Chapter 2　Ultraviolet and Visible Spectroscopy

2.1　Introductions of Ultraviolet and Visible Spectroscopy

When continuous radiation passes through a transparent object，a portion of the radiation may be absorbed.If that occurs，the residual radiation，when it passes through a prism，yields a spectrum with gaps in it，called an absorption spectroscopy^[33].Ultraviolet and Visible Spectroscopy^[34] （UV）is a kind of molecular absorption spectroscopy.In structure identification^[35]，it is used to detect the presence of chromophores^[36]like dienes，aromatics，polyenes，and conjugated ketones，etc.

Electromagnetic radiation^[37] such as visible light is commonly regarded as a wave phenomenon，characterized by a wavelength or frequency.Ultraviolet and Visible wavelengths cover a range from approximately 1 to 800 nm.The longest visible wavelength is red and the shortest is violet.The visible spectrum constitutes but a small part of the total radiation spectrum.Most of the radiation that surrounds us cannot be seen，but can be detected by dedicated sensing instruments.This electro-magnetic spectrum ranges from very short wavelengths（including gamma and X-rays）to very long wavelengths（including microwaves and broadcast radio waves）.The following Fig.2-1 displays many of the important regions of this spectrum.

Fig.2-1　The different regions of electromagnetic spectrum and the corresponding spectroscopy

图2-1　电磁波的不同区域及对应的波谱学分类

Ultraviolet and visible （UV-Vis） absorption spectroscopy is the measurement of the absorption of a beam of light after it passes through a sample.Absorption measurements can be at a single wavelength or over an extended spectral range， and is a plot of absorbance （A） versus wavelength （λ）.Applications of UV-Vis spectroscopy mainly includes：

1） It is useful to detect the conjugation of the compounds.

2） Detection of geometrical isomers.In case of geometrical isomers， trans isomers exhibits λ_max at slightly longer wavelength and have larger extinction coefficient than the cis isomers.

3） It is useful in quantitative analysis of the compounds.

4） Detection of impurities.UV absorption spectroscopy is the one of the best methods for detecting impurities in organic compounds.