The excitation wavelength can have an effect on the quantum yield and fluorescence lifetime of a commercial fluorescent dye. However, it is important to note that the effect may not be significant in all cases, and it depends on the specific dye and its properties.Quantum yield is the ratio of the number of photons emitted as fluorescence to the number of photons absorbed by the dye. It is a measure of the efficiency of the fluorescence process. The excitation wavelength can affect the quantum yield if the dye has different absorption bands with different quantum yields. In such cases, exciting the dye at different wavelengths may result in different quantum yields.Fluorescence lifetime is the average time a fluorophore spends in the excited state before returning to the ground state through the emission of a photon. The fluorescence lifetime is generally independent of the excitation wavelength, as it is determined by the rate constants of the radiative fluorescence and non-radiative e.g., internal conversion, intersystem crossing, and quenching processes that depopulate the excited state.The mechanism behind the effect of excitation wavelength on quantum yield can be explained by considering the Jablonski diagram, which represents the energy levels and transitions in a fluorescent molecule. When a dye absorbs a photon, it is promoted from the ground state S0 to an excited state usually S1 or S2 . The molecule can then undergo various processes, such as internal conversion, intersystem crossing, or fluorescence, to return to the ground state.If the dye has multiple absorption bands with different quantum yields, exciting the dye at different wavelengths may populate different excited states with different probabilities of undergoing fluorescence. This can result in different quantum yields for different excitation wavelengths.In summary, the excitation wavelength can have an effect on the quantum yield of a commercial fluorescent dye, particularly if the dye has multiple absorption bands with different quantum yields. However, the fluorescence lifetime is generally independent of the excitation wavelength and is determined by the rate constants of the radiative and non-radiative processes that depopulate the excited state.