Excitation Equilibria in an Argon Surface-Wave-Produced and -Sustained Plasmas

In this paper the population of the excited levels in a high frequency surface-wave-produced and -sustained argon plasma, has been measured in the pressure range 0.2–2.8 Torr. The experimental procedure is based on different techniques; for the radiative transitions between 4p–4s argon excited levels a self-absorption method has been implemented, whereas for the rest of the transitions (involving 5d, 5p, 6d and 6s levels) the plasma has been assumed optically thin and an optical emission spectroscopy technique has been used. The results show that the plasma remains very far from thermodynamic equilibrium. The excited states present an overpopulation when compared to those obtained through equilibrium relations (to an order of magnitude of 105, or even higher). The plasma is, therefore, an ionizing plasma; so a classification of the excited levels according to the dominant collisional processes has been established. The dependence of these populations on the effective quantum number, p, is proportional to p-6 and their overpopulation is proportional to p-5.