Spectral properties of luminescent materials

Fluorescence is photoluminescence, and under the action of optical radiation, fluorescent substances emit fluorescence with different wavelengths. Any fluorescent compound has two characteristic fluorescence spectra, namely excitation spectrum and emission spectrum. A fluorescence emission wavelength is selected to record the fluorescence emission intensity as a function of the excitation wavelength, and the fluorescence excitation spectrum is obtained. It reflects the relative efficiency of fluorescence caused by excitation light with different wavelengths. The excitation spectrum can identify the fluorescent substance, and select the appropriate excitation wavelength when measuring fluorescence. The fluorescence excitation spectrum is similar to the ultraviolet-visible absorption spectrum. Select a light with a fixed wavelength to excite the sample, and record the functional relationship between the fluorescence emission intensity and the emission wavelength generated in the sample, so as to obtain the fluorescence emission spectrum. The shape of the emission spectrum has nothing to do with the excitation wavelength: the excitation spectrum of a molecule may contain several excitation bands, but the emission spectrum only contains one emission band. Even if the molecule is excited to an electronic state higher than S1, it will fall to the lowest vibration level and rotation level of S1 electronic state due to extremely fast internal conversion and vibration relaxation, and then release energy in the form of radiation to return to the ground state. The fluorescence spectrum has the following characteristics. Stokes shift: Compared with excitation harmonic, the wavelength of fluorescence harmonic always appears at a longer wavelength; Stokes discovered this phenomenon in 1852, so it was named Stokes displacement. Stokes displacement shows that there is a certain energy loss between excitation and emission. The rapid decay of excited molecules to the lowest vibrational energy level of S electronic state due to internal conversion and vibrational relaxation is the main reason for Stokes shift. The fluorescence spectrum is independent of the excitation wavelength. Although the electronic absorption spectrum of a molecule may contain several absorption bands, its fluorescence spectrum only contains one emission band. Because fluorescence emission occurs at the lowest vibrational level of the excited state of the first electron, it is independent of which electronic state it is excited to, so the shape of the spectrum is usually independent of the excitation wavelength. If у=250nm or у=350nm is used as the excitation light source, the obtained fluorescence spectrum shape and peak position are the same. The absorption spectrum and emission spectrum are roughly mirror images. Compared with its absorption light, the emission spectrum of fluorescent substance has a mirror image relationship.