Femtosecond laser tagging for velocimetry in argon and nitrogen gas mixtures

Tagging is demonstrated in argon and nitrogen gases using a femtosecond laser with pulse energies of approximately 70 μJ through a nonresonant ionization process at 267 nm. The signal fluorescence lifetime in pure argon and nitrogen-argon mixtures are measured and found to be long enough to make mean velocity and turbulence measurements in a subsonic flow. In pure argon, the dominating processes involve atomic transitions between 700 and 900 nm. In argon-nitrogen mixtures, nitrogen quenches atomic argon species and the dominant radiating processes are transitions in the nitrogen second positive system. In pure nitrogen, emission on the microsecond time scale comes from the nitrogen first positive system. Lower energy density is needed for tagging and narrower tagged lines are produced using 267 nm as compared to femtosecond laser tagging in argon and nitrogen using 400 nm or 800 nm. Velocimetry using the 267 nm line is demonstrated in a turbulent argon pipe flow and the Taylor microscale of the flow is determined.