Abstract
The standoff detection of energetic materials via laser-induced fluorescence of vapors has received relatively little attention due to spectrally broad fluorescence emission from aerosols and unwanted background molecules. This unwanted broad emission can obscure fluorescence from the molecule of interest. When multiphoton excitation is used, the problem can be avoided by blue-shifting the emission from the target molecule relative to the unwanted broad emission. As a precursor to the detection of explosives, we demonstrate coherent multiphoton excitation via stimulated Raman adiabatic passage (STIRAP) on sodium vapor in an argon buffer gas as a function of argon pressure. Results indicate that STIRAP can be performed in a buffer gas at atmospheric pressure with a minimal eduction in STIRAP efficiency. The 15 ps long light pulses used for the pump and Stokes pulses were produced by two synchronously pumped OPO/OPAs tuned to the 3p (<sup>2</sup>P<sub>1/2</sub>) ← 3s (<sup>2</sup>S<sub>1/2</sub>) transition for the pump pulse and the 5s (<sup>2</sup>S<sub>1/2</sub>) ← 3p (<sup>2</sup>P<sub>1/2</sub>) for the Stokes pulse.
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