You should take wavelength correspondence to max. Absorption because it represent your band gap of material
In the material you are studying the band structure has a density distribution. Depending on the excitation wavelength you will hit a different energy of the band structure and the difference in the density distribution will have an influence on how the PL response looks like.
To convince yourself you can do PLE (Photoluminescence excitation) experiments. Set the detection side at a fixed wavelength, then vary the source. At that point you can see the density distribution in the conduction band (in the case of a semi conductor).
Check you sample using uv-vis, the wavelength of peak obtained can be used a your excitation which will give a significant result on PL.
Thanks for information. One more thing i wish to ask what may be the consequences if we excite less wavelength than the max absorption
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