Microwave re-excitation of femtosecond laser tagging for highly flexible velocimetry.

Molecular tagging velocimetry is typically species specific and limited by excited state/species lifetimes. We utilize laser-generated ionization, long-lived anions, and a time-delayed microwave pulse to monitor the tagged region up to several milliseconds. This non-resonant excitation and microwave interaction is demonstrated in a range of gas mixtures. Signal levels show up to 1000-fold improvement, and the flexibility in interrogation time allows for velocity measurements over a large dynamic range (1-100 m/s) with single-shot precision of <5%. This approach has the potential for wide application over a range of relevant gas compositions, temperatures, and pressures.