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The present work deals with investigating the loss kinetics of atomic hydrogen in an H 2 /CH 4 pulsed microwave discharge used for diamond chemical vapour deposition. The temporal evolution of the hydrogen atom density, particularly in the afterglow phase, was investigated in order to determine the fundamental loss processes occurring in such discharge. Time resolved two-photon laser induced fluorescence (LIF) measurements of the relative concentration of H-atoms in the ground state and observed by H α transition have been established. The gas temperature was estimated from the Doppler broadening of the LIF Gaussian profile obtained by scanning the laser frequency. The wavelength calibration and the estimation of the effective bandwidth of the LIF profile were accomplished by measuring the spectral response of atomic hydrogen and deuterium. The decay of H-atom density in the afterglow is characterized by a fast decrease in the early afterglow up to 2 ms, followed by a slow evolution. The effect of the gas cooling on the diffusion coefficient and then on the atom density decay in the afterglow was studied. It is found that the fundamental loss processes of atomic hydrogen are governed by the diffusion phenomenon and surface recombinations, especially on the diamond substrate.

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In this work, we show the feasibility of applying time-resolved broadband optical absorption spectroscopy using an LED light source as a valuable diagnostic tool for characterizing the electrical arc's current-interruption process. With our setup we have shown the capability of following in a single measurement the time evolution of the diffusion of chromium (Cr) neutral atoms around and after current zero of an electrical arc that was ignited between two Cu-Cr contacts. This is achieved by analyzing the absorption spectrum of CrI resonance lines near the central wavelength of the broadband LED at 425 nm, and using the so-called fast-kinetics acquisition mode of the spectrometer, with a time resolution of 415 µs. As a result, by relying on the optical thinness of our lines, we could estimate the time evolution of the ground-state population of neutral Cr atoms outside the contact gap.