Ultrafast ps–TALIF and streak camera diagnostics of atomic hydrogen in a helium microplasma jet

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Abstract Picosecond two-photon absorption laser induced fluorescence (ps–TALIF) is combined with a streak camera (∼1 ps highest temporal resolution) to probe the effective lifetime ( τ eff H ( n = 3 ) ) and absolute density ( N H ) of atomic hydrogen in the effluent of a helium atmospheric-pressure microplasma jet ( μ APPJ). This approach allows for improved temporal resolution compared to conventional nanosecond diagnostics, enabling measurements of τ eff H ( n = 3 ) as small as 50 ps, the corresponding N H reaching down to 10 14 cm −3 . Atomic densities are calibrated by performing identical ps–TALIF measurements in krypton gas contained in a custom-built low-pressure cuvette. Physical mechanisms that may be involved in the TALIF scheme such as photoionisation (PIN) and/or stimulated emission (SE) are also assessed, ensuring reliable studies. The determined τ eff H ( n = 3 ) and N H increase with the helium flow rate ( Q He ) in the range Q He = 0.3–1 slm. Both quantities are maximised near the exit nozzle of the μ APPJ, obtaining values below 400 ps and 6 × 10 14 cm −3 , respectively. As the axial distance increases, τ eff H ( n = 3 ) declines with a rate of ≈ 65 ps mm −1 for Q He = 1 slm which is about 3 times smaller than for Q He = 0.3 slm. These findings reveal a strong correlation between the experimentally-measured τ eff H

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