Standoff Methods for the Detection of Threat Agents: A Review of Several Promising Laser-Based Techniques

Figure 5

Energy-level diagram of three states in a molecule or atom. represents the angular frequency of the pump photons which couple states and . represents the angular frequency of the Stokes photons which couple states and . In the off-resonance diagram the photon energies are detuned by an amount for the pump photons, and for the Stokes photons yielding a two-photon detuning of for the combination. The heavy black horizontal lines correspond to energy states in the atom or molecule. The grey lines, which are present for illustration only, do not correspond to energy states.