Two-photon absorption laser-induced fluorescence of H atoms: A probe for heterogeneous processes in hydrogen plasmas

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Abstract
Two-photon absorption laser-induced fluorescence has been used to obtain the spatial distribution of H atoms in the interelectrode space of a parallel plate rf reactor. Continuous and pulsed operation of the discharge allows the monitoring of H atoms, which are produced at the sheath boundary and are removed on the confining surfaces, at rates that depend on the conditions of the discharge and the nature of the surfaces. A variety of metallic and semiconducting surfaces have been loaded on the ground electrode resulting in widely varying H concentration decay rates and concentration gradients characteristic of the surfaces, which range from significantly absorbing to nearly reflecting. A simple analytical model that simulates spatial and temporal profiles determines from the experimental data surface loss coefficients γ in the range 0.1%–20% for H atoms. Finally, we demonstrate the sensitivity of the method in probing, in situ and in real-time, surface modifications induced by energetic plasmas.