Depopulation mechanisms of atomic hydrogen in the n=3 level following two-photon excitation by a picosecond laser

International audiencetes

Characterization of a helium micro-plasma jet by means of ps-TALIF and a streak camera

Atmospheric pressure plasma jets (APPJ) present rich gas-phase chemistry,transient electric fields and electron densities, low gas temperatures, etc., whichmake them very promising in various novel applications such as ambient ionizationmass spectrometry [1]. Due to their operation at atmospheric pressure, collisionalquenching of generated reactive species becomes significant, and, in some cases(e.g., excited atomic hydrogen), the corresponding effective lifetimes may fall to subnstimescales [2]. In most published studies, classic optical diagnostics (such asnanosecond –ns– TALIF and ICCD cameras) are employed to understand thekinetics of reactive atoms in APPJ. However, precise measurements of theirdensities using ns-TALIF may become challenging at atmospheric and higherpressures [2]. In this case, the use of picosecond (ps) or femtosecond (fs) TALIF aswell as ultrafast detectors for atomic density and lifetime determination is a bettersolution [2,3].This work focuses on the investigation of a μs-pulsed microtube helium APPJ bymeans of ps-TALIF (laser: Ekspla®; pulse width: ~10 ps) and a streak camera(C1091005, Hamamatsu®; few ps time resolution). The achieved spatial resolutioncan be as low as 400 μm, while we were able to measure laser-excited H-atomlifetimes down to a few hundred ps (Figure 1). The obtained H-atom density andlifetime values depend on the distance from the tube exit and the APPJ parameters(voltage, helium flow rate, …). These results are supported by electrical and OESdiagnostics. This study provides useful information for APPJ kinetic models while ithelps in the optimisation of APPJ for novel applications [1].[1] S. Brandt et al., Anal. Chim. Acta 951, 16–31, 2017[2] K. Gazeli et al., Plasma 4, 145–71, 2021[3] S-J. Klose et al., Plasma Sources Sci. Technol. 29, 125018, 202