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. 2021 Jan 14;154(2):024104. doi: 10.1063/5.0034933.
Tip-enhanced Raman spectroscopy in combination with scanning tunneling microscopy could produce ultrahigh-resolution Raman spectra and images for single-molecule vibrations. Furthermore, a recent experimental study successfully decoupled the interaction between the molecule and the substrate/tip to investigate the intrinsic properties of molecules and their near-field interactions by Raman spectroscopy. In such a circumstance, more explicit treatments of the near field and molecular interactions beyond the dipole approximation would be desirable. Here, we propose a theoretical method based on the multipolar Hamiltonian that considers full spatial distribution of the electric field under the framework of real-time time-dependent density functional theory. This approach allows us to treat the on- and off-resonance Raman phenomena on the same footing. For demonstration, a model for the on- and off-resonance tip-enhanced Raman process in benzene was constructed. The obtained Raman spectra are well understood by considering both the spatial structure of the near field and the molecular vibration in the off-resonance condition. For the on-resonance condition, the Raman spectra are governed by the transition moment, in addition to the selection rule of off-resonance Raman. Interestingly, on-resonance Raman can be activated even when the near field forbids the π-π transition at equilibrium geometry due to vibronic couplings originating from structural distortions.
针尖增强拉曼光谱与扫描隧道显微镜相结合，可以产生用于单分子振动的超高分辨率拉曼光谱和图像。此外，最近的一项实验研究成功地解耦了分子与基底/针尖之间的相互作用，以通过拉曼光谱研究分子的本征性质及其近场相互作用。在这种情况下，需要对偶极近似之外的近场和分子相互作用进行更明确的处理。在这里，我们提出了一种基于多极哈密顿量的理论方法，该方法在实时含时密度泛函理论框架下考虑电场的全空间分布。这种方法使我们能够在相同基础上处理共振和非共振拉曼现象。为了进行演示，构建了一个苯中共振和非共振针尖增强拉曼过程的模型。通过考虑近场的空间结构和非共振条件下的分子振动，可以很好地理解所获得的拉曼光谱。对于共振条件，除了非共振拉曼的选择规则外，拉曼光谱还受跃迁矩的支配。有趣的是，即使在平衡几何构型下近场由于结构畸变引起的振动耦合而禁止π-π跃迁时，共振拉曼也可以被激活。
