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

被引：0

作者：

Duluard, Corinne Y. ^{[1
]}

Invernizzi, Laurent ^{[1
]}

Hassouni, Khaled ^{[1
]}

Lombardi, Guillaume ^{[1
]}

Gazeli, Kristaq ^{[1
]}

Prasanna, Swaminathan ^{[1
]}

机构：

[1] Laboratoire des Sciences des Procédés et des Matériaux, LSPM (UPR 3407), CNRS, Université Sorbonne Paris Nord, Villetaneuse,93430, France

来源：

Plasma Sources Science and Technology | 2024年 / 33卷 / 01期

关键词：

Atoms - Collisional plasmas - Electric discharges - Hydrogen - Laser excitation - Laser optics - Laser produced plasmas - Photons - Quenching - Two photon processes;

D O I：

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学科分类号：

摘要：

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 ( τ fluo 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 τ fluo 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 τ fluo 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 Λ 31 decreases from 0.94-0.98 down to 0.58-0.86 while the measured atomic hydrogen density rises from 8 ± 5 × 10 19 m − 3 to 9 ± 6 × 10 20 m − 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, σ Q H n = 3 / H 2 , is not valid for our measurements. We used two different physics-based approaches, and show that the quenching cross-section σ Q H n = 3 / H 2 lies in the range 90- 106 × 10 − 20 m 2 , which can be averaged as 98 ± 8 × 10 − 20 m 2 . This substantially improved estimation of σ Q H n = 3 / H 2 obtained in this work will be useful for the accurate estimation of H(n = 3) fluorescence decay times and therefore the atomic hydrogen densities. © 2024 IOP Publishing Ltd.

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