Tip-enhanced Raman spectroscopy (TERS) is a versatile tool for chemical analysis at the nanoscale. In earlier TERS experiments, Raman modes with components parallel to the tip were studied based on the strong electric field enhancement along the tip. Perpendicular modes were usually neglected. Here, we investigate an isolated copper naphthalocyanine molecule adsorbed on a triple-layer NaCl on Ag(111) using scanning tunnelling microscope TERS imaging. For flat-lying molecules on NaCl, the Raman images present different patterns depending on the symmetry of the vibrational mode. Our results reveal that components of the electric field perpendicular to the tip should be considered aside from the parallel components. Moreover, under resonance excitation conditions, the perpendicular components can play a substantial role in the enhancement. This single-molecule study in a well-defined environment provides insights into the Raman process at the plasmonic nanocavity, which may be useful in the nanoscale metrology of various molecular systems.

尖端增强拉曼光谱 (TERS) 是一种用于纳米级化学分析的通用工具。在早期的 TERS 实验中，基于沿尖端的强电场增强研究了具有平行于尖端的组件的拉曼模式。垂直模式通常被忽略。在这里，我们使用扫描隧道显微镜 TERS 成像研究了吸附在 Ag(111) 上的三层 NaCl 上的分离的铜萘花青分子。对于 NaCl 上的平躺分子，拉曼图像根据振动模式的对称性呈现不同的模式。我们的结果表明，除了平行分量之外，还应考虑垂直于尖端的电场分量。此外，在共振激发条件下，垂直分量可以在增强中发挥重要作用。