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4255298 
Journal Article 
Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air 
Michael, JB; Edwards, MR; Dogariu, A; Miles, RB 
2011 
Yes 
Applied Optics
ISSN: 0003-6935
EISSN: 1539-4522 
50 
26 
5158-5162 
English 
21946998 
WOS:000294726700014 
Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.