The peak which is sharp  can be selected as its wavelength.

A safe procedure is to measure the excitation spectrum of the material. In this photoluminescence experiment, you set  the detection wavelength at the expected emssion peak, and then you scan the excitation wavelength. Recording the emission intensity as a function of the excitation wavelength you get a clear picture of the suitable excitation ranges. Of course you should normalize the emission data for the dependence of excitation intensity on the wavelength.  

@ hema...what will be if the a peak as maximum absorption. sharp peak means maximum absorption or different ?

Dear Sumitra you can choose the sharp peak or the broad peak, it depends on your requirement. Sharp peak does not meant maximum. A broad peak may have high intensity then a sharp one. Usually, the peak (sharp or broad) which have highest intensity is selected as excitation wavelength. 

@ sanoujam ...thanks....means we consider that peak which have highest intensity either which is sharp or broad ?

i do agree with Metei, it is always advisable to consider a peak of maximum intensity irrespective of its breadth. 

First of all make a  minimum concentration of solution of the substance  in a suitable solvent which does not have absorbance.then scan the solution in uv range. the peak which is having the maximum absorption and also sharp compared to other peaks can be considered as wavelength.

we have glasses in powder form. it is not soluble easily in any solvent ? what we do ?

If you have the suitable equipment follow the Falconieri's advice. An excitation spectrum is a more correct mean to choose the necessary emission.