紫外-可见光谱Ultraviolet–visible spectroscopy

σ→σ*（能量最高，< 200 nm，饱和烃，真空紫外区）；n→σ*（约 150–250 nm，含杂原子的饱和化合物如醇、胺）；π→π*（不饱和和共轭体系，ε 大，共轭越长 λ_max 越长：乙烯 165 nm，1,3-丁二烯 217 nm，β-胡萝卜素 450 nm 呈橙色）；n→π*（羰基等，约 270–300 nm，ε 小）；还有过渡金属配合物的 d-d 跃迁和电荷转移跃迁（如 MnO₄⁻ 呈紫色）。由于振动和转动能级叠加，分子紫外光谱是宽的谱带。

σ→σ* transitions need the most energy (below 200 nm, in the vacuum UV, for saturated hydrocarbons); n→σ* occur at about 150–250 nm in saturated compounds with heteroatoms such as alcohols and amines; π→π* occur in unsaturated and conjugated systems with large ε, and longer conjugation shifts λ_max longer (ethylene 165 nm, 1,3-butadiene 217 nm, β-carotene 450 nm and orange); n→π* occur in carbonyls and similar groups at about 270–300 nm with small ε; and transition-metal complexes add d–d and charge-transfer bands (MnO₄⁻ is purple from the latter). Superimposed vibrational and rotational levels make molecular UV spectra broad bands.

1941 年伍德沃德总结了共轭二烯和 α,β-不饱和酮 λ_max 的经验计算规则，后经菲泽尔推广（伍德沃德-菲泽尔规则），在核磁共振普及前是推测天然产物结构的重要手段。物质的颜色是其吸收光的互补色：吸收蓝光（约 450 nm）显黄色，吸收绿光显紫红色。

In 1941 Woodward set out empirical rules for calculating λ_max of conjugated dienes and α,β-unsaturated ketones, later extended by Fieser (the Woodward–Fieser rules), an important tool for deducing natural-product structures before NMR. A substance's colour is complementary to the light it absorbs: absorbing blue (about 450 nm) it looks yellow, absorbing green it looks purple-red.

UV-Vis 是最常用的定量分析方法之一，用于测定金属离子（显色后）、药物含量、核酸和蛋白质浓度、酶活性、反应动力学和平衡常数（如 pKa）；作为 HPLC 的检测器；表征共轭聚合物、量子点、金属纳米颗粒的表面等离子体共振和半导体禁带宽度（陶克图）。

UV-Vis is among the most used quantitative methods: metal ions after colour development, drug content, nucleic acids and proteins, enzyme activity, reaction kinetics and equilibrium constants such as pKa; it is the standard HPLC detector; and it characterises conjugated polymers, quantum dots, plasmon resonance of metal nanoparticles and semiconductor band gaps (Tauc plots).