拉曼光谱Raman spectroscopy

绝大多数散射光频率不变（瑞利散射）；约 10⁻⁶–10⁻⁸ 的光子与分子交换振动能量：散射光频率降低的称斯托克斯线，升高的称反斯托克斯线（较弱）。拉曼活性要求振动使分子极化率发生变化，而红外活性要求偶极矩变化。对于有对称中心的分子，红外活性和拉曼活性互相排斥（互斥规则），如 CO₂ 对称伸缩只有拉曼活性。对称性高、非极性的键（C=C、S–S、C≡C）拉曼信号强，水的拉曼信号很弱，所以拉曼适合水溶液样品。

Almost all scattered light keeps its frequency (Rayleigh scattering); about one photon in 10⁶–10⁸ exchanges vibrational energy with a molecule, giving Stokes lines at lower frequency and weaker anti-Stokes lines at higher. Raman activity requires a vibration to change the polarisability, whereas IR activity requires a change in dipole. In centrosymmetric molecules IR and Raman activity are mutually exclusive, so CO₂'s symmetric stretch is Raman-active only. Symmetric, non-polar bonds such as C=C, S–S and C≡C give strong Raman signals, and water is a weak scatterer, so Raman suits aqueous samples.

1923 年斯梅卡尔从理论上预言了非弹性光散射。1928 年印度物理学家钱德拉塞卡拉·拉曼和克里希南用太阳光和滤光片在液体中观察到这一现象，拉曼因此获 1930 年诺贝尔物理学奖，是首位获得科学类诺贝尔奖的亚洲人。同年苏联的兰兹贝格和曼德尔施塔姆在晶体中独立发现了同一效应。1960 年代激光的发明使拉曼光谱成为实用技术。

Smekal predicted inelastic light scattering theoretically in 1923. In 1928 the Indian physicists C. V. Raman and K. S. Krishnan observed it in liquids using sunlight and filters; Raman received the 1930 Nobel Prize in Physics, the first Asian to win a science Nobel. Landsberg and Mandelstam in the Soviet Union independently found the same effect in crystals that year. The laser, invented in the 1960s, made Raman spectroscopy practical.

表面增强拉曼散射（SERS，1974 年发现）利用金、银纳米结构的局域表面等离子体使信号增强 10⁶–10¹⁰ 倍，可检测单分子；共振拉曼、相干反斯托克斯拉曼（CARS）、受激拉曼成像用于生物组织成像；尖端增强拉曼达到纳米级空间分辨。拉曼用于碳材料表征（石墨烯 G 峰和 2D 峰、碳纳米管）、半导体应力分析、药物晶型鉴别、文物颜料无损分析、宝石鉴定、毒品和爆炸物现场检测。

Surface-enhanced Raman scattering (SERS, discovered in 1974) uses localised plasmons on gold and silver nanostructures to boost signals 10⁶–10¹⁰-fold, down to single molecules; resonance Raman, coherent anti-Stokes Raman (CARS) and stimulated Raman imaging image biological tissue; and tip-enhanced Raman reaches nanometre resolution. Raman characterises carbon materials (the G and 2D bands of graphene, carbon nanotubes), maps stress in semiconductors, identifies drug polymorphs, analyses pigments in artworks non-destructively, identifies gems, and detects drugs and explosives in the field.