The theory of surface-enhanced Raman scattering
The theory of surface-enhanced Raman scattering
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DOI：
10.1063/1.3698292
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发表时间：
2012-04-14
影响因子：
4.4
通讯作者：
Birke, Ronald L.
中科院分区：
文献类型：
--
作者：
Lombardi, John R.;Birke, Ronald L.
通过考虑分子和金属形成一个连接系统，我们推导了表面增强拉曼散射中观测到的拉曼光谱的表达式。金属能级被认为是由一个连续体组成的，其中能级填满了费米能级，上面是空的，而分子的能级是离散的，填满了最高已占据轨道，上面是空的。假定金属的费米能级位于分子的最高填充能级和最低未填充能级之间。然后在填充和未填充的能级上，将分子能级耦合到金属连续统上，利用Fano给出的问题解，导出了分子-金属体系在基态和激发态之间的跃迁幅度表达式。结果表明，三种共振对整体增强有贡献；即表面等离子体共振，分子共振，以及分子与金属之间的电荷转移共振。此外，这些共振通过分子中的项连接起来，从而产生SERS选择规则。这些关联的共振不能分离，这解释了许多观察到的SERS现象。分子-金属耦合用变形势来解释，变形势与赫茨伯格-泰勒振动耦合常数相比较。我们证明了总和中的一项涉及表面等离子体跃迁偶极子和分子跃迁偶极子之间的耦合。它们通过连接到电荷转移态的变形势耦合。另一项涉及到电荷转移跃迁和分子跃迁偶极子之间的耦合。这些是由连接到等离子体共振态的变形势耦合的。通过将选择规则应用于二聚体和三聚体纳米粒子的情况下，我们表明SERS光谱可以随激发波长变化很大，这取决于激发的等离子体和/或电荷转移共振。(C) 2012年美国物理研究所。[http://dx.doi.org/10.1063/1.3698292]
By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that. It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule. The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena. The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to plasmon resonance states. By applying the selection rules to the cases of dimer and trimer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which plasmon and/or charge-transfer resonance is excited. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3698292]