Two Step Excitation or Double Resonance:

The double resonance fluorescence includes a two step excitation procedures using two dye lasers. The first one is laser excites the analyte from ground state to an excited state from where it additional gets excited to another higher excited state along with the help of a second laser. The de-excitation of this higher excited state to a lower energy state is accompanied through fluorescence emission. This is known as double resonance fluorescence. The excitation and de-excitation transitions responsible for double resonance fluorescence are displays schematically.