CW laser absorption spectroscopy for the diagnostic of I 2 molecules and I atoms in iodine plasmas

Abstract This paper details the implementation of optical diagnostics in a simplified system constituted of a radio frequency capacitive plasma discharge in a cylindrical glass cell with fixed initial density to probe the ground state of the neutral species of iodine plasmas: the molecules $\rm{I}_2$ and the atoms $\rm{I}$. Three distinct diagnostics are implemented. First, the temperature and density of the iodine atoms is measured via laser absorption by a hyperfine component of the forbidden transition between the two fine-structure levels $^2\rm{P}^o_{3/2}$ and $^2\rm{P}^o_{1/2}$. Second, saturated absorption spectroscopy is implemented to measure the hyperfine structure of the absorption line $\rm{P}(28)\,42-0$ of the iodine molecule. The knowledge of this structure allows to measure the temperature and density of a particular ro-vibrational state by laser absorption spectroscopy. Third, the diffuse region of the absorption spectrum, above the dissociation limit, is probed by photo-dissociation in order to measure the absolute molecular density. Measurements are taken in the center of the plasma cell for a range of plasma power as well as along the two main radial directions. It is found that the measured translational temperatures of the two probed species are different, the atoms being hotter than the molecules in the whole cell, this discrepancy being attributed to spatial gradients of temperature and density. In fact, the comparison of two absorption transitions shows confirms that the population of the ro-vibrational states of the molecule in the plasma is described by the Maxwell Boltzmann equilibrium and that the translational and rotational temperatures are in equilibrium for the molecules. The proportion of excited atomic state $^2\rm{P}^o_{1/2}$ is estimated to be between 5~\% and 10~\% in the tested plasma conditions. Additionally, radial measurements highlight large radial gradients of temperature, density and dissociation fraction in the cell while the isobaric condition is always satisfied.