[KAIST] IR / Raman / SERS spectroscopy and selection rule

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1. IR/Raman/SERS Spectroscopy and Selection Rules

IR/Raman/SERS 분광법과 선택 규칙에 대해 설명합니다. SERS 실험에서 일반 라만 분광법에 비해 관찰되는 큰 증강 효과를 설명하는 두 가지 메커니즘이 제안됩니다. 전자기 이론에 따르면 입사 전자기파에 의해 국부 표면 플라즈몬이 여기되어 입사광 강도가 증가하고 라만 산란 신호가 더 증강됩니다. 화학 이론에 따르면 금속 표면과 흡착질 간의 전하 이동이 라만 산란 강도를 증가시킵니다. IR 분광법과 라만 분광법의 주요 차이는 IR은 흡수 분광법인 반면 라만은 진동 모드에 의한 비탄성 산란을 검출한다는 점입니다. IR 활성 진동은 쌍극자 모멘트 변화를 겪는 반면 라만은 분극률 변화를 검출합니다. 따라서 극성 결합은 IR에서 활성이지만 라만에서는 약한 반면, 비극성 결합은 라만에서 강하고 IR에서는 비활성입니다. 시료 요구사항도 다릅니다. IR은 IR 영역에서 투명한 매질이 필요하지만 라만은 용액 상태로도 가능합니다. 은 나노입자는 약 10nm 크기로 SERS에 사용되는 400-1800nm 파장보다 훨씬 작아 집단적으로 전자 분포가 왜곡되어 표면 플라즈몬이 증폭됩니다. 이러한 은 나노입자는 질산은을 붕소수소화나트륨으로 환원하여 합성할 수 있습니다.

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1. IR/Raman/SERS Spectroscopy and Selection Rules

IR (Infrared), Raman, and SERS (Surface-Enhanced Raman Spectroscopy) are powerful spectroscopic techniques that provide valuable information about the molecular structure and composition of materials. Each technique has its own selection rules that govern the allowed transitions and the intensity of the observed signals. Understanding these selection rules is crucial for interpreting the spectra and gaining insights into the properties of the studied systems. IR spectroscopy is based on the absorption of infrared radiation by molecules, which causes changes in the vibrational energy levels. The selection rules for IR spectroscopy state that a change in the dipole moment of the molecule must occur during the vibrational transition for the transition to be IR-active. This means that only molecules with a non-zero dipole moment, or those that undergo a change in dipole moment during the vibration, will exhibit strong IR absorption bands. Raman spectroscopy, on the other hand, relies on the inelastic scattering of monochromatic light, typically from a laser source. The selection rules for Raman spectroscopy state that a change in the polarizability of the molecule must occur during the vibrational transition for the transition to be Raman-active. Polarizability is a measure of how easily the electron cloud of a molecule can be distorted by an external electric field. Molecules with a change in polarizability during a vibration will exhibit Raman-active bands. SERS is a variant of Raman spectroscopy that enhances the Raman signal by several orders of magnitude when the analyte is adsorbed on or in close proximity to a roughened metal surface, typically silver or gold. The selection rules for SERS are similar to those for Raman spectroscopy, but the enhancement mechanism involves the interaction between the analyte and the metal surface, which can lead to additional selection rules and the observation of previously forbidden transitions. Understanding the selection rules for these spectroscopic techniques is crucial for interpreting the observed spectra, identifying the molecular species present, and gaining insights into the structure and dynamics of the studied systems. By carefully considering the selection rules, researchers can design experiments, select appropriate techniques, and extract more meaningful information from the spectroscopic data.