Fluorescence Spectrum:
You have learnt that the fluorescence arises while the electronically excited molecule relaxes back from S1 state to S0 state accompanied through an emission of radiation. You would have noticed from Figure that while this radiative relaxation takes place, the molecule could come to any of the vibration levels of S0 state. This denotes that the emitted radiation is constituted of different wavelengths. A plot of the emitted radiation as a function of wavelength for any given excitation wavelength is called as the emission fluorescence spectrum. In Figure, the excitation radiation is displays to have caused the transition from S0 to S2 state. What do you think would happen to the fluorescence emission if we modify the wavelength of the excitation radiation?
Yes, you thought it right; an emission spectrum would still remain the similar because the fluorescence emission commonly occurs only from the first excited singlet state irrespective of the excited singlet state produced initially. You would observe in Figure in which the fluorescence emission remains the similar for both the excitations viz., S0 → S1 and S0 → S2. Thus, the intensity of the emission would be expected to change since the excitation radiation is absorbed to different extents (recall the UV- VIS spectra). Now if we change the wavelength of the exciting light and plot the emission from the sample at a provide emission wavelength against the wavelength of exciting radiation, the result is known as the excitation spectrum.