Atomic density distribution over the excited-states in high-pressure mercury plasmas

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Summary form only given, as follows. Determination of the excited-state distribution of atoms in lighting plasmas is of paramount importance in the physics and the development of radiation sources. The development of a self-reversed line spectroscopy (SRLS) method, which is independent of plasma-equilibrium assumptions, made it possible to determine the electron temperature by identifying it to the distribution temperature between the mercury metastable levels as well as to deduced the densities of the ground and low-lying levels in a mercury discharge. Comparing to active methods, observation of emission lines is most accessible experimentally and do not introduce plasma perturbation. This work is aimed at applying SRLS for determining the excited-state distribution of atoms up to the ionization level. Two mercury discharges at 2 and 5 bar with 1 cm diameter operated on ac (50 Hz, 100 W/cm) were used. Side-on-phase resolved radiation intensity measurements were performed by means of an automated experimental set-up. The local values of the level-population temperatures referenced to the ground state were found as well as the ionization temperature at the moments of the maximum and the minimum emission phases. This enabled us to determine the nonequilibrium factor, i.e., the actual level density normalized to the Saha-equilibrium density at the electron temperature and the electron density prevailing in the discharge. Then, measuring the density of a sufficiently high-lying level by Abel-inverting an optically thin line, the density distribution of atoms over the excited-states at various radial positions was deduced. It was found that the population temperature decreases as we go up the excitation space of the atom (nonequilibrium excitation), and the ground state is over-populated with respect to its Saha-equilibrium density. The deviation of the plasma parameters from those obtained assuming equilibrium conditions is also discussed.