Always UV spectra to be taken first. Because if it does not have any absorption; it can not have any photoluminescence. Generally, excitation wavelength is where absorption value is maximum (absorption maxima). One can fix the excitation wavelength at any value, however at absorption maxima, intensity of PL will be highest

Absorption study should be done before fluorescence generally. If you are expert, only fluorescence can be taken directly with the help emission and excitation spectra.

Absorption spectrum should be taken first and then fluorescence or any other measurement. Once you know the absorption wavelength then you can decide the excitation wavelength for taking the fluorescence spectra.

Absorbance spectroscopy measures how much of a particular wavelength of light gets absorbed by a sample. Fluorescence microscopy measures the fluorescence of a particular compound when given a particular wavelength. So absorption spectroscopy should be done first to get particular wavelength and then go for the fluorescence spectra.

first you have to do UV absorption test to clear whether your material is uv light responsive or visible light responsive.then you should calculate the bnad gap of this materials by using Diffused UV spectra.Then you can do PL measurments.for deciding the excitation wavelength you can use such wavelength from which your graph starts to go upward from DUV absorption spectra

Thank You everyone, i got it. I have done UV first and then done PL by varying excitation wavelength corresponding to the absorption region and got the correct data.

I also have a query , how UV Or PL give information about unknown material whether it is direct band gap or Indirect band gap?

First UV absorption spectrum and then PL Spectrum. The wave length at which absorption is maximum, is taken as excitation wavelength for PL