Investigation of reactive plasma species created in SO_2 by an inductively coupled RF discharge in E- and H-mode

摘要

Optical emission spectroscopy (OES) and mass spectrometry were used to investigate the gas phase and surface reactions in inductively coupled SO_2 plasma at various radiofrequency discharge powers up to 1000 W and gas pressures from 30 to 100 Pa. At such conditions, the plasma was created either in E- or in H-mode. In the E-mode, extensive radiation in the UV range was observed due to transitions of SO_2 and SO molecules to the ground electronic states, whereas the other spectral features were marginal. At elevated powers, an abrupt transition to the H-mode occurred, where the total radiation increased for several orders of magnitude. Strong hysteresis was observed in the behaviour of all OES spectral features at the transitions between the E- and H-modes. In the H-mode, the atomic lines prevailed because of the relaxation of highly excited O and S atoms to the lower excited states, indicating high density of atoms. UV continuum was very weak and governed only by transitions of the SO radicals to the ground state. Thus, it was concluded that in the E-mode, predominantly SO and O radicals are formed during the partial dissociation of SO_2 molecules, whereas in the H-mode, high dissociation to S and O atoms occurred, leading to the negligible concentration of SO_2. However, in the flowing afterglow, the final gas composition was predominantly always SO_2. The concentration of O_2 was only approximately 3%, whereas the concentration of SO_3 was marginal. This was explained by the recombination of the reactive plasma species formed in the plasma back to SO_2 molecules on the surfaces of the remote plasma reactor.