What are the principles of fluorescence spectroscopy?

Fluorescence spectroscopy is based on the principle that electrons create an emission spectrum when they transition from one state of energy to another. The transition from a higher energy state to a lower energy level is accompanied by radiation. 

Before the emission phenomenon, molecules have to be excited to bring them into a higher state. This is because molecules can only emit fluorescence in their excited forms. 

On excitation, molecules attain different levels of rotational and vibrational energy at closely related wavelength values, which causes the emitted radiation to appear as a band spectrum. Because the fluorescence properties of a molecule depend on the features of the molecules, this technique can be used for identifying and quantifying the analyte concentration in a sample.