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4.1 introduction源语言可能是:英语4.1 引言When certain substances are illuminated by a certain wavelength, the wavelength of the incident light is longer than that of the incident light. In 1852, Stokes illustrates the mechanism of the fluorescence emission, that the fluorescence is due to material absorption of light and re issued by the different wavelengths of light, and by a hair fluorescent mineral name for fluorescence. We usually say of fluorescence is refers to the material in the ultraviolet a longer wavelength UV fluorescent or visible fluorescence, and the shorter wavelengths of visible light absorption after a longer wavelength visible fluorescence. In addition to the ultraviolet fluorescence and visible fluorescence, as there are infrared fluorescence, X ray fluorescence, etc.. This is not the content of this chapter. Fluorescence spectrum has two main advantages: the first is the high sensitivity. Because the wavelength of the fluorescence emission is more than that of the excitation wavelength, the fluorescence frequency is different from that of the incident light. In addition, the fluorescence spectrum is an optical emission spectroscopy can be detected in and the incident light at right angles to the direction, so that fluorescence without interference from the bottom of the luminescence of the. The sensitivity is greatly higher than that of UV Vis absorption spectra. Second, the fluorescence spectrum can detect some of the processes that can not be detected by UV Vis absorption spectra. UV and visible fluorescence are involved in the transition between the electron energy levels, including two processes: absorption and emission. Each time and transition frequency of the countdown is the same time order of magnitude (about 10-15 seconds), but two processes have a time delay, is about 10-9 seconds, the molecules within this period of time in the excited state. The lifetime of the excited state depends on the competition between the radiation and the radiation.