Flow tagging helium by multiphoton excitation
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1997, American Institute of Aeronautics and Astronautics, Inc.. We present a theoretical analysis of a new non-intrusive diagnostic technique based on tagging helium by multiphoton excitation and tracking by laser induced fluorescence. Helium is a particularly interesting gas to tag since it is a carrier gas for many fluid and combustion processes and is of great interest for fundamental turbulence measurements. By frequency tripling a narrow linewidth Titanium: Sapphire laser at 721.2 nm we can achieve an ultraviolet source at 240.6 nm. This source is then used for fourphoton excitation of helium from the ground state Is2s 1S0 to the metastable state 2s2s 1S0 state. The tagged helium can be tracked by resonant laser induced fluorescence at 501.6 nm. A theoretical model based on rate equation is used to describe the tagging process. The four photon laser absorption cross section at 240.6 nm has been determined to be 3x10-111 cm8s3 by nonlinear perturbation theory.
