The absorption maximum is usually about the same as the excitation maximum for fluorophores in solution. I don't have any experience with carbon dots, however. It is possible that the absorption spectrum is distorted by light scattering from the nanoparticles, in which case the absorption maximum may be at a longer wavelength than it appears to be. In any case, it is not necessary to know the excitation maximum exactly in order to do an emission scan to determine the emission maximum. The height of the emission peak is affected but not the wavelength. That assumes that there is only one excitation peak and one emisson peak.

After you find the emission peak, you can go back and do an excitation scan to find the true excitation peak.

"For a given solution, the homogeneity of the size of quantum dots is correlated to the relationship between the fluorescence maximum position and the quantum dot size." See

https://www.researchgate.net/publication/51049415_Determination_of_the_size_of_quantum_dots_by_fluorescence_spectroscopy [accessed Dec 01 2018].

Thanks Adam and Yuri for your valuable experience in this field.

Respected sir Adam B Shapiro

Your explanation will not fit for carbon dots.

In most of the cases carbon dots exhibit excitation dependent emission behavior (red shifted emission) and their optimal excitation wavelength will be higher than the absorption maxima. Hence Dear Abhimanyu Dev you need to scan CDs to find out the optimal excitation wavelength. Mostly, this will be around 360 nm.

Dear rajkumar

Thanks for your answer... I understand that it's quite obvious that excitation maximum could be obtained through scanning of the respective sample... But question remains why excitation maxima will be different from absorption maxima obtained by uv-vis spectrophotometer...

I more or less agree with the explanation by Mr. Adam B. Shapiro that excitation maxima may differ from absorption maxima due to scattering phenomenon.. Otherwise what will be the reason for difference in absorption maxima and excitation maxima??

See

https://www.researchgate.net/post/Why_is_l_excitation_of_a_fluorescent_compound_different_from_its_l_absorption

Thanks yuri for the valuable answer.. I got my answer.. It means in case of mono chromophoric system, absorption and excitation spectra are same but in the evening of multi chromophoric system, absorption and excitation spectra will be different...