Takenaka Masato, Taketsugu Tetsuya, Iwasa Takeshi
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan.
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.
J Chem Phys. 2020 Apr 30;152(16):164103. doi: 10.1063/1.5143855.
Surface enhanced spectroscopy, which enhances the signal intensity of molecules on a surface, facilitates the study of molecular properties, even down to a single-molecule level if a scanning probe is used. To realize the full potential of surface enhanced spectroscopy, a clear theoretical understanding is indispensable. However, quantum chemical calculations for surface enhanced spectroscopy are not simple because of the violation of the widely used dipole approximation. The spatial structure of electric near-field in the close proximity of a surface strongly depends on the geometry of the metal nanostructure as well as on the incident wavelength. Therefore, in principle, a universal model for electric near-field cannot exist. To address this issue, we have developed a generalized light-matter interaction model from first-principles quantum chemical calculations by using the multipolar Hamiltonian, in which the spatial structure of the electric field is fully considered. Here, we incorporate computational electrodynamics for surface enhanced infrared (IR) absorption spectroscopy in the model, where electric near-field around a Ag ellipsoid is obtained and used for IR calculations. Furthermore, we have devised a method to successfully reproduce the peak selectivity observed experimentally.
表面增强光谱法可增强表面分子的信号强度，有助于研究分子特性，若使用扫描探针，甚至能研究单分子水平的特性。为充分发挥表面增强光谱法的潜力，清晰的理论理解必不可少。然而，由于违反了广泛使用的偶极近似，表面增强光谱法的量子化学计算并不简单。表面附近电近场的空间结构强烈依赖于金属纳米结构的几何形状以及入射波长。因此，原则上不可能存在电近场的通用模型。为解决这个问题，我们通过使用多极哈密顿量从第一性原理量子化学计算中开发了一个广义的光与物质相互作用模型，其中充分考虑了电场的空间结构。在此，我们将表面增强红外（IR）吸收光谱的计算电动力学纳入模型，在该模型中获得了银椭球体周围的电近场并用于红外计算。此外，我们设计了一种方法，成功再现了实验观察到的峰选择性。
