The graphical representation of the fluorescence intensity as a function of the emission wavelength is called the fluorescence spectrum. It is one of the important physical properties of the excited state and can be measured using a fluorescence spectrometer. The fluorescence spectrum can also reveal the relevant properties of the excited state, such as the energy of the S state, which can be determined by the position of the 0-0 band.
Fluorescence emission is the reverse process of light absorption. The fluorescence spectrum has a similar mirror-like relationship to the absorption spectrum, but the fluorescence spectrum always shifts redward compared to the corresponding absorption spectrum (see Figure 2.2), which is called the Stokes shift (Stoke’s shift). The main reasons for the Stokes shift are two: one is that the excited-state molecules that have transitioned to the high vibrational energy level of the excited state first undergo a faster vibrational relaxation (with a rate in the order of 10^13 s^-1), losing some energy and reaching the zero vibrational energy level; the other is that the configuration of the excited-state molecules will quickly be further adjusted to reach a stable state at a lower energy level, which also loses some energy. Both of these lead to the energy of the fluorescence quantum being lower than the energy of the absorbed photon. That is, compared to the absorption spectrum, the fluorescence spectrum will shift towards the long-wavelength direction. However, sometimes an anti-Stokes shift phenomenon can be observed at high temperatures, that is, the fluorescence spectrum shifts towards the short-wavelength direction of the absorption spectrum. This is because high temperatures cause more excited-state molecules to be in the high vibrational energy level, and fluorescence mainly emits from the high vibrational energy level of the excited state.
If the molecular configuration of the excited-state molecules is significantly different from that of the ground-state molecules, then no fluorescence spectrum and absorption spectrum can be observed.