Resonance-enhanced, rare-gas-assistedfemtosecond-laser electronic-excitationtagging in argon/nitrogen mixtures

Multiphoton-resonance enhancement of a rare-gas-assisted nitrogen femtosecond-laser electronic-excitation-tagging (FLEET) signal is demonstrated. The FLEET signal is ideal for velocimetric tracking of nitrogen gas in flow environments by virtue of its long-lived nature. By tuning to three-photon-resonant transitions of argon, energy can be more efficiently deposited into the mixture, thereby producing a stronger and longer-lived FLEET signal following subsequent efficient energy transfer from excited-state argon to the C (3Πu) excited state of nitrogen. Such resonant excitation exhibits as much as an order of magnitude increase in this rare-gas-assisted FLEET signal, compared to near-resonance excitation of seeded argon demonstrated in previous work, while reducing the required input excitation-pulse energies by two orders of magnitude compared to traditional FLEET.