于大于1 ML的胶片，微点2PPE图像变得均匀，这反映了共振激发的猝灭。这些结果表明，空间分辨和能量分辨的微点2PPE可能是阐明分子/底物界面处的纳米级几何和电子结构的有力工具。