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

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One of the major constraints of measurements of atomic hydrogen densities using two-photon absorption laser induced fluorescence in most plasma and combustion environments is the determination of fluorescence decay times ( τfluo H ), especially when using nanosecond-lasers or slow acquisition systems. Therefore, it is necessary to identify the depopulation processes of the laser excited level in order to correctly estimate τfluo H . In this study, depopulation mechanisms of atomic hydrogen excited by two-photon absorption to the n = 3 level (H(n = 3)) have been investigated using a picosecond-laser excitation and acquisition of fluorescence by a streak camera, which allowed for direct measurement of τfluo H and hence, the atomic hydrogen densities, in an H2 microwave plasma operating in the pressure range 20–300 Pa. By combining these measurements with a detailed H(n = 3) collisional radiative depopulation model, it was found that full mixing amongst the H(n = 3) sub-levels occurs in our discharge conditions, even at a pressure as low as 20 Pa. Moreover, it is also seen that the Lyman β line is only partially trapped, as its escape factor Λ31 decreases from 0.94–0.98 down to 0.58–0.86 while the measured atomic hydrogen density rises from 8±5×1019m−3 to 9±6×1020m−3 . This means that the radiative decay rate of H(n = 3) atoms varies with pressure and the classical Stern–Volmer method used to determine the quenching cross-section of excited H(n = 3) in collisions with H2 molecules, σQHn=3/H2 , is not valid for our measurements. We used two different physics-based approaches, and show that the quenching cross-section σQHn=3/H2 lies in the range 90– 106×10−20m2 , which can be averaged as 98±8×10−20m2 . This substantially improved estimation of σQHn=3/H2 obtained in this work will be useful for the accurate estimation of H(n = 3) fluorescence decay times and therefore the atomic hydrogen densities.