LADDERLIKE IONIZATION OF THE MERCURY ATOM IN HG-AR LOW-PRESSURE DISCHARGE AND ITS MODELING

A collisional-radiative model for hydrogen plasma has been reviewed in order to evaluate the ionization process through the high-lying levels of the mercury atom. Experimentally observed population densities of the mercury atom near its series limit have shown an x*-6 dependence (x* is the effective principle quantum number) as is predicted from the collisional-radiative model for an ionizing plasma. This fact suggests that a ladderlike ionization process accounts for a missing ionization process found in a previous model calculation for the mercury-argon low-pressure discharge [J. Appl. Phys. 68, 5052 (1990)]. An effective rate coefficient for the ladderlike excitation and ionization has been determined and introduced into the model equation. Electric and optical characteristics as well as the population densities of the excited atoms in the positive column have been calculated and compared with the experimental result. Significant improvement in agreement is obtained. The ladderlike excitation and ionization via high-lying levels of the mercury atom have been shown to occupy 10%-20% of the total ionization rate in the discharge considered.