Hi Mohammad

You can change the excited wavelength with 20 nm interval, for example (380,400,... 700 nm) and measure the emission fluorescence intensities for each excited wavelength. Then you can choose the optimized emission and excitation wavelength for highest fluorescence intensity. 

You first recorded the absortion spectra, identify the excitonic peak (the maxima absorção), and choose the excitation wavelength near the maxima (20-50 nm lower). In general the fluorescence will be maxim for the excitonic emission.

The absorption or photoluminescence spectra can give you idea about the adequate wavelength